summaryrefslogtreecommitdiffstats
blob: 26e153c6ba9e509d8bb5a9ca9470d38d3694e3cd (plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
752
753
754
755
756
757
758
759
760
761
762
763
764
765
766
767
768
769
770
771
772
773
774
775
776
777
778
779
780
781
782
783
784
785
786
787
788
789
790
791
792
793
794
795
796
797
798
799
800
801
802
803
804
805
806
807
808
809
810
811
812
813
814
815
816
817
818
819
820
821
822
823
824
825
826
827
828
829
830
831
832
833
834
835
836
837
838
839
840
841
842
843
844
845
846
847
848
849
850
851
852
853
854
855
856
857
858
859
860
861
862
863
864
865
866
867
868
869
870
871
872
873
874
875
876
877
878
879
880
881
882
883
884
885
886
887
888
889
890
891
892
893
894
895
896
897
898
899
900
901
902
903
904
905
906
907
908
909
910
911
912
913
914
915
916
917
918
919
920
921
922
923
924
925
926
927
928
929
930
931
932
933
934
935
936
937
938
939
940
941
942
943
944
945
946
947
948
949
950
951
952
953
954
955
956
957
958
959
960
961
962
963
964
965
966
967
968
969
970
971
972
973
974
975
976
977
978
979
980
981
982
983
984
985
986
987
988
989
990
991
992
993
994
995
996
997
998
999
1000
1001
1002
1003
1004
1005
1006
1007
1008
1009
1010
1011
1012
1013
1014
1015
1016
1017
1018
1019
1020
1021
1022
1023
1024
1025
1026
1027
1028
1029
1030
1031
1032
1033
1034
1035
1036
1037
1038
1039
1040
1041
1042
1043
1044
1045
1046
1047
1048
1049
1050
1051
1052
1053
1054
1055
1056
1057
1058
1059
1060
1061
1062
1063
1064
1065
1066
1067
1068
1069
1070
1071
1072
1073
1074
1075
1076
1077
1078
1079
1080
1081
1082
1083
1084
1085
1086
1087
1088
1089
1090
1091
1092
1093
1094
1095
1096
1097
1098
1099
1100
1101
1102
1103
1104
1105
1106
1107
1108
1109
1110
1111
1112
1113
1114
1115
1116
1117
1118
1119
1120
1121
1122
1123
1124
1125
1126
1127
1128
1129
1130
1131
1132
1133
1134
1135
1136
1137
1138
1139
1140
1141
1142
1143
1144
1145
1146
1147
1148
1149
1150
1151
1152
1153
1154
1155
1156
1157
1158
1159
1160
1161
1162
1163
1164
1165
1166
1167
1168
1169
1170
1171
1172
1173
1174
1175
1176
1177
1178
1179
1180
1181
1182
1183
1184
1185
1186
1187
1188
1189
1190
1191
1192
1193
1194
1195
1196
1197
1198
1199
1200
1201
1202
1203
1204
1205
1206
1207
1208
1209
1210
1211
1212
1213
1214
1215
1216
1217
1218
1219
1220
1221
1222
1223
1224
1225
1226
1227
1228
1229
1230
1231
1232
1233
1234
1235
1236
1237
1238
1239
1240
1241
1242
1243
1244
1245
1246
1247
1248
1249
1250
1251
1252
1253
1254
1255
1256
1257
1258
1259
1260
1261
1262
1263
1264
1265
1266
1267
1268
1269
1270
1271
1272
1273
1274
1275
1276
1277
1278
1279
1280
1281
1282
1283
1284
1285
1286
1287
1288
1289
1290
1291
1292
1293
1294
1295
1296
1297
1298
1299
1300
1301
1302
1303
1304
1305
1306
1307
1308
1309
1310
1311
1312
1313
1314
1315
1316
1317
1318
1319
1320
1321
1322
1323
1324
1325
1326
1327
1328
1329
1330
1331
1332
1333
1334
1335
1336
1337
1338
1339
1340
1341
1342
1343
1344
1345
1346
1347
1348
1349
1350
1351
1352
1353
1354
1355
1356
1357
1358
1359
1360
1361
1362
1363
1364
1365
1366
1367
1368
1369
1370
1371
1372
1373
1374
1375
1376
1377
1378
1379
1380
1381
1382
1383
1384
1385
1386
1387
1388
1389
1390
1391
1392
1393
1394
1395
1396
1397
1398
1399
1400
1401
1402
1403
1404
1405
1406
1407
1408
1409
1410
1411
1412
1413
1414
1415
1416
1417
1418
1419
1420
1421
1422
1423
1424
1425
1426
1427
1428
1429
1430
1431
1432
1433
1434
1435
1436
1437
1438
1439
1440
1441
1442
1443
1444
1445
1446
1447
1448
1449
1450
1451
1452
1453
1454
1455
1456
1457
1458
1459
1460
1461
1462
1463
1464
1465
1466
1467
1468
1469
1470
1471
1472
1473
1474
1475
1476
1477
1478
1479
1480
1481
1482
1483
1484
1485
1486
1487
1488
1489
1490
1491
1492
1493
1494
1495
1496
1497
1498
1499
1500
1501
1502
1503
1504
1505
1506
1507
1508
1509
1510
1511
1512
1513
1514
1515
1516
1517
1518
1519
1520
1521
1522
1523
1524
1525
1526
1527
1528
1529
1530
1531
1532
1533
1534
1535
1536
1537
1538
1539
1540
1541
1542
1543
1544
1545
1546
1547
1548
1549
1550
1551
1552
1553
1554
1555
1556
1557
1558
1559
1560
1561
1562
1563
1564
1565
1566
1567
1568
1569
1570
1571
1572
1573
1574
1575
1576
1577
1578
1579
1580
1581
1582
1583
1584
1585
1586
1587
1588
1589
1590
1591
1592
1593
1594
1595
1596
1597
1598
1599
1600
1601
1602
1603
1604
1605
1606
1607
1608
1609
1610
1611
1612
1613
1614
1615
1616
1617
1618
1619
1620
1621
1622
1623
1624
1625
1626
1627
1628
1629
1630
1631
1632
1633
1634
1635
1636
1637
1638
1639
1640
1641
1642
1643
1644
1645
1646
1647
1648
1649
1650
1651
1652
1653
1654
1655
1656
1657
1658
1659
1660
1661
1662
1663
1664
1665
1666
1667
1668
1669
1670
1671
1672
1673
1674
1675
1676
1677
1678
1679
1680
1681
1682
1683
1684
1685
1686
1687
1688
1689
1690
1691
1692
1693
1694
1695
1696
1697
1698
1699
1700
1701
1702
1703
1704
1705
1706
1707
1708
1709
1710
1711
1712
1713
1714
1715
1716
1717
1718
1719
1720
1721
1722
1723
1724
1725
1726
1727
1728
1729
1730
1731
1732
1733
1734
1735
1736
1737
1738
1739
1740
1741
1742
1743
1744
1745
1746
1747
1748
1749
1750
1751
1752
1753
1754
1755
1756
1757
1758
1759
1760
1761
1762
1763
1764
1765
1766
1767
1768
1769
1770
1771
1772
1773
1774
1775
1776
1777
1778
1779
1780
1781
1782
1783
1784
1785
1786
1787
1788
1789
1790
1791
1792
1793
1794
1795
1796
1797
1798
1799
1800
1801
1802
1803
1804
1805
1806
1807
1808
1809
1810
1811
1812
1813
1814
1815
1816
1817
1818
1819
1820
1821
1822
1823
1824
1825
1826
1827
1828
1829
1830
1831
1832
1833
1834
1835
1836
1837
1838
1839
1840
1841
1842
1843
1844
1845
1846
1847
1848
1849
1850
1851
1852
1853
1854
1855
1856
1857
1858
1859
1860
1861
1862
1863
1864
1865
1866
1867
1868
1869
1870
1871
1872
1873
1874
1875
1876
1877
1878
1879
1880
1881
1882
1883
1884
1885
1886
1887
1888
1889
1890
1891
1892
1893
1894
1895
1896
1897
1898
1899
1900
1901
1902
1903
1904
1905
1906
1907
1908
1909
1910
1911
1912
1913
1914
1915
1916
1917
1918
1919
1920
1921
1922
1923
1924
1925
1926
1927
1928
1929
1930
1931
1932
1933
1934
1935
1936
1937
1938
1939
1940
1941
1942
1943
1944
1945
1946
1947
1948
1949
1950
1951
1952
1953
1954
1955
1956
1957
1958
1959
1960
1961
1962
1963
1964
1965
1966
1967
1968
1969
1970
1971
1972
1973
1974
1975
1976
1977
1978
1979
1980
1981
1982
1983
1984
1985
1986
1987
1988
1989
1990
1991
1992
1993
1994
1995
1996
1997
1998
1999
2000
2001
2002
2003
2004
2005
2006
2007
2008
2009
2010
2011
2012
2013
2014
2015
2016
2017
2018
2019
2020
2021
2022
2023
2024
2025
2026
2027
2028
2029
2030
2031
2032
2033
2034
2035
2036
2037
2038
2039
2040
2041
2042
2043
2044
2045
2046
2047
2048
2049
2050
2051
2052
2053
2054
2055
2056
2057
2058
2059
2060
2061
2062
2063
2064
2065
2066
2067
2068
2069
2070
2071
2072
2073
2074
2075
2076
2077
2078
2079
2080
2081
2082
2083
2084
2085
2086
2087
2088
2089
2090
2091
2092
2093
2094
2095
2096
2097
2098
2099
2100
2101
2102
2103
2104
2105
2106
2107
2108
2109
2110
2111
2112
2113
2114
2115
2116
2117
2118
2119
2120
2121
2122
2123
2124
2125
2126
2127
2128
2129
2130
2131
2132
2133
2134
2135
2136
2137
2138
2139
2140
2141
2142
2143
2144
2145
2146
2147
2148
2149
2150
2151
2152
2153
2154
2155
2156
2157
2158
2159
2160
2161
2162
2163
2164
2165
2166
2167
2168
2169
2170
2171
2172
2173
2174
2175
2176
2177
2178
2179
2180
2181
2182
2183
2184
2185
2186
2187
2188
2189
2190
2191
2192
2193
2194
2195
2196
2197
2198
2199
2200
2201
2202
2203
2204
2205
2206
2207
2208
2209
2210
2211
2212
2213
2214
2215
2216
2217
2218
2219
2220
2221
2222
2223
2224
2225
2226
2227
2228
2229
2230
2231
2232
2233
2234
2235
2236
2237
2238
2239
2240
2241
2242
2243
2244
2245
2246
2247
2248
2249
2250
2251
2252
2253
2254
2255
2256
2257
2258
2259
2260
2261
2262
2263
2264
2265
2266
2267
2268
2269
2270
2271
2272
2273
2274
2275
2276
2277
2278
2279
2280
2281
2282
2283
2284
2285
2286
2287
2288
2289
2290
2291
2292
2293
2294
2295
2296
2297
2298
2299
2300
2301
2302
2303
2304
2305
2306
2307
2308
2309
2310
2311
2312
2313
2314
2315
2316
2317
2318
2319
2320
2321
2322
2323
2324
2325
2326
2327
2328
2329
2330
2331
2332
2333
2334
2335
2336
2337
2338
2339
2340
2341
2342
2343
2344
2345
2346
2347
2348
2349
2350
2351
2352
2353
2354
2355
2356
2357
2358
2359
2360
2361
2362
2363
2364
2365
2366
2367
2368
2369
2370
2371
2372
2373
2374
2375
2376
2377
2378
2379
2380
2381
2382
2383
2384
2385
2386
2387
2388
2389
2390
2391
2392
2393
2394
2395
2396
2397
2398
2399
2400
2401
2402
2403
2404
2405
2406
2407
2408
2409
2410
2411
2412
2413
2414
2415
2416
2417
2418
2419
2420
2421
2422
2423
2424
2425
2426
2427
2428
2429
2430
2431
2432
2433
2434
2435
2436
2437
2438
2439
2440
2441
2442
2443
2444
2445
2446
2447
2448
2449
2450
2451
2452
2453
2454
2455
2456
2457
2458
2459
2460
2461
2462
2463
2464
2465
2466
2467
2468
2469
2470
2471
2472
2473
2474
2475
2476
2477
2478
2479
2480
2481
2482
2483
2484
2485
2486
2487
2488
2489
2490
2491
2492
2493
2494
2495
2496
2497
2498
2499
2500
2501
2502
2503
2504
2505
2506
2507
2508
2509
2510
2511
2512
2513
2514
2515
2516
2517
2518
2519
2520
2521
2522
2523
2524
2525
2526
2527
2528
2529
2530
2531
2532
2533
2534
2535
2536
2537
2538
2539
2540
2541
2542
2543
2544
2545
2546
2547
2548
2549
2550
2551
2552
2553
2554
2555
2556
2557
2558
2559
2560
2561
2562
2563
2564
2565
2566
2567
2568
2569
2570
2571
2572
2573
2574
2575
2576
2577
2578
2579
2580
2581
2582
2583
2584
2585
2586
2587
2588
2589
2590
2591
2592
2593
2594
2595
2596
2597
2598
2599
2600
2601
2602
2603
2604
2605
2606
2607
2608
2609
2610
2611
2612
2613
2614
2615
2616
2617
2618
2619
2620
2621
2622
2623
2624
2625
2626
2627
2628
2629
2630
2631
2632
2633
2634
2635
2636
2637
2638
2639
2640
2641
2642
2643
2644
2645
2646
2647
2648
2649
2650
2651
2652
2653
2654
2655
2656
2657
2658
2659
2660
2661
2662
2663
2664
2665
2666
2667
2668
2669
2670
2671
2672
2673
2674
2675
2676
2677
2678
2679
2680
2681
2682
2683
2684
2685
2686
2687
2688
2689
2690
2691
2692
2693
2694
2695
2696
2697
2698
2699
2700
2701
2702
2703
2704
2705
2706
2707
2708
2709
2710
2711
2712
2713
2714
2715
2716
2717
2718
2719
2720
2721
2722
2723
2724
2725
2726
2727
2728
2729
2730
2731
2732
2733
2734
2735
2736
2737
2738
2739
2740
2741
2742
2743
2744
2745
2746
2747
2748
2749
2750
2751
2752
2753
2754
2755
2756
2757
2758
2759
2760
2761
2762
2763
2764
2765
2766
2767
2768
2769
2770
2771
2772
2773
2774
2775
2776
2777
2778
2779
2780
2781
2782
2783
2784
2785
2786
2787
2788
2789
2790
2791
2792
2793
2794
2795
2796
2797
2798
2799
2800
2801
2802
2803
2804
2805
2806
2807
2808
2809
2810
2811
2812
2813
2814
2815
2816
2817
2818
2819
2820
2821
2822
2823
2824
2825
2826
2827
2828
2829
2830
2831
2832
2833
2834
2835
2836
2837
2838
2839
2840
2841
2842
2843
2844
2845
2846
2847
2848
2849
2850
2851
2852
2853
2854
2855
2856
2857
2858
2859
2860
2861
2862
2863
2864
2865
2866
2867
2868
2869
2870
2871
2872
2873
2874
2875
2876
2877
2878
2879
2880
2881
2882
2883
2884
2885
2886
2887
2888
2889
2890
2891
2892
2893
2894
2895
2896
2897
2898
2899
2900
2901
2902
2903
2904
2905
2906
2907
2908
2909
2910
2911
2912
2913
2914
2915
2916
2917
2918
2919
2920
2921
2922
2923
2924
2925
2926
2927
2928
2929
2930
2931
2932
2933
2934
2935
2936
2937
2938
2939
2940
2941
2942
2943
2944
2945
2946
2947
2948
2949
2950
2951
2952
2953
2954
2955
2956
2957
2958
2959
2960
2961
2962
2963
2964
2965
2966
2967
2968
2969
2970
2971
2972
2973
2974
2975
2976
2977
2978
2979
2980
2981
2982
2983
2984
2985
2986
2987
2988
2989
2990
2991
2992
2993
2994
2995
2996
2997
2998
2999
3000
3001
3002
3003
3004
3005
3006
3007
3008
3009
3010
3011
3012
3013
3014
3015
3016
3017
3018
3019
3020
3021
3022
3023
3024
3025
3026
3027
3028
3029
3030
3031
3032
3033
3034
3035
3036
3037
3038
3039
3040
3041
3042
3043
3044
3045
3046
3047
3048
3049
3050
3051
3052
3053
3054
3055
3056
3057
3058
3059
3060
3061
3062
3063
3064
3065
3066
3067
3068
3069
3070
3071
3072
3073
3074
3075
3076
3077
3078
3079
3080
3081
3082
3083
3084
3085
3086
3087
3088
3089
3090
3091
3092
3093
3094
3095
3096
3097
3098
3099
3100
3101
3102
3103
3104
3105
3106
3107
3108
3109
3110
3111
3112
3113
3114
3115
3116
3117
3118
3119
3120
3121
3122
3123
3124
3125
3126
3127
3128
3129
3130
3131
3132
3133
3134
3135
3136
3137
3138
3139
3140
3141
3142
3143
3144
3145
3146
3147
3148
3149
3150
3151
3152
3153
3154
3155
3156
3157
3158
3159
3160
3161
3162
3163
3164
3165
3166
3167
3168
3169
3170
3171
3172
3173
3174
3175
3176
3177
3178
3179
3180
3181
3182
3183
3184
3185
3186
3187
3188
3189
3190
3191
3192
3193
3194
3195
3196
3197
3198
3199
3200
3201
3202
3203
3204
3205
3206
3207
3208
3209
3210
3211
3212
3213
3214
3215
3216
3217
3218
3219
3220
3221
3222
3223
3224
3225
3226
3227
3228
3229
3230
3231
3232
3233
3234
3235
3236
3237
3238
3239
3240
3241
3242
3243
3244
3245
3246
3247
3248
3249
3250


You can find recipes for using Google Mock here. If you haven't yet,
please read the [ForDummies](V1_5_ForDummies.md) document first to make sure you understand
the basics.

**Note:** Google Mock lives in the `testing` name space. For
readability, it is recommended to write `using ::testing::Foo;` once in
your file before using the name `Foo` defined by Google Mock. We omit
such `using` statements in this page for brevity, but you should do it
in your own code.

# Creating Mock Classes #

## Mocking Private or Protected Methods ##

You must always put a mock method definition (`MOCK_METHOD*`) in a
`public:` section of the mock class, regardless of the method being
mocked being `public`, `protected`, or `private` in the base class.
This allows `ON_CALL` and `EXPECT_CALL` to reference the mock function
from outside of the mock class.  (Yes, C++ allows a subclass to change
the access level of a virtual function in the base class.)  Example:

```
class Foo {
 public:
  ...
  virtual bool Transform(Gadget* g) = 0;

 protected:
  virtual void Resume();

 private:
  virtual int GetTimeOut();
};

class MockFoo : public Foo {
 public:
  ...
  MOCK_METHOD1(Transform, bool(Gadget* g));

  // The following must be in the public section, even though the
  // methods are protected or private in the base class.
  MOCK_METHOD0(Resume, void());
  MOCK_METHOD0(GetTimeOut, int());
};
```

## Mocking Overloaded Methods ##

You can mock overloaded functions as usual. No special attention is required:

```
class Foo {
  ...

  // Must be virtual as we'll inherit from Foo.
  virtual ~Foo();

  // Overloaded on the types and/or numbers of arguments.
  virtual int Add(Element x);
  virtual int Add(int times, Element x);

  // Overloaded on the const-ness of this object.
  virtual Bar& GetBar();
  virtual const Bar& GetBar() const;
};

class MockFoo : public Foo {
  ...
  MOCK_METHOD1(Add, int(Element x));
  MOCK_METHOD2(Add, int(int times, Element x);

  MOCK_METHOD0(GetBar, Bar&());
  MOCK_CONST_METHOD0(GetBar, const Bar&());
};
```

**Note:** if you don't mock all versions of the overloaded method, the
compiler will give you a warning about some methods in the base class
being hidden. To fix that, use `using` to bring them in scope:

```
class MockFoo : public Foo {
  ...
  using Foo::Add;
  MOCK_METHOD1(Add, int(Element x));
  // We don't want to mock int Add(int times, Element x);
  ...
};
```

## Mocking Class Templates ##

To mock a class template, append `_T` to the `MOCK_*` macros:

```
template <typename Elem>
class StackInterface {
  ...
  // Must be virtual as we'll inherit from StackInterface.
  virtual ~StackInterface();

  virtual int GetSize() const = 0;
  virtual void Push(const Elem& x) = 0;
};

template <typename Elem>
class MockStack : public StackInterface<Elem> {
  ...
  MOCK_CONST_METHOD0_T(GetSize, int());
  MOCK_METHOD1_T(Push, void(const Elem& x));
};
```

## Mocking Nonvirtual Methods ##

Google Mock can mock non-virtual functions to be used in what we call _hi-perf
dependency injection_.

In this case, instead of sharing a common base class with the real
class, your mock class will be _unrelated_ to the real class, but
contain methods with the same signatures.  The syntax for mocking
non-virtual methods is the _same_ as mocking virtual methods:

```
// A simple packet stream class.  None of its members is virtual.
class ConcretePacketStream {
 public:
  void AppendPacket(Packet* new_packet);
  const Packet* GetPacket(size_t packet_number) const;
  size_t NumberOfPackets() const;
  ...
};

// A mock packet stream class.  It inherits from no other, but defines
// GetPacket() and NumberOfPackets().
class MockPacketStream {
 public:
  MOCK_CONST_METHOD1(GetPacket, const Packet*(size_t packet_number));
  MOCK_CONST_METHOD0(NumberOfPackets, size_t());
  ...
};
```

Note that the mock class doesn't define `AppendPacket()`, unlike the
real class. That's fine as long as the test doesn't need to call it.

Next, you need a way to say that you want to use
`ConcretePacketStream` in production code, and use `MockPacketStream`
in tests.  Since the functions are not virtual and the two classes are
unrelated, you must specify your choice at _compile time_ (as opposed
to run time).

One way to do it is to templatize your code that needs to use a packet
stream.  More specifically, you will give your code a template type
argument for the type of the packet stream.  In production, you will
instantiate your template with `ConcretePacketStream` as the type
argument.  In tests, you will instantiate the same template with
`MockPacketStream`.  For example, you may write:

```
template <class PacketStream>
void CreateConnection(PacketStream* stream) { ... }

template <class PacketStream>
class PacketReader {
 public:
  void ReadPackets(PacketStream* stream, size_t packet_num);
};
```

Then you can use `CreateConnection<ConcretePacketStream>()` and
`PacketReader<ConcretePacketStream>` in production code, and use
`CreateConnection<MockPacketStream>()` and
`PacketReader<MockPacketStream>` in tests.

```
  MockPacketStream mock_stream;
  EXPECT_CALL(mock_stream, ...)...;
  .. set more expectations on mock_stream ...
  PacketReader<MockPacketStream> reader(&mock_stream);
  ... exercise reader ...
```

## Mocking Free Functions ##

It's possible to use Google Mock to mock a free function (i.e. a
C-style function or a static method).  You just need to rewrite your
code to use an interface (abstract class).

Instead of calling a free function (say, `OpenFile`) directly,
introduce an interface for it and have a concrete subclass that calls
the free function:

```
class FileInterface {
 public:
  ...
  virtual bool Open(const char* path, const char* mode) = 0;
};

class File : public FileInterface {
 public:
  ...
  virtual bool Open(const char* path, const char* mode) {
    return OpenFile(path, mode);
  }
};
```

Your code should talk to `FileInterface` to open a file.  Now it's
easy to mock out the function.

This may seem much hassle, but in practice you often have multiple
related functions that you can put in the same interface, so the
per-function syntactic overhead will be much lower.

If you are concerned about the performance overhead incurred by
virtual functions, and profiling confirms your concern, you can
combine this with the recipe for [mocking non-virtual methods](#Mocking_Nonvirtual_Methods.md).

## Nice Mocks and Strict Mocks ##

If a mock method has no `EXPECT_CALL` spec but is called, Google Mock
will print a warning about the "uninteresting call". The rationale is:

  * New methods may be added to an interface after a test is written. We shouldn't fail a test just because a method it doesn't know about is called.
  * However, this may also mean there's a bug in the test, so Google Mock shouldn't be silent either. If the user believes these calls are harmless, he can add an `EXPECT_CALL()` to suppress the warning.

However, sometimes you may want to suppress all "uninteresting call"
warnings, while sometimes you may want the opposite, i.e. to treat all
of them as errors. Google Mock lets you make the decision on a
per-mock-object basis.

Suppose your test uses a mock class `MockFoo`:

```
TEST(...) {
  MockFoo mock_foo;
  EXPECT_CALL(mock_foo, DoThis());
  ... code that uses mock_foo ...
}
```

If a method of `mock_foo` other than `DoThis()` is called, it will be
reported by Google Mock as a warning. However, if you rewrite your
test to use `NiceMock<MockFoo>` instead, the warning will be gone,
resulting in a cleaner test output:

```
using ::testing::NiceMock;

TEST(...) {
  NiceMock<MockFoo> mock_foo;
  EXPECT_CALL(mock_foo, DoThis());
  ... code that uses mock_foo ...
}
```

`NiceMock<MockFoo>` is a subclass of `MockFoo`, so it can be used
wherever `MockFoo` is accepted.

It also works if `MockFoo`'s constructor takes some arguments, as
`NiceMock<MockFoo>` "inherits" `MockFoo`'s constructors:

```
using ::testing::NiceMock;

TEST(...) {
  NiceMock<MockFoo> mock_foo(5, "hi");  // Calls MockFoo(5, "hi").
  EXPECT_CALL(mock_foo, DoThis());
  ... code that uses mock_foo ...
}
```

The usage of `StrictMock` is similar, except that it makes all
uninteresting calls failures:

```
using ::testing::StrictMock;

TEST(...) {
  StrictMock<MockFoo> mock_foo;
  EXPECT_CALL(mock_foo, DoThis());
  ... code that uses mock_foo ...

  // The test will fail if a method of mock_foo other than DoThis()
  // is called.
}
```

There are some caveats though (I don't like them just as much as the
next guy, but sadly they are side effects of C++'s limitations):

  1. `NiceMock<MockFoo>` and `StrictMock<MockFoo>` only work for mock methods defined using the `MOCK_METHOD*` family of macros **directly** in the `MockFoo` class. If a mock method is defined in a **base class** of `MockFoo`, the "nice" or "strict" modifier may not affect it, depending on the compiler. In particular, nesting `NiceMock` and `StrictMock` (e.g. `NiceMock<StrictMock<MockFoo> >`) is **not** supported.
  1. The constructors of the base mock (`MockFoo`) cannot have arguments passed by non-const reference, which happens to be banned by the [Google C++ style guide](http://google-styleguide.googlecode.com/svn/trunk/cppguide.xml).
  1. During the constructor or destructor of `MockFoo`, the mock object is _not_ nice or strict.  This may cause surprises if the constructor or destructor calls a mock method on `this` object. (This behavior, however, is consistent with C++'s general rule: if a constructor or destructor calls a virtual method of `this` object, that method is treated as non-virtual.  In other words, to the base class's constructor or destructor, `this` object behaves like an instance of the base class, not the derived class.  This rule is required for safety.  Otherwise a base constructor may use members of a derived class before they are initialized, or a base destructor may use members of a derived class after they have been destroyed.)

Finally, you should be **very cautious** when using this feature, as the
decision you make applies to **all** future changes to the mock
class. If an important change is made in the interface you are mocking
(and thus in the mock class), it could break your tests (if you use
`StrictMock`) or let bugs pass through without a warning (if you use
`NiceMock`). Therefore, try to specify the mock's behavior using
explicit `EXPECT_CALL` first, and only turn to `NiceMock` or
`StrictMock` as the last resort.

## Simplifying the Interface without Breaking Existing Code ##

Sometimes a method has a long list of arguments that is mostly
uninteresting. For example,

```
class LogSink {
 public:
  ...
  virtual void send(LogSeverity severity, const char* full_filename,
                    const char* base_filename, int line,
                    const struct tm* tm_time,
                    const char* message, size_t message_len) = 0;
};
```

This method's argument list is lengthy and hard to work with (let's
say that the `message` argument is not even 0-terminated). If we mock
it as is, using the mock will be awkward. If, however, we try to
simplify this interface, we'll need to fix all clients depending on
it, which is often infeasible.

The trick is to re-dispatch the method in the mock class:

```
class ScopedMockLog : public LogSink {
 public:
  ...
  virtual void send(LogSeverity severity, const char* full_filename,
                    const char* base_filename, int line, const tm* tm_time,
                    const char* message, size_t message_len) {
    // We are only interested in the log severity, full file name, and
    // log message.
    Log(severity, full_filename, std::string(message, message_len));
  }

  // Implements the mock method:
  //
  //   void Log(LogSeverity severity,
  //            const string& file_path,
  //            const string& message);
  MOCK_METHOD3(Log, void(LogSeverity severity, const string& file_path,
                         const string& message));
};
```

By defining a new mock method with a trimmed argument list, we make
the mock class much more user-friendly.

## Alternative to Mocking Concrete Classes ##

Often you may find yourself using classes that don't implement
interfaces. In order to test your code that uses such a class (let's
call it `Concrete`), you may be tempted to make the methods of
`Concrete` virtual and then mock it.

Try not to do that.

Making a non-virtual function virtual is a big decision. It creates an
extension point where subclasses can tweak your class' behavior. This
weakens your control on the class because now it's harder to maintain
the class' invariants. You should make a function virtual only when
there is a valid reason for a subclass to override it.

Mocking concrete classes directly is problematic as it creates a tight
coupling between the class and the tests - any small change in the
class may invalidate your tests and make test maintenance a pain.

To avoid such problems, many programmers have been practicing "coding
to interfaces": instead of talking to the `Concrete` class, your code
would define an interface and talk to it. Then you implement that
interface as an adaptor on top of `Concrete`. In tests, you can easily
mock that interface to observe how your code is doing.

This technique incurs some overhead:

  * You pay the cost of virtual function calls (usually not a problem).
  * There is more abstraction for the programmers to learn.

However, it can also bring significant benefits in addition to better
testability:

  * `Concrete`'s API may not fit your problem domain very well, as you may not be the only client it tries to serve. By designing your own interface, you have a chance to tailor it to your need - you may add higher-level functionalities, rename stuff, etc instead of just trimming the class. This allows you to write your code (user of the interface) in a more natural way, which means it will be more readable, more maintainable, and you'll be more productive.
  * If `Concrete`'s implementation ever has to change, you don't have to rewrite everywhere it is used. Instead, you can absorb the change in your implementation of the interface, and your other code and tests will be insulated from this change.

Some people worry that if everyone is practicing this technique, they
will end up writing lots of redundant code. This concern is totally
understandable. However, there are two reasons why it may not be the
case:

  * Different projects may need to use `Concrete` in different ways, so the best interfaces for them will be different. Therefore, each of them will have its own domain-specific interface on top of `Concrete`, and they will not be the same code.
  * If enough projects want to use the same interface, they can always share it, just like they have been sharing `Concrete`. You can check in the interface and the adaptor somewhere near `Concrete` (perhaps in a `contrib` sub-directory) and let many projects use it.

You need to weigh the pros and cons carefully for your particular
problem, but I'd like to assure you that the Java community has been
practicing this for a long time and it's a proven effective technique
applicable in a wide variety of situations. :-)

## Delegating Calls to a Fake ##

Some times you have a non-trivial fake implementation of an
interface. For example:

```
class Foo {
 public:
  virtual ~Foo() {}
  virtual char DoThis(int n) = 0;
  virtual void DoThat(const char* s, int* p) = 0;
};

class FakeFoo : public Foo {
 public:
  virtual char DoThis(int n) {
    return (n > 0) ? '+' :
        (n < 0) ? '-' : '0';
  }

  virtual void DoThat(const char* s, int* p) {
    *p = strlen(s);
  }
};
```

Now you want to mock this interface such that you can set expectations
on it. However, you also want to use `FakeFoo` for the default
behavior, as duplicating it in the mock object is, well, a lot of
work.

When you define the mock class using Google Mock, you can have it
delegate its default action to a fake class you already have, using
this pattern:

```
using ::testing::_;
using ::testing::Invoke;

class MockFoo : public Foo {
 public:
  // Normal mock method definitions using Google Mock.
  MOCK_METHOD1(DoThis, char(int n));
  MOCK_METHOD2(DoThat, void(const char* s, int* p));

  // Delegates the default actions of the methods to a FakeFoo object.
  // This must be called *before* the custom ON_CALL() statements.
  void DelegateToFake() {
    ON_CALL(*this, DoThis(_))
        .WillByDefault(Invoke(&fake_, &FakeFoo::DoThis));
    ON_CALL(*this, DoThat(_, _))
        .WillByDefault(Invoke(&fake_, &FakeFoo::DoThat));
  }
 private:
  FakeFoo fake_;  // Keeps an instance of the fake in the mock.
};
```

With that, you can use `MockFoo` in your tests as usual. Just remember
that if you don't explicitly set an action in an `ON_CALL()` or
`EXPECT_CALL()`, the fake will be called upon to do it:

```
using ::testing::_;

TEST(AbcTest, Xyz) {
  MockFoo foo;
  foo.DelegateToFake(); // Enables the fake for delegation.

  // Put your ON_CALL(foo, ...)s here, if any.

  // No action specified, meaning to use the default action.
  EXPECT_CALL(foo, DoThis(5));
  EXPECT_CALL(foo, DoThat(_, _));

  int n = 0;
  EXPECT_EQ('+', foo.DoThis(5));  // FakeFoo::DoThis() is invoked.
  foo.DoThat("Hi", &n);           // FakeFoo::DoThat() is invoked.
  EXPECT_EQ(2, n);
}
```

**Some tips:**

  * If you want, you can still override the default action by providing your own `ON_CALL()` or using `.WillOnce()` / `.WillRepeatedly()` in `EXPECT_CALL()`.
  * In `DelegateToFake()`, you only need to delegate the methods whose fake implementation you intend to use.
  * The general technique discussed here works for overloaded methods, but you'll need to tell the compiler which version you mean. To disambiguate a mock function (the one you specify inside the parentheses of `ON_CALL()`), see the "Selecting Between Overloaded Functions" section on this page; to disambiguate a fake function (the one you place inside `Invoke()`), use a `static_cast` to specify the function's type.
  * Having to mix a mock and a fake is often a sign of something gone wrong. Perhaps you haven't got used to the interaction-based way of testing yet. Or perhaps your interface is taking on too many roles and should be split up. Therefore, **don't abuse this**. We would only recommend to do it as an intermediate step when you are refactoring your code.

Regarding the tip on mixing a mock and a fake, here's an example on
why it may be a bad sign: Suppose you have a class `System` for
low-level system operations. In particular, it does file and I/O
operations. And suppose you want to test how your code uses `System`
to do I/O, and you just want the file operations to work normally. If
you mock out the entire `System` class, you'll have to provide a fake
implementation for the file operation part, which suggests that
`System` is taking on too many roles.

Instead, you can define a `FileOps` interface and an `IOOps` interface
and split `System`'s functionalities into the two. Then you can mock
`IOOps` without mocking `FileOps`.

## Delegating Calls to a Real Object ##

When using testing doubles (mocks, fakes, stubs, and etc), sometimes
their behaviors will differ from those of the real objects. This
difference could be either intentional (as in simulating an error such
that you can test the error handling code) or unintentional. If your
mocks have different behaviors than the real objects by mistake, you
could end up with code that passes the tests but fails in production.

You can use the _delegating-to-real_ technique to ensure that your
mock has the same behavior as the real object while retaining the
ability to validate calls. This technique is very similar to the
delegating-to-fake technique, the difference being that we use a real
object instead of a fake. Here's an example:

```
using ::testing::_;
using ::testing::AtLeast;
using ::testing::Invoke;

class MockFoo : public Foo {
 public:
  MockFoo() {
    // By default, all calls are delegated to the real object.
    ON_CALL(*this, DoThis())
        .WillByDefault(Invoke(&real_, &Foo::DoThis));
    ON_CALL(*this, DoThat(_))
        .WillByDefault(Invoke(&real_, &Foo::DoThat));
    ...
  }
  MOCK_METHOD0(DoThis, ...);
  MOCK_METHOD1(DoThat, ...);
  ...
 private:
  Foo real_;
};
...

  MockFoo mock;

  EXPECT_CALL(mock, DoThis())
      .Times(3);
  EXPECT_CALL(mock, DoThat("Hi"))
      .Times(AtLeast(1));
  ... use mock in test ...
```

With this, Google Mock will verify that your code made the right calls
(with the right arguments, in the right order, called the right number
of times, etc), and a real object will answer the calls (so the
behavior will be the same as in production). This gives you the best
of both worlds.

## Delegating Calls to a Parent Class ##

Ideally, you should code to interfaces, whose methods are all pure
virtual. In reality, sometimes you do need to mock a virtual method
that is not pure (i.e, it already has an implementation). For example:

```
class Foo {
 public:
  virtual ~Foo();

  virtual void Pure(int n) = 0;
  virtual int Concrete(const char* str) { ... }
};

class MockFoo : public Foo {
 public:
  // Mocking a pure method.
  MOCK_METHOD1(Pure, void(int n));
  // Mocking a concrete method.  Foo::Concrete() is shadowed.
  MOCK_METHOD1(Concrete, int(const char* str));
};
```

Sometimes you may want to call `Foo::Concrete()` instead of
`MockFoo::Concrete()`. Perhaps you want to do it as part of a stub
action, or perhaps your test doesn't need to mock `Concrete()` at all
(but it would be oh-so painful to have to define a new mock class
whenever you don't need to mock one of its methods).

The trick is to leave a back door in your mock class for accessing the
real methods in the base class:

```
class MockFoo : public Foo {
 public:
  // Mocking a pure method.
  MOCK_METHOD1(Pure, void(int n));
  // Mocking a concrete method.  Foo::Concrete() is shadowed.
  MOCK_METHOD1(Concrete, int(const char* str));

  // Use this to call Concrete() defined in Foo.
  int FooConcrete(const char* str) { return Foo::Concrete(str); }
};
```

Now, you can call `Foo::Concrete()` inside an action by:

```
using ::testing::_;
using ::testing::Invoke;
...
  EXPECT_CALL(foo, Concrete(_))
      .WillOnce(Invoke(&foo, &MockFoo::FooConcrete));
```

or tell the mock object that you don't want to mock `Concrete()`:

```
using ::testing::Invoke;
...
  ON_CALL(foo, Concrete(_))
      .WillByDefault(Invoke(&foo, &MockFoo::FooConcrete));
```

(Why don't we just write `Invoke(&foo, &Foo::Concrete)`? If you do
that, `MockFoo::Concrete()` will be called (and cause an infinite
recursion) since `Foo::Concrete()` is virtual. That's just how C++
works.)

# Using Matchers #

## Matching Argument Values Exactly ##

You can specify exactly which arguments a mock method is expecting:

```
using ::testing::Return;
...
  EXPECT_CALL(foo, DoThis(5))
      .WillOnce(Return('a'));
  EXPECT_CALL(foo, DoThat("Hello", bar));
```

## Using Simple Matchers ##

You can use matchers to match arguments that have a certain property:

```
using ::testing::Ge;
using ::testing::NotNull;
using ::testing::Return;
...
  EXPECT_CALL(foo, DoThis(Ge(5)))  // The argument must be >= 5.
      .WillOnce(Return('a'));
  EXPECT_CALL(foo, DoThat("Hello", NotNull()));
  // The second argument must not be NULL.
```

A frequently used matcher is `_`, which matches anything:

```
using ::testing::_;
using ::testing::NotNull;
...
  EXPECT_CALL(foo, DoThat(_, NotNull()));
```

## Combining Matchers ##

You can build complex matchers from existing ones using `AllOf()`,
`AnyOf()`, and `Not()`:

```
using ::testing::AllOf;
using ::testing::Gt;
using ::testing::HasSubstr;
using ::testing::Ne;
using ::testing::Not;
...
  // The argument must be > 5 and != 10.
  EXPECT_CALL(foo, DoThis(AllOf(Gt(5),
                                Ne(10))));

  // The first argument must not contain sub-string "blah".
  EXPECT_CALL(foo, DoThat(Not(HasSubstr("blah")),
                          NULL));
```

## Casting Matchers ##

Google Mock matchers are statically typed, meaning that the compiler
can catch your mistake if you use a matcher of the wrong type (for
example, if you use `Eq(5)` to match a `string` argument). Good for
you!

Sometimes, however, you know what you're doing and want the compiler
to give you some slack. One example is that you have a matcher for
`long` and the argument you want to match is `int`. While the two
types aren't exactly the same, there is nothing really wrong with
using a `Matcher<long>` to match an `int` - after all, we can first
convert the `int` argument to a `long` before giving it to the
matcher.

To support this need, Google Mock gives you the
`SafeMatcherCast<T>(m)` function. It casts a matcher `m` to type
`Matcher<T>`. To ensure safety, Google Mock checks that (let `U` be the
type `m` accepts):

  1. Type `T` can be implicitly cast to type `U`;
  1. When both `T` and `U` are built-in arithmetic types (`bool`, integers, and floating-point numbers), the conversion from `T` to `U` is not lossy (in other words, any value representable by `T` can also be represented by `U`); and
  1. When `U` is a reference, `T` must also be a reference (as the underlying matcher may be interested in the address of the `U` value).

The code won't compile if any of these conditions isn't met.

Here's one example:

```
using ::testing::SafeMatcherCast;

// A base class and a child class.
class Base { ... };
class Derived : public Base { ... };

class MockFoo : public Foo {
 public:
  MOCK_METHOD1(DoThis, void(Derived* derived));
};
...

  MockFoo foo;
  // m is a Matcher<Base*> we got from somewhere.
  EXPECT_CALL(foo, DoThis(SafeMatcherCast<Derived*>(m)));
```

If you find `SafeMatcherCast<T>(m)` too limiting, you can use a similar
function `MatcherCast<T>(m)`. The difference is that `MatcherCast` works
as long as you can `static_cast` type `T` to type `U`.

`MatcherCast` essentially lets you bypass C++'s type system
(`static_cast` isn't always safe as it could throw away information,
for example), so be careful not to misuse/abuse it.

## Selecting Between Overloaded Functions ##

If you expect an overloaded function to be called, the compiler may
need some help on which overloaded version it is.

To disambiguate functions overloaded on the const-ness of this object,
use the `Const()` argument wrapper.

```
using ::testing::ReturnRef;

class MockFoo : public Foo {
  ...
  MOCK_METHOD0(GetBar, Bar&());
  MOCK_CONST_METHOD0(GetBar, const Bar&());
};
...

  MockFoo foo;
  Bar bar1, bar2;
  EXPECT_CALL(foo, GetBar())         // The non-const GetBar().
      .WillOnce(ReturnRef(bar1));
  EXPECT_CALL(Const(foo), GetBar())  // The const GetBar().
      .WillOnce(ReturnRef(bar2));
```

(`Const()` is defined by Google Mock and returns a `const` reference
to its argument.)

To disambiguate overloaded functions with the same number of arguments
but different argument types, you may need to specify the exact type
of a matcher, either by wrapping your matcher in `Matcher<type>()`, or
using a matcher whose type is fixed (`TypedEq<type>`, `An<type>()`,
etc):

```
using ::testing::An;
using ::testing::Lt;
using ::testing::Matcher;
using ::testing::TypedEq;

class MockPrinter : public Printer {
 public:
  MOCK_METHOD1(Print, void(int n));
  MOCK_METHOD1(Print, void(char c));
};

TEST(PrinterTest, Print) {
  MockPrinter printer;

  EXPECT_CALL(printer, Print(An<int>()));            // void Print(int);
  EXPECT_CALL(printer, Print(Matcher<int>(Lt(5))));  // void Print(int);
  EXPECT_CALL(printer, Print(TypedEq<char>('a')));   // void Print(char);

  printer.Print(3);
  printer.Print(6);
  printer.Print('a');
}
```

## Performing Different Actions Based on the Arguments ##

When a mock method is called, the _last_ matching expectation that's
still active will be selected (think "newer overrides older"). So, you
can make a method do different things depending on its argument values
like this:

```
using ::testing::_;
using ::testing::Lt;
using ::testing::Return;
...
  // The default case.
  EXPECT_CALL(foo, DoThis(_))
      .WillRepeatedly(Return('b'));

  // The more specific case.
  EXPECT_CALL(foo, DoThis(Lt(5)))
      .WillRepeatedly(Return('a'));
```

Now, if `foo.DoThis()` is called with a value less than 5, `'a'` will
be returned; otherwise `'b'` will be returned.

## Matching Multiple Arguments as a Whole ##

Sometimes it's not enough to match the arguments individually. For
example, we may want to say that the first argument must be less than
the second argument. The `With()` clause allows us to match
all arguments of a mock function as a whole. For example,

```
using ::testing::_;
using ::testing::Lt;
using ::testing::Ne;
...
  EXPECT_CALL(foo, InRange(Ne(0), _))
      .With(Lt());
```

says that the first argument of `InRange()` must not be 0, and must be
less than the second argument.

The expression inside `With()` must be a matcher of type
`Matcher<tr1::tuple<A1, ..., An> >`, where `A1`, ..., `An` are the
types of the function arguments.

You can also write `AllArgs(m)` instead of `m` inside `.With()`. The
two forms are equivalent, but `.With(AllArgs(Lt()))` is more readable
than `.With(Lt())`.

You can use `Args<k1, ..., kn>(m)` to match the `n` selected arguments
against `m`. For example,

```
using ::testing::_;
using ::testing::AllOf;
using ::testing::Args;
using ::testing::Lt;
...
  EXPECT_CALL(foo, Blah(_, _, _))
      .With(AllOf(Args<0, 1>(Lt()), Args<1, 2>(Lt())));
```

says that `Blah()` will be called with arguments `x`, `y`, and `z` where
`x < y < z`.

As a convenience and example, Google Mock provides some matchers for
2-tuples, including the `Lt()` matcher above. See the [CheatSheet](V1_5_CheatSheet.md) for
the complete list.

## Using Matchers as Predicates ##

Have you noticed that a matcher is just a fancy predicate that also
knows how to describe itself? Many existing algorithms take predicates
as arguments (e.g. those defined in STL's `<algorithm>` header), and
it would be a shame if Google Mock matchers are not allowed to
participate.

Luckily, you can use a matcher where a unary predicate functor is
expected by wrapping it inside the `Matches()` function. For example,

```
#include <algorithm>
#include <vector>

std::vector<int> v;
...
// How many elements in v are >= 10?
const int count = count_if(v.begin(), v.end(), Matches(Ge(10)));
```

Since you can build complex matchers from simpler ones easily using
Google Mock, this gives you a way to conveniently construct composite
predicates (doing the same using STL's `<functional>` header is just
painful). For example, here's a predicate that's satisfied by any
number that is >= 0, <= 100, and != 50:

```
Matches(AllOf(Ge(0), Le(100), Ne(50)))
```

## Using Matchers in Google Test Assertions ##

Since matchers are basically predicates that also know how to describe
themselves, there is a way to take advantage of them in
[Google Test](http://code.google.com/p/googletest/) assertions. It's
called `ASSERT_THAT` and `EXPECT_THAT`:

```
  ASSERT_THAT(value, matcher);  // Asserts that value matches matcher.
  EXPECT_THAT(value, matcher);  // The non-fatal version.
```

For example, in a Google Test test you can write:

```
#include <gmock/gmock.h>

using ::testing::AllOf;
using ::testing::Ge;
using ::testing::Le;
using ::testing::MatchesRegex;
using ::testing::StartsWith;
...

  EXPECT_THAT(Foo(), StartsWith("Hello"));
  EXPECT_THAT(Bar(), MatchesRegex("Line \\d+"));
  ASSERT_THAT(Baz(), AllOf(Ge(5), Le(10)));
```

which (as you can probably guess) executes `Foo()`, `Bar()`, and
`Baz()`, and verifies that:

  * `Foo()` returns a string that starts with `"Hello"`.
  * `Bar()` returns a string that matches regular expression `"Line \\d+"`.
  * `Baz()` returns a number in the range [5, 10].

The nice thing about these macros is that _they read like
English_. They generate informative messages too. For example, if the
first `EXPECT_THAT()` above fails, the message will be something like:

```
Value of: Foo()
  Actual: "Hi, world!"
Expected: starts with "Hello"
```

**Credit:** The idea of `(ASSERT|EXPECT)_THAT` was stolen from the
[Hamcrest](http://code.google.com/p/hamcrest/) project, which adds
`assertThat()` to JUnit.

## Using Predicates as Matchers ##

Google Mock provides a built-in set of matchers. In case you find them
lacking, you can use an arbitray unary predicate function or functor
as a matcher - as long as the predicate accepts a value of the type
you want. You do this by wrapping the predicate inside the `Truly()`
function, for example:

```
using ::testing::Truly;

int IsEven(int n) { return (n % 2) == 0 ? 1 : 0; }
...

  // Bar() must be called with an even number.
  EXPECT_CALL(foo, Bar(Truly(IsEven)));
```

Note that the predicate function / functor doesn't have to return
`bool`. It works as long as the return value can be used as the
condition in statement `if (condition) ...`.

## Matching Arguments that Are Not Copyable ##

When you do an `EXPECT_CALL(mock_obj, Foo(bar))`, Google Mock saves
away a copy of `bar`. When `Foo()` is called later, Google Mock
compares the argument to `Foo()` with the saved copy of `bar`. This
way, you don't need to worry about `bar` being modified or destroyed
after the `EXPECT_CALL()` is executed. The same is true when you use
matchers like `Eq(bar)`, `Le(bar)`, and so on.

But what if `bar` cannot be copied (i.e. has no copy constructor)? You
could define your own matcher function and use it with `Truly()`, as
the previous couple of recipes have shown. Or, you may be able to get
away from it if you can guarantee that `bar` won't be changed after
the `EXPECT_CALL()` is executed. Just tell Google Mock that it should
save a reference to `bar`, instead of a copy of it. Here's how:

```
using ::testing::Eq;
using ::testing::ByRef;
using ::testing::Lt;
...
  // Expects that Foo()'s argument == bar.
  EXPECT_CALL(mock_obj, Foo(Eq(ByRef(bar))));

  // Expects that Foo()'s argument < bar.
  EXPECT_CALL(mock_obj, Foo(Lt(ByRef(bar))));
```

Remember: if you do this, don't change `bar` after the
`EXPECT_CALL()`, or the result is undefined.

## Validating a Member of an Object ##

Often a mock function takes a reference to object as an argument. When
matching the argument, you may not want to compare the entire object
against a fixed object, as that may be over-specification. Instead,
you may need to validate a certain member variable or the result of a
certain getter method of the object. You can do this with `Field()`
and `Property()`. More specifically,

```
Field(&Foo::bar, m)
```

is a matcher that matches a `Foo` object whose `bar` member variable
satisfies matcher `m`.

```
Property(&Foo::baz, m)
```

is a matcher that matches a `Foo` object whose `baz()` method returns
a value that satisfies matcher `m`.

For example:

> | `Field(&Foo::number, Ge(3))` | Matches `x` where `x.number >= 3`. |
|:-----------------------------|:-----------------------------------|
> | `Property(&Foo::name, StartsWith("John "))` | Matches `x` where `x.name()` starts with `"John "`. |

Note that in `Property(&Foo::baz, ...)`, method `baz()` must take no
argument and be declared as `const`.

BTW, `Field()` and `Property()` can also match plain pointers to
objects. For instance,

```
Field(&Foo::number, Ge(3))
```

matches a plain pointer `p` where `p->number >= 3`. If `p` is `NULL`,
the match will always fail regardless of the inner matcher.

What if you want to validate more than one members at the same time?
Remember that there is `AllOf()`.

## Validating the Value Pointed to by a Pointer Argument ##

C++ functions often take pointers as arguments. You can use matchers
like `NULL`, `NotNull()`, and other comparison matchers to match a
pointer, but what if you want to make sure the value _pointed to_ by
the pointer, instead of the pointer itself, has a certain property?
Well, you can use the `Pointee(m)` matcher.

`Pointee(m)` matches a pointer iff `m` matches the value the pointer
points to. For example:

```
using ::testing::Ge;
using ::testing::Pointee;
...
  EXPECT_CALL(foo, Bar(Pointee(Ge(3))));
```

expects `foo.Bar()` to be called with a pointer that points to a value
greater than or equal to 3.

One nice thing about `Pointee()` is that it treats a `NULL` pointer as
a match failure, so you can write `Pointee(m)` instead of

```
  AllOf(NotNull(), Pointee(m))
```

without worrying that a `NULL` pointer will crash your test.

Also, did we tell you that `Pointee()` works with both raw pointers
**and** smart pointers (`linked_ptr`, `shared_ptr`, `scoped_ptr`, and
etc)?

What if you have a pointer to pointer? You guessed it - you can use
nested `Pointee()` to probe deeper inside the value. For example,
`Pointee(Pointee(Lt(3)))` matches a pointer that points to a pointer
that points to a number less than 3 (what a mouthful...).

## Testing a Certain Property of an Object ##

Sometimes you want to specify that an object argument has a certain
property, but there is no existing matcher that does this. If you want
good error messages, you should define a matcher. If you want to do it
quick and dirty, you could get away with writing an ordinary function.

Let's say you have a mock function that takes an object of type `Foo`,
which has an `int bar()` method and an `int baz()` method, and you
want to constrain that the argument's `bar()` value plus its `baz()`
value is a given number. Here's how you can define a matcher to do it:

```
using ::testing::MatcherInterface;
using ::testing::MatchResultListener;

class BarPlusBazEqMatcher : public MatcherInterface<const Foo&> {
 public:
  explicit BarPlusBazEqMatcher(int expected_sum)
      : expected_sum_(expected_sum) {}

  virtual bool MatchAndExplain(const Foo& foo,
                               MatchResultListener* listener) const {
    return (foo.bar() + foo.baz()) == expected_sum_;
  }

  virtual void DescribeTo(::std::ostream* os) const {
    *os << "bar() + baz() equals " << expected_sum_;
  }

  virtual void DescribeNegationTo(::std::ostream* os) const {
    *os << "bar() + baz() does not equal " << expected_sum_;
  }
 private:
  const int expected_sum_;
};

inline Matcher<const Foo&> BarPlusBazEq(int expected_sum) {
  return MakeMatcher(new BarPlusBazEqMatcher(expected_sum));
}

...

  EXPECT_CALL(..., DoThis(BarPlusBazEq(5)))...;
```

## Matching Containers ##

Sometimes an STL container (e.g. list, vector, map, ...) is passed to
a mock function and you may want to validate it. Since most STL
containers support the `==` operator, you can write
`Eq(expected_container)` or simply `expected_container` to match a
container exactly.

Sometimes, though, you may want to be more flexible (for example, the
first element must be an exact match, but the second element can be
any positive number, and so on). Also, containers used in tests often
have a small number of elements, and having to define the expected
container out-of-line is a bit of a hassle.

You can use the `ElementsAre()` matcher in such cases:

```
using ::testing::_;
using ::testing::ElementsAre;
using ::testing::Gt;
...

  MOCK_METHOD1(Foo, void(const vector<int>& numbers));
...

  EXPECT_CALL(mock, Foo(ElementsAre(1, Gt(0), _, 5)));
```

The above matcher says that the container must have 4 elements, which
must be 1, greater than 0, anything, and 5 respectively.

`ElementsAre()` is overloaded to take 0 to 10 arguments. If more are
needed, you can place them in a C-style array and use
`ElementsAreArray()` instead:

```
using ::testing::ElementsAreArray;
...

  // ElementsAreArray accepts an array of element values.
  const int expected_vector1[] = { 1, 5, 2, 4, ... };
  EXPECT_CALL(mock, Foo(ElementsAreArray(expected_vector1)));

  // Or, an array of element matchers.
  Matcher<int> expected_vector2 = { 1, Gt(2), _, 3, ... };
  EXPECT_CALL(mock, Foo(ElementsAreArray(expected_vector2)));
```

In case the array needs to be dynamically created (and therefore the
array size cannot be inferred by the compiler), you can give
`ElementsAreArray()` an additional argument to specify the array size:

```
using ::testing::ElementsAreArray;
...
  int* const expected_vector3 = new int[count];
  ... fill expected_vector3 with values ...
  EXPECT_CALL(mock, Foo(ElementsAreArray(expected_vector3, count)));
```

**Tips:**

  * `ElementAre*()` works with _any_ container that implements the STL iterator concept (i.e. it has a `const_iterator` type and supports `begin()/end()`) and supports `size()`, not just the ones defined in STL. It will even work with container types yet to be written - as long as they follows the above pattern.
  * You can use nested `ElementAre*()` to match nested (multi-dimensional) containers.
  * If the container is passed by pointer instead of by reference, just write `Pointee(ElementsAre*(...))`.
  * The order of elements _matters_ for `ElementsAre*()`. Therefore don't use it with containers whose element order is undefined (e.g. `hash_map`).

## Sharing Matchers ##

Under the hood, a Google Mock matcher object consists of a pointer to
a ref-counted implementation object. Copying matchers is allowed and
very efficient, as only the pointer is copied. When the last matcher
that references the implementation object dies, the implementation
object will be deleted.

Therefore, if you have some complex matcher that you want to use again
and again, there is no need to build it everytime. Just assign it to a
matcher variable and use that variable repeatedly! For example,

```
  Matcher<int> in_range = AllOf(Gt(5), Le(10));
  ... use in_range as a matcher in multiple EXPECT_CALLs ...
```

# Setting Expectations #

## Ignoring Uninteresting Calls ##

If you are not interested in how a mock method is called, just don't
say anything about it. In this case, if the method is ever called,
Google Mock will perform its default action to allow the test program
to continue. If you are not happy with the default action taken by
Google Mock, you can override it using `DefaultValue<T>::Set()`
(described later in this document) or `ON_CALL()`.

Please note that once you expressed interest in a particular mock
method (via `EXPECT_CALL()`), all invocations to it must match some
expectation. If this function is called but the arguments don't match
any `EXPECT_CALL()` statement, it will be an error.

## Disallowing Unexpected Calls ##

If a mock method shouldn't be called at all, explicitly say so:

```
using ::testing::_;
...
  EXPECT_CALL(foo, Bar(_))
      .Times(0);
```

If some calls to the method are allowed, but the rest are not, just
list all the expected calls:

```
using ::testing::AnyNumber;
using ::testing::Gt;
...
  EXPECT_CALL(foo, Bar(5));
  EXPECT_CALL(foo, Bar(Gt(10)))
      .Times(AnyNumber());
```

A call to `foo.Bar()` that doesn't match any of the `EXPECT_CALL()`
statements will be an error.

## Expecting Ordered Calls ##

Although an `EXPECT_CALL()` statement defined earlier takes precedence
when Google Mock tries to match a function call with an expectation,
by default calls don't have to happen in the order `EXPECT_CALL()`
statements are written. For example, if the arguments match the
matchers in the third `EXPECT_CALL()`, but not those in the first two,
then the third expectation will be used.

If you would rather have all calls occur in the order of the
expectations, put the `EXPECT_CALL()` statements in a block where you
define a variable of type `InSequence`:

```
  using ::testing::_;
  using ::testing::InSequence;

  {
    InSequence s;

    EXPECT_CALL(foo, DoThis(5));
    EXPECT_CALL(bar, DoThat(_))
        .Times(2);
    EXPECT_CALL(foo, DoThis(6));
  }
```

In this example, we expect a call to `foo.DoThis(5)`, followed by two
calls to `bar.DoThat()` where the argument can be anything, which are
in turn followed by a call to `foo.DoThis(6)`. If a call occurred
out-of-order, Google Mock will report an error.

## Expecting Partially Ordered Calls ##

Sometimes requiring everything to occur in a predetermined order can
lead to brittle tests. For example, we may care about `A` occurring
before both `B` and `C`, but aren't interested in the relative order
of `B` and `C`. In this case, the test should reflect our real intent,
instead of being overly constraining.

Google Mock allows you to impose an arbitrary DAG (directed acyclic
graph) on the calls. One way to express the DAG is to use the
[After](V1_5_CheatSheet#The_After_Clause.md) clause of `EXPECT_CALL`.

Another way is via the `InSequence()` clause (not the same as the
`InSequence` class), which we borrowed from jMock 2. It's less
flexible than `After()`, but more convenient when you have long chains
of sequential calls, as it doesn't require you to come up with
different names for the expectations in the chains.  Here's how it
works:

If we view `EXPECT_CALL()` statements as nodes in a graph, and add an
edge from node A to node B wherever A must occur before B, we can get
a DAG. We use the term "sequence" to mean a directed path in this
DAG. Now, if we decompose the DAG into sequences, we just need to know
which sequences each `EXPECT_CALL()` belongs to in order to be able to
reconstruct the orginal DAG.

So, to specify the partial order on the expectations we need to do two
things: first to define some `Sequence` objects, and then for each
`EXPECT_CALL()` say which `Sequence` objects it is part
of. Expectations in the same sequence must occur in the order they are
written. For example,

```
  using ::testing::Sequence;

  Sequence s1, s2;

  EXPECT_CALL(foo, A())
      .InSequence(s1, s2);
  EXPECT_CALL(bar, B())
      .InSequence(s1);
  EXPECT_CALL(bar, C())
      .InSequence(s2);
  EXPECT_CALL(foo, D())
      .InSequence(s2);
```

specifies the following DAG (where `s1` is `A -> B`, and `s2` is `A ->
C -> D`):

```
       +---> B
       |
  A ---|
       |
       +---> C ---> D
```

This means that A must occur before B and C, and C must occur before
D. There's no restriction about the order other than these.

## Controlling When an Expectation Retires ##

When a mock method is called, Google Mock only consider expectations
that are still active. An expectation is active when created, and
becomes inactive (aka _retires_) when a call that has to occur later
has occurred. For example, in

```
  using ::testing::_;
  using ::testing::Sequence;

  Sequence s1, s2;

  EXPECT_CALL(log, Log(WARNING, _, "File too large."))     // #1
      .Times(AnyNumber())
      .InSequence(s1, s2);
  EXPECT_CALL(log, Log(WARNING, _, "Data set is empty."))  // #2
      .InSequence(s1);
  EXPECT_CALL(log, Log(WARNING, _, "User not found."))     // #3
      .InSequence(s2);
```

as soon as either #2 or #3 is matched, #1 will retire. If a warning
`"File too large."` is logged after this, it will be an error.

Note that an expectation doesn't retire automatically when it's
saturated. For example,

```
using ::testing::_;
...
  EXPECT_CALL(log, Log(WARNING, _, _));                  // #1
  EXPECT_CALL(log, Log(WARNING, _, "File too large."));  // #2
```

says that there will be exactly one warning with the message `"File
too large."`. If the second warning contains this message too, #2 will
match again and result in an upper-bound-violated error.

If this is not what you want, you can ask an expectation to retire as
soon as it becomes saturated:

```
using ::testing::_;
...
  EXPECT_CALL(log, Log(WARNING, _, _));                 // #1
  EXPECT_CALL(log, Log(WARNING, _, "File too large."))  // #2
      .RetiresOnSaturation();
```

Here #2 can be used only once, so if you have two warnings with the
message `"File too large."`, the first will match #2 and the second
will match #1 - there will be no error.

# Using Actions #

## Returning References from Mock Methods ##

If a mock function's return type is a reference, you need to use
`ReturnRef()` instead of `Return()` to return a result:

```
using ::testing::ReturnRef;

class MockFoo : public Foo {
 public:
  MOCK_METHOD0(GetBar, Bar&());
};
...

  MockFoo foo;
  Bar bar;
  EXPECT_CALL(foo, GetBar())
      .WillOnce(ReturnRef(bar));
```

## Combining Actions ##

Want to do more than one thing when a function is called? That's
fine. `DoAll()` allow you to do sequence of actions every time. Only
the return value of the last action in the sequence will be used.

```
using ::testing::DoAll;

class MockFoo : public Foo {
 public:
  MOCK_METHOD1(Bar, bool(int n));
};
...

  EXPECT_CALL(foo, Bar(_))
      .WillOnce(DoAll(action_1,
                      action_2,
                      ...
                      action_n));
```

## Mocking Side Effects ##

Sometimes a method exhibits its effect not via returning a value but
via side effects. For example, it may change some global state or
modify an output argument. To mock side effects, in general you can
define your own action by implementing `::testing::ActionInterface`.

If all you need to do is to change an output argument, the built-in
`SetArgumentPointee()` action is convenient:

```
using ::testing::SetArgumentPointee;

class MockMutator : public Mutator {
 public:
  MOCK_METHOD2(Mutate, void(bool mutate, int* value));
  ...
};
...

  MockMutator mutator;
  EXPECT_CALL(mutator, Mutate(true, _))
      .WillOnce(SetArgumentPointee<1>(5));
```

In this example, when `mutator.Mutate()` is called, we will assign 5
to the `int` variable pointed to by argument #1
(0-based).

`SetArgumentPointee()` conveniently makes an internal copy of the
value you pass to it, removing the need to keep the value in scope and
alive. The implication however is that the value must have a copy
constructor and assignment operator.

If the mock method also needs to return a value as well, you can chain
`SetArgumentPointee()` with `Return()` using `DoAll()`:

```
using ::testing::_;
using ::testing::Return;
using ::testing::SetArgumentPointee;

class MockMutator : public Mutator {
 public:
  ...
  MOCK_METHOD1(MutateInt, bool(int* value));
};
...

  MockMutator mutator;
  EXPECT_CALL(mutator, MutateInt(_))
      .WillOnce(DoAll(SetArgumentPointee<0>(5),
                      Return(true)));
```

If the output argument is an array, use the
`SetArrayArgument<N>(first, last)` action instead. It copies the
elements in source range `[first, last)` to the array pointed to by
the `N`-th (0-based) argument:

```
using ::testing::NotNull;
using ::testing::SetArrayArgument;

class MockArrayMutator : public ArrayMutator {
 public:
  MOCK_METHOD2(Mutate, void(int* values, int num_values));
  ...
};
...

  MockArrayMutator mutator;
  int values[5] = { 1, 2, 3, 4, 5 };
  EXPECT_CALL(mutator, Mutate(NotNull(), 5))
      .WillOnce(SetArrayArgument<0>(values, values + 5));
```

This also works when the argument is an output iterator:

```
using ::testing::_;
using ::testing::SeArrayArgument;

class MockRolodex : public Rolodex {
 public:
  MOCK_METHOD1(GetNames, void(std::back_insert_iterator<vector<string> >));
  ...
};
...

  MockRolodex rolodex;
  vector<string> names;
  names.push_back("George");
  names.push_back("John");
  names.push_back("Thomas");
  EXPECT_CALL(rolodex, GetNames(_))
      .WillOnce(SetArrayArgument<0>(names.begin(), names.end()));
```

## Changing a Mock Object's Behavior Based on the State ##

If you expect a call to change the behavior of a mock object, you can use `::testing::InSequence` to specify different behaviors before and after the call:

```
using ::testing::InSequence;
using ::testing::Return;

...
  {
    InSequence seq;
    EXPECT_CALL(my_mock, IsDirty())
        .WillRepeatedly(Return(true));
    EXPECT_CALL(my_mock, Flush());
    EXPECT_CALL(my_mock, IsDirty())
        .WillRepeatedly(Return(false));
  }
  my_mock.FlushIfDirty();
```

This makes `my_mock.IsDirty()` return `true` before `my_mock.Flush()` is called and return `false` afterwards.

If the behavior change is more complex, you can store the effects in a variable and make a mock method get its return value from that variable:

```
using ::testing::_;
using ::testing::SaveArg;
using ::testing::Return;

ACTION_P(ReturnPointee, p) { return *p; }
...
  int previous_value = 0;
  EXPECT_CALL(my_mock, GetPrevValue())
      .WillRepeatedly(ReturnPointee(&previous_value));
  EXPECT_CALL(my_mock, UpdateValue(_))
      .WillRepeatedly(SaveArg<0>(&previous_value));
  my_mock.DoSomethingToUpdateValue();
```

Here `my_mock.GetPrevValue()` will always return the argument of the last `UpdateValue()` call.

## Setting the Default Value for a Return Type ##

If a mock method's return type is a built-in C++ type or pointer, by
default it will return 0 when invoked. You only need to specify an
action if this default value doesn't work for you.

Sometimes, you may want to change this default value, or you may want
to specify a default value for types Google Mock doesn't know
about. You can do this using the `::testing::DefaultValue` class
template:

```
class MockFoo : public Foo {
 public:
  MOCK_METHOD0(CalculateBar, Bar());
};
...

  Bar default_bar;
  // Sets the default return value for type Bar.
  DefaultValue<Bar>::Set(default_bar);

  MockFoo foo;

  // We don't need to specify an action here, as the default
  // return value works for us.
  EXPECT_CALL(foo, CalculateBar());

  foo.CalculateBar();  // This should return default_bar.

  // Unsets the default return value.
  DefaultValue<Bar>::Clear();
```

Please note that changing the default value for a type can make you
tests hard to understand. We recommend you to use this feature
judiciously. For example, you may want to make sure the `Set()` and
`Clear()` calls are right next to the code that uses your mock.

## Setting the Default Actions for a Mock Method ##

You've learned how to change the default value of a given
type. However, this may be too coarse for your purpose: perhaps you
have two mock methods with the same return type and you want them to
have different behaviors. The `ON_CALL()` macro allows you to
customize your mock's behavior at the method level:

```
using ::testing::_;
using ::testing::AnyNumber;
using ::testing::Gt;
using ::testing::Return;
...
  ON_CALL(foo, Sign(_))
      .WillByDefault(Return(-1));
  ON_CALL(foo, Sign(0))
      .WillByDefault(Return(0));
  ON_CALL(foo, Sign(Gt(0)))
      .WillByDefault(Return(1));

  EXPECT_CALL(foo, Sign(_))
      .Times(AnyNumber());

  foo.Sign(5);   // This should return 1.
  foo.Sign(-9);  // This should return -1.
  foo.Sign(0);   // This should return 0.
```

As you may have guessed, when there are more than one `ON_CALL()`
statements, the news order take precedence over the older ones. In
other words, the **last** one that matches the function arguments will
be used. This matching order allows you to set up the common behavior
in a mock object's constructor or the test fixture's set-up phase and
specialize the mock's behavior later.

## Using Functions/Methods/Functors as Actions ##

If the built-in actions don't suit you, you can easily use an existing
function, method, or functor as an action:

```
using ::testing::_;
using ::testing::Invoke;

class MockFoo : public Foo {
 public:
  MOCK_METHOD2(Sum, int(int x, int y));
  MOCK_METHOD1(ComplexJob, bool(int x));
};

int CalculateSum(int x, int y) { return x + y; }

class Helper {
 public:
  bool ComplexJob(int x);
};
...

  MockFoo foo;
  Helper helper;
  EXPECT_CALL(foo, Sum(_, _))
      .WillOnce(Invoke(CalculateSum));
  EXPECT_CALL(foo, ComplexJob(_))
      .WillOnce(Invoke(&helper, &Helper::ComplexJob));

  foo.Sum(5, 6);       // Invokes CalculateSum(5, 6).
  foo.ComplexJob(10);  // Invokes helper.ComplexJob(10);
```

The only requirement is that the type of the function, etc must be
_compatible_ with the signature of the mock function, meaning that the
latter's arguments can be implicitly converted to the corresponding
arguments of the former, and the former's return type can be
implicitly converted to that of the latter. So, you can invoke
something whose type is _not_ exactly the same as the mock function,
as long as it's safe to do so - nice, huh?

## Invoking a Function/Method/Functor Without Arguments ##

`Invoke()` is very useful for doing actions that are more complex. It
passes the mock function's arguments to the function or functor being
invoked such that the callee has the full context of the call to work
with. If the invoked function is not interested in some or all of the
arguments, it can simply ignore them.

Yet, a common pattern is that a test author wants to invoke a function
without the arguments of the mock function. `Invoke()` allows her to
do that using a wrapper function that throws away the arguments before
invoking an underlining nullary function. Needless to say, this can be
tedious and obscures the intent of the test.

`InvokeWithoutArgs()` solves this problem. It's like `Invoke()` except
that it doesn't pass the mock function's arguments to the
callee. Here's an example:

```
using ::testing::_;
using ::testing::InvokeWithoutArgs;

class MockFoo : public Foo {
 public:
  MOCK_METHOD1(ComplexJob, bool(int n));
};

bool Job1() { ... }
...

  MockFoo foo;
  EXPECT_CALL(foo, ComplexJob(_))
      .WillOnce(InvokeWithoutArgs(Job1));

  foo.ComplexJob(10);  // Invokes Job1().
```

## Invoking an Argument of the Mock Function ##

Sometimes a mock function will receive a function pointer or a functor
(in other words, a "callable") as an argument, e.g.

```
class MockFoo : public Foo {
 public:
  MOCK_METHOD2(DoThis, bool(int n, bool (*fp)(int)));
};
```

and you may want to invoke this callable argument:

```
using ::testing::_;
...
  MockFoo foo;
  EXPECT_CALL(foo, DoThis(_, _))
      .WillOnce(...);
  // Will execute (*fp)(5), where fp is the
  // second argument DoThis() receives.
```

Arghh, you need to refer to a mock function argument but C++ has no
lambda (yet), so you have to define your own action. :-( Or do you
really?

Well, Google Mock has an action to solve _exactly_ this problem:

```
  InvokeArgument<N>(arg_1, arg_2, ..., arg_m)
```

will invoke the `N`-th (0-based) argument the mock function receives,
with `arg_1`, `arg_2`, ..., and `arg_m`. No matter if the argument is
a function pointer or a functor, Google Mock handles them both.

With that, you could write:

```
using ::testing::_;
using ::testing::InvokeArgument;
...
  EXPECT_CALL(foo, DoThis(_, _))
      .WillOnce(InvokeArgument<1>(5));
  // Will execute (*fp)(5), where fp is the
  // second argument DoThis() receives.
```

What if the callable takes an argument by reference? No problem - just
wrap it inside `ByRef()`:

```
...
  MOCK_METHOD1(Bar, bool(bool (*fp)(int, const Helper&)));
...
using ::testing::_;
using ::testing::ByRef;
using ::testing::InvokeArgument;
...

  MockFoo foo;
  Helper helper;
  ...
  EXPECT_CALL(foo, Bar(_))
      .WillOnce(InvokeArgument<0>(5, ByRef(helper)));
  // ByRef(helper) guarantees that a reference to helper, not a copy of it,
  // will be passed to the callable.
```

What if the callable takes an argument by reference and we do **not**
wrap the argument in `ByRef()`? Then `InvokeArgument()` will _make a
copy_ of the argument, and pass a _reference to the copy_, instead of
a reference to the original value, to the callable. This is especially
handy when the argument is a temporary value:

```
...
  MOCK_METHOD1(DoThat, bool(bool (*f)(const double& x, const string& s)));
...
using ::testing::_;
using ::testing::InvokeArgument;
...

  MockFoo foo;
  ...
  EXPECT_CALL(foo, DoThat(_))
      .WillOnce(InvokeArgument<0>(5.0, string("Hi")));
  // Will execute (*f)(5.0, string("Hi")), where f is the function pointer
  // DoThat() receives.  Note that the values 5.0 and string("Hi") are
  // temporary and dead once the EXPECT_CALL() statement finishes.  Yet
  // it's fine to perform this action later, since a copy of the values
  // are kept inside the InvokeArgument action.
```

## Ignoring an Action's Result ##

Sometimes you have an action that returns _something_, but you need an
action that returns `void` (perhaps you want to use it in a mock
function that returns `void`, or perhaps it needs to be used in
`DoAll()` and it's not the last in the list). `IgnoreResult()` lets
you do that. For example:

```
using ::testing::_;
using ::testing::Invoke;
using ::testing::Return;

int Process(const MyData& data);
string DoSomething();

class MockFoo : public Foo {
 public:
  MOCK_METHOD1(Abc, void(const MyData& data));
  MOCK_METHOD0(Xyz, bool());
};
...

  MockFoo foo;
  EXPECT_CALL(foo, Abc(_))
  // .WillOnce(Invoke(Process));
  // The above line won't compile as Process() returns int but Abc() needs
  // to return void.
      .WillOnce(IgnoreResult(Invoke(Process)));

  EXPECT_CALL(foo, Xyz())
      .WillOnce(DoAll(IgnoreResult(Invoke(DoSomething)),
      // Ignores the string DoSomething() returns.
                      Return(true)));
```

Note that you **cannot** use `IgnoreResult()` on an action that already
returns `void`. Doing so will lead to ugly compiler errors.

## Selecting an Action's Arguments ##

Say you have a mock function `Foo()` that takes seven arguments, and
you have a custom action that you want to invoke when `Foo()` is
called. Trouble is, the custom action only wants three arguments:

```
using ::testing::_;
using ::testing::Invoke;
...
  MOCK_METHOD7(Foo, bool(bool visible, const string& name, int x, int y,
                         const map<pair<int, int>, double>& weight,
                         double min_weight, double max_wight));
...

bool IsVisibleInQuadrant1(bool visible, int x, int y) {
  return visible && x >= 0 && y >= 0;
}
...

  EXPECT_CALL(mock, Foo(_, _, _, _, _, _, _))
      .WillOnce(Invoke(IsVisibleInQuadrant1));  // Uh, won't compile. :-(
```

To please the compiler God, you can to define an "adaptor" that has
the same signature as `Foo()` and calls the custom action with the
right arguments:

```
using ::testing::_;
using ::testing::Invoke;

bool MyIsVisibleInQuadrant1(bool visible, const string& name, int x, int y,
                            const map<pair<int, int>, double>& weight,
                            double min_weight, double max_wight) {
  return IsVisibleInQuadrant1(visible, x, y);
}
...

  EXPECT_CALL(mock, Foo(_, _, _, _, _, _, _))
      .WillOnce(Invoke(MyIsVisibleInQuadrant1));  // Now it works.
```

But isn't this awkward?

Google Mock provides a generic _action adaptor_, so you can spend your
time minding more important business than writing your own
adaptors. Here's the syntax:

```
  WithArgs<N1, N2, ..., Nk>(action)
```

creates an action that passes the arguments of the mock function at
the given indices (0-based) to the inner `action` and performs
it. Using `WithArgs`, our original example can be written as:

```
using ::testing::_;
using ::testing::Invoke;
using ::testing::WithArgs;
...
  EXPECT_CALL(mock, Foo(_, _, _, _, _, _, _))
      .WillOnce(WithArgs<0, 2, 3>(Invoke(IsVisibleInQuadrant1)));
      // No need to define your own adaptor.
```

For better readability, Google Mock also gives you:

  * `WithoutArgs(action)` when the inner `action` takes _no_ argument, and
  * `WithArg<N>(action)` (no `s` after `Arg`) when the inner `action` takes _one_ argument.

As you may have realized, `InvokeWithoutArgs(...)` is just syntactic
sugar for `WithoutArgs(Inovke(...))`.

Here are more tips:

  * The inner action used in `WithArgs` and friends does not have to be `Invoke()` -- it can be anything.
  * You can repeat an argument in the argument list if necessary, e.g. `WithArgs<2, 3, 3, 5>(...)`.
  * You can change the order of the arguments, e.g. `WithArgs<3, 2, 1>(...)`.
  * The types of the selected arguments do _not_ have to match the signature of the inner action exactly. It works as long as they can be implicitly converted to the corresponding arguments of the inner action. For example, if the 4-th argument of the mock function is an `int` and `my_action` takes a `double`, `WithArg<4>(my_action)` will work.

## Ignoring Arguments in Action Functions ##

The selecting-an-action's-arguments recipe showed us one way to make a
mock function and an action with incompatible argument lists fit
together. The downside is that wrapping the action in
`WithArgs<...>()` can get tedious for people writing the tests.

If you are defining a function, method, or functor to be used with
`Invoke*()`, and you are not interested in some of its arguments, an
alternative to `WithArgs` is to declare the uninteresting arguments as
`Unused`. This makes the definition less cluttered and less fragile in
case the types of the uninteresting arguments change. It could also
increase the chance the action function can be reused. For example,
given

```
  MOCK_METHOD3(Foo, double(const string& label, double x, double y));
  MOCK_METHOD3(Bar, double(int index, double x, double y));
```

instead of

```
using ::testing::_;
using ::testing::Invoke;

double DistanceToOriginWithLabel(const string& label, double x, double y) {
  return sqrt(x*x + y*y);
}

double DistanceToOriginWithIndex(int index, double x, double y) {
  return sqrt(x*x + y*y);
}
...

  EXEPCT_CALL(mock, Foo("abc", _, _))
      .WillOnce(Invoke(DistanceToOriginWithLabel));
  EXEPCT_CALL(mock, Bar(5, _, _))
      .WillOnce(Invoke(DistanceToOriginWithIndex));
```

you could write

```
using ::testing::_;
using ::testing::Invoke;
using ::testing::Unused;

double DistanceToOrigin(Unused, double x, double y) {
  return sqrt(x*x + y*y);
}
...

  EXEPCT_CALL(mock, Foo("abc", _, _))
      .WillOnce(Invoke(DistanceToOrigin));
  EXEPCT_CALL(mock, Bar(5, _, _))
      .WillOnce(Invoke(DistanceToOrigin));
```

## Sharing Actions ##

Just like matchers, a Google Mock action object consists of a pointer
to a ref-counted implementation object. Therefore copying actions is
also allowed and very efficient. When the last action that references
the implementation object dies, the implementation object will be
deleted.

If you have some complex action that you want to use again and again,
you may not have to build it from scratch everytime. If the action
doesn't have an internal state (i.e. if it always does the same thing
no matter how many times it has been called), you can assign it to an
action variable and use that variable repeatedly. For example:

```
  Action<bool(int*)> set_flag = DoAll(SetArgumentPointee<0>(5),
                                      Return(true));
  ... use set_flag in .WillOnce() and .WillRepeatedly() ...
```

However, if the action has its own state, you may be surprised if you
share the action object. Suppose you have an action factory
`IncrementCounter(init)` which creates an action that increments and
returns a counter whose initial value is `init`, using two actions
created from the same expression and using a shared action will
exihibit different behaviors. Example:

```
  EXPECT_CALL(foo, DoThis())
      .WillRepeatedly(IncrementCounter(0));
  EXPECT_CALL(foo, DoThat())
      .WillRepeatedly(IncrementCounter(0));
  foo.DoThis();  // Returns 1.
  foo.DoThis();  // Returns 2.
  foo.DoThat();  // Returns 1 - Blah() uses a different
                 // counter than Bar()'s.
```

versus

```
  Action<int()> increment = IncrementCounter(0);

  EXPECT_CALL(foo, DoThis())
      .WillRepeatedly(increment);
  EXPECT_CALL(foo, DoThat())
      .WillRepeatedly(increment);
  foo.DoThis();  // Returns 1.
  foo.DoThis();  // Returns 2.
  foo.DoThat();  // Returns 3 - the counter is shared.
```

# Misc Recipes on Using Google Mock #

## Forcing a Verification ##

When it's being destoyed, your friendly mock object will automatically
verify that all expectations on it have been satisfied, and will
generate [Google Test](http://code.google.com/p/googletest/) failures
if not. This is convenient as it leaves you with one less thing to
worry about. That is, unless you are not sure if your mock object will
be destoyed.

How could it be that your mock object won't eventually be destroyed?
Well, it might be created on the heap and owned by the code you are
testing. Suppose there's a bug in that code and it doesn't delete the
mock object properly - you could end up with a passing test when
there's actually a bug.

Using a heap checker is a good idea and can alleviate the concern, but
its implementation may not be 100% reliable. So, sometimes you do want
to _force_ Google Mock to verify a mock object before it is
(hopefully) destructed. You can do this with
`Mock::VerifyAndClearExpectations(&mock_object)`:

```
TEST(MyServerTest, ProcessesRequest) {
  using ::testing::Mock;

  MockFoo* const foo = new MockFoo;
  EXPECT_CALL(*foo, ...)...;
  // ... other expectations ...

  // server now owns foo.
  MyServer server(foo);
  server.ProcessRequest(...);

  // In case that server's destructor will forget to delete foo,
  // this will verify the expectations anyway.
  Mock::VerifyAndClearExpectations(foo);
}  // server is destroyed when it goes out of scope here.
```

**Tip:** The `Mock::VerifyAndClearExpectations()` function returns a
`bool` to indicate whether the verification was successful (`true` for
yes), so you can wrap that function call inside a `ASSERT_TRUE()` if
there is no point going further when the verification has failed.

## Using Check Points ##

Sometimes you may want to "reset" a mock object at various check
points in your test: at each check point, you verify that all existing
expectations on the mock object have been satisfied, and then you set
some new expectations on it as if it's newly created. This allows you
to work with a mock object in "phases" whose sizes are each
manageable.

One such scenario is that in your test's `SetUp()` function, you may
want to put the object you are testing into a certain state, with the
help from a mock object. Once in the desired state, you want to clear
all expectations on the mock, such that in the `TEST_F` body you can
set fresh expectations on it.

As you may have figured out, the `Mock::VerifyAndClearExpectations()`
function we saw in the previous recipe can help you here. Or, if you
are using `ON_CALL()` to set default actions on the mock object and
want to clear the default actions as well, use
`Mock::VerifyAndClear(&mock_object)` instead. This function does what
`Mock::VerifyAndClearExpectations(&mock_object)` does and returns the
same `bool`, **plus** it clears the `ON_CALL()` statements on
`mock_object` too.

Another trick you can use to achieve the same effect is to put the
expectations in sequences and insert calls to a dummy "check-point"
function at specific places. Then you can verify that the mock
function calls do happen at the right time. For example, if you are
exercising code:

```
Foo(1);
Foo(2);
Foo(3);
```

and want to verify that `Foo(1)` and `Foo(3)` both invoke
`mock.Bar("a")`, but `Foo(2)` doesn't invoke anything. You can write:

```
using ::testing::MockFunction;

TEST(FooTest, InvokesBarCorrectly) {
  MyMock mock;
  // Class MockFunction<F> has exactly one mock method.  It is named
  // Call() and has type F.
  MockFunction<void(string check_point_name)> check;
  {
    InSequence s;

    EXPECT_CALL(mock, Bar("a"));
    EXPECT_CALL(check, Call("1"));
    EXPECT_CALL(check, Call("2"));
    EXPECT_CALL(mock, Bar("a"));
  }
  Foo(1);
  check.Call("1");
  Foo(2);
  check.Call("2");
  Foo(3);
}
```

The expectation spec says that the first `Bar("a")` must happen before
check point "1", the second `Bar("a")` must happen after check point "2",
and nothing should happen between the two check points. The explicit
check points make it easy to tell which `Bar("a")` is called by which
call to `Foo()`.

## Mocking Destructors ##

Sometimes you want to make sure a mock object is destructed at the
right time, e.g. after `bar->A()` is called but before `bar->B()` is
called. We already know that you can specify constraints on the order
of mock function calls, so all we need to do is to mock the destructor
of the mock function.

This sounds simple, except for one problem: a destructor is a special
function with special syntax and special semantics, and the
`MOCK_METHOD0` macro doesn't work for it:

```
  MOCK_METHOD0(~MockFoo, void());  // Won't compile!
```

The good news is that you can use a simple pattern to achieve the same
effect. First, add a mock function `Die()` to your mock class and call
it in the destructor, like this:

```
class MockFoo : public Foo {
  ...
  // Add the following two lines to the mock class.
  MOCK_METHOD0(Die, void());
  virtual ~MockFoo() { Die(); }
};
```

(If the name `Die()` clashes with an existing symbol, choose another
name.) Now, we have translated the problem of testing when a `MockFoo`
object dies to testing when its `Die()` method is called:

```
  MockFoo* foo = new MockFoo;
  MockBar* bar = new MockBar;
  ...
  {
    InSequence s;

    // Expects *foo to die after bar->A() and before bar->B().
    EXPECT_CALL(*bar, A());
    EXPECT_CALL(*foo, Die());
    EXPECT_CALL(*bar, B());
  }
```

And that's that.

## Using Google Mock and Threads ##

**IMPORTANT NOTE:** What we describe in this recipe is **NOT** true yet,
as Google Mock is not currently thread-safe.  However, all we need to
make it thread-safe is to implement some synchronization operations in
`<gtest/internal/gtest-port.h>` - and then the information below will
become true.

In a **unit** test, it's best if you could isolate and test a piece of
code in a single-threaded context. That avoids race conditions and
dead locks, and makes debugging your test much easier.

Yet many programs are multi-threaded, and sometimes to test something
we need to pound on it from more than one thread. Google Mock works
for this purpose too.

Remember the steps for using a mock:

  1. Create a mock object `foo`.
  1. Set its default actions and expectations using `ON_CALL()` and `EXPECT_CALL()`.
  1. The code under test calls methods of `foo`.
  1. Optionally, verify and reset the mock.
  1. Destroy the mock yourself, or let the code under test destroy it. The destructor will automatically verify it.

If you follow the following simple rules, your mocks and threads can
live happily togeter:

  * Execute your _test code_ (as opposed to the code being tested) in _one_ thread. This makes your test easy to follow.
  * Obviously, you can do step #1 without locking.
  * When doing step #2 and #5, make sure no other thread is accessing `foo`. Obvious too, huh?
  * #3 and #4 can be done either in one thread or in multiple threads - anyway you want. Google Mock takes care of the locking, so you don't have to do any - unless required by your test logic.

If you violate the rules (for example, if you set expectations on a
mock while another thread is calling its methods), you get undefined
behavior. That's not fun, so don't do it.

Google Mock guarantees that the action for a mock function is done in
the same thread that called the mock function. For example, in

```
  EXPECT_CALL(mock, Foo(1))
      .WillOnce(action1);
  EXPECT_CALL(mock, Foo(2))
      .WillOnce(action2);
```

if `Foo(1)` is called in thread 1 and `Foo(2)` is called in thread 2,
Google Mock will execute `action1` in thread 1 and `action2` in thread
2.

Google Mock does _not_ impose a sequence on actions performed in
different threads (doing so may create deadlocks as the actions may
need to cooperate). This means that the execution of `action1` and
`action2` in the above example _may_ interleave. If this is a problem,
you should add proper synchronization logic to `action1` and `action2`
to make the test thread-safe.


Also, remember that `DefaultValue<T>` is a global resource that
potentially affects _all_ living mock objects in your
program. Naturally, you won't want to mess with it from multiple
threads or when there still are mocks in action.

## Controlling How Much Information Google Mock Prints ##

When Google Mock sees something that has the potential of being an
error (e.g. a mock function with no expectation is called, a.k.a. an
uninteresting call, which is allowed but perhaps you forgot to
explicitly ban the call), it prints some warning messages, including
the arguments of the function and the return value. Hopefully this
will remind you to take a look and see if there is indeed a problem.

Sometimes you are confident that your tests are correct and may not
appreciate such friendly messages. Some other times, you are debugging
your tests or learning about the behavior of the code you are testing,
and wish you could observe every mock call that happens (including
argument values and the return value). Clearly, one size doesn't fit
all.

You can control how much Google Mock tells you using the
`--gmock_verbose=LEVEL` command-line flag, where `LEVEL` is a string
with three possible values:

  * `info`: Google Mock will print all informational messages, warnings, and errors (most verbose). At this setting, Google Mock will also log any calls to the `ON_CALL/EXPECT_CALL` macros.
  * `warning`: Google Mock will print both warnings and errors (less verbose). This is the default.
  * `error`: Google Mock will print errors only (least verbose).

Alternatively, you can adjust the value of that flag from within your
tests like so:

```
  ::testing::FLAGS_gmock_verbose = "error";
```

Now, judiciously use the right flag to enable Google Mock serve you better!

## Running Tests in Emacs ##

If you build and run your tests in Emacs, the source file locations of
Google Mock and [Google Test](http://code.google.com/p/googletest/)
errors will be highlighted. Just press `<Enter>` on one of them and
you'll be taken to the offending line. Or, you can just type `C-x ``
to jump to the next error.

To make it even easier, you can add the following lines to your
`~/.emacs` file:

```
(global-set-key "\M-m"   'compile)  ; m is for make
(global-set-key [M-down] 'next-error)
(global-set-key [M-up]   '(lambda () (interactive) (next-error -1)))
```

Then you can type `M-m` to start a build, or `M-up`/`M-down` to move
back and forth between errors.

## Fusing Google Mock Source Files ##

Google Mock's implementation consists of dozens of files (excluding
its own tests).  Sometimes you may want them to be packaged up in
fewer files instead, such that you can easily copy them to a new
machine and start hacking there.  For this we provide an experimental
Python script `fuse_gmock_files.py` in the `scripts/` directory
(starting with release 1.2.0).  Assuming you have Python 2.4 or above
installed on your machine, just go to that directory and run
```
python fuse_gmock_files.py OUTPUT_DIR
```

and you should see an `OUTPUT_DIR` directory being created with files
`gtest/gtest.h`, `gmock/gmock.h`, and `gmock-gtest-all.cc` in it.
These three files contain everything you need to use Google Mock (and
Google Test).  Just copy them to anywhere you want and you are ready
to write tests and use mocks.  You can use the
[scrpts/test/Makefile](http://code.google.com/p/googlemock/source/browse/trunk/scripts/test/Makefile) file as an example on how to compile your tests
against them.

# Extending Google Mock #

## Writing New Matchers Quickly ##

The `MATCHER*` family of macros can be used to define custom matchers
easily.  The syntax:

```
MATCHER(name, "description string") { statements; }
```

will define a matcher with the given name that executes the
statements, which must return a `bool` to indicate if the match
succeeds.  Inside the statements, you can refer to the value being
matched by `arg`, and refer to its type by `arg_type`.

The description string documents what the matcher does, and is used to
generate the failure message when the match fails.  Since a
`MATCHER()` is usually defined in a header file shared by multiple C++
source files, we require the description to be a C-string _literal_ to
avoid possible side effects.  It can be empty (`""`), in which case
Google Mock will use the sequence of words in the matcher name as the
description.

For example:
```
MATCHER(IsDivisibleBy7, "") { return (arg % 7) == 0; }
```
allows you to write
```
  // Expects mock_foo.Bar(n) to be called where n is divisible by 7.
  EXPECT_CALL(mock_foo, Bar(IsDivisibleBy7()));
```
or,
```
  // Verifies that the value of some_expression is divisible by 7.
  EXPECT_THAT(some_expression, IsDivisibleBy7());
```
If the above assertion fails, it will print something like:
```
  Value of: some_expression
  Expected: is divisible by 7
    Actual: 27
```
where the description `"is divisible by 7"` is automatically calculated from the
matcher name `IsDivisibleBy7`.

Optionally, you can stream additional information to a hidden argument
named `result_listener` to explain the match result. For example, a
better definition of `IsDivisibleBy7` is:
```
MATCHER(IsDivisibleBy7, "") {
  if ((arg % 7) == 0)
    return true;

  *result_listener << "the remainder is " << (arg % 7);
  return false;
}
```

With this definition, the above assertion will give a better message:
```
  Value of: some_expression
  Expected: is divisible by 7
    Actual: 27 (the remainder is 6)
```

You should let `MatchAndExplain()` print _any additional information_
that can help a user understand the match result. Note that it should
explain why the match succeeds in case of a success (unless it's
obvious) - this is useful when the matcher is used inside
`Not()`. There is no need to print the argument value itself, as
Google Mock already prints it for you.

**Notes:**

  1. The type of the value being matched (`arg_type`) is determined by the context in which you use the matcher and is supplied to you by the compiler, so you don't need to worry about declaring it (nor can you).  This allows the matcher to be polymorphic.  For example, `IsDivisibleBy7()` can be used to match any type where the value of `(arg % 7) == 0` can be implicitly converted to a `bool`.  In the `Bar(IsDivisibleBy7())` example above, if method `Bar()` takes an `int`, `arg_type` will be `int`; if it takes an `unsigned long`, `arg_type` will be `unsigned long`; and so on.
  1. Google Mock doesn't guarantee when or how many times a matcher will be invoked. Therefore the matcher logic must be _purely functional_ (i.e. it cannot have any side effect, and the result must not depend on anything other than the value being matched and the matcher parameters). This requirement must be satisfied no matter how you define the matcher (e.g. using one of the methods described in the following recipes). In particular, a matcher can never call a mock function, as that will affect the state of the mock object and Google Mock.

## Writing New Parameterized Matchers Quickly ##

Sometimes you'll want to define a matcher that has parameters.  For that you
can use the macro:
```
MATCHER_P(name, param_name, "description string") { statements; }
```

For example:
```
MATCHER_P(HasAbsoluteValue, value, "") { return abs(arg) == value; }
```
will allow you to write:
```
  EXPECT_THAT(Blah("a"), HasAbsoluteValue(n));
```
which may lead to this message (assuming `n` is 10):
```
  Value of: Blah("a")
  Expected: has absolute value 10
    Actual: -9
```

Note that both the matcher description and its parameter are
printed, making the message human-friendly.

In the matcher definition body, you can write `foo_type` to
reference the type of a parameter named `foo`.  For example, in the
body of `MATCHER_P(HasAbsoluteValue, value)` above, you can write
`value_type` to refer to the type of `value`.

Google Mock also provides `MATCHER_P2`, `MATCHER_P3`, ..., up to
`MATCHER_P10` to support multi-parameter matchers:
```
MATCHER_Pk(name, param_1, ..., param_k, "description string") { statements; }
```

Please note that the custom description string is for a particular
**instance** of the matcher, where the parameters have been bound to
actual values.  Therefore usually you'll want the parameter values to
be part of the description.  Google Mock lets you do that using
Python-style interpolations.  The following syntaxes are supported
currently:

| `%%` | a single `%` character |
|:-----|:-----------------------|
| `%(*)s` | all parameters of the matcher printed as a tuple |
| `%(foo)s` | value of the matcher parameter named `foo` |

For example,
```
  MATCHER_P2(InClosedRange, low, hi, "is in range [%(low)s, %(hi)s]") {
    return low <= arg && arg <= hi;
  }
  ...
  EXPECT_THAT(3, InClosedRange(4, 6));
```
would generate a failure that contains the message:
```
  Expected: is in range [4, 6]
```

If you specify `""` as the description, the failure message will
contain the sequence of words in the matcher name followed by the
parameter values printed as a tuple.  For example,
```
  MATCHER_P2(InClosedRange, low, hi, "") { ... }
  ...
  EXPECT_THAT(3, InClosedRange(4, 6));
```
would generate a failure that contains the text:
```
  Expected: in closed range (4, 6)
```

For the purpose of typing, you can view
```
MATCHER_Pk(Foo, p1, ..., pk, "description string") { ... }
```
as shorthand for
```
template <typename p1_type, ..., typename pk_type>
FooMatcherPk<p1_type, ..., pk_type>
Foo(p1_type p1, ..., pk_type pk) { ... }
```

When you write `Foo(v1, ..., vk)`, the compiler infers the types of
the parameters `v1`, ..., and `vk` for you.  If you are not happy with
the result of the type inference, you can specify the types by
explicitly instantiating the template, as in `Foo<long, bool>(5, false)`.
As said earlier, you don't get to (or need to) specify
`arg_type` as that's determined by the context in which the matcher
is used.

You can assign the result of expression `Foo(p1, ..., pk)` to a
variable of type `FooMatcherPk<p1_type, ..., pk_type>`.  This can be
useful when composing matchers.  Matchers that don't have a parameter
or have only one parameter have special types: you can assign `Foo()`
to a `FooMatcher`-typed variable, and assign `Foo(p)` to a
`FooMatcherP<p_type>`-typed variable.

While you can instantiate a matcher template with reference types,
passing the parameters by pointer usually makes your code more
readable.  If, however, you still want to pass a parameter by
reference, be aware that in the failure message generated by the
matcher you will see the value of the referenced object but not its
address.

You can overload matchers with different numbers of parameters:
```
MATCHER_P(Blah, a, "description string 1") { ... }
MATCHER_P2(Blah, a, b, "description string 2") { ... }
```

While it's tempting to always use the `MATCHER*` macros when defining
a new matcher, you should also consider implementing
`MatcherInterface` or using `MakePolymorphicMatcher()` instead (see
the recipes that follow), especially if you need to use the matcher a
lot.  While these approaches require more work, they give you more
control on the types of the value being matched and the matcher
parameters, which in general leads to better compiler error messages
that pay off in the long run.  They also allow overloading matchers
based on parameter types (as opposed to just based on the number of
parameters).

## Writing New Monomorphic Matchers ##

A matcher of argument type `T` implements
`::testing::MatcherInterface<T>` and does two things: it tests whether a
value of type `T` matches the matcher, and can describe what kind of
values it matches. The latter ability is used for generating readable
error messages when expectations are violated.

The interface looks like this:

```
class MatchResultListener {
 public:
  ...
  // Streams x to the underlying ostream; does nothing if the ostream
  // is NULL.
  template <typename T>
  MatchResultListener& operator<<(const T& x);

  // Returns the underlying ostream.
  ::std::ostream* stream();
};

template <typename T>
class MatcherInterface {
 public:
  virtual ~MatcherInterface();

  // Returns true iff the matcher matches x; also explains the match
  // result to 'listener'.
  virtual bool MatchAndExplain(T x, MatchResultListener* listener) const = 0;

  // Describes this matcher to an ostream.
  virtual void DescribeTo(::std::ostream* os) const = 0;

  // Describes the negation of this matcher to an ostream.
  virtual void DescribeNegationTo(::std::ostream* os) const;
};
```

If you need a custom matcher but `Truly()` is not a good option (for
example, you may not be happy with the way `Truly(predicate)`
describes itself, or you may want your matcher to be polymorphic as
`Eq(value)` is), you can define a matcher to do whatever you want in
two steps: first implement the matcher interface, and then define a
factory function to create a matcher instance. The second step is not
strictly needed but it makes the syntax of using the matcher nicer.

For example, you can define a matcher to test whether an `int` is
divisible by 7 and then use it like this:
```
using ::testing::MakeMatcher;
using ::testing::Matcher;
using ::testing::MatcherInterface;
using ::testing::MatchResultListener;

class DivisibleBy7Matcher : public MatcherInterface<int> {
 public:
  virtual bool MatchAndExplain(int n, MatchResultListener* listener) const {
    return (n % 7) == 0;
  }

  virtual void DescribeTo(::std::ostream* os) const {
    *os << "is divisible by 7";
  }

  virtual void DescribeNegationTo(::std::ostream* os) const {
    *os << "is not divisible by 7";
  }
};

inline Matcher<int> DivisibleBy7() {
  return MakeMatcher(new DivisibleBy7Matcher);
}
...

  EXPECT_CALL(foo, Bar(DivisibleBy7()));
```

You may improve the matcher message by streaming additional
information to the `listener` argument in `MatchAndExplain()`:

```
class DivisibleBy7Matcher : public MatcherInterface<int> {
 public:
  virtual bool MatchAndExplain(int n,
                               MatchResultListener* listener) const {
    const int remainder = n % 7;
    if (remainder != 0) {
      *listener << "the remainder is " << remainder;
    }
    return remainder == 0;
  }
  ...
};
```

Then, `EXPECT_THAT(x, DivisibleBy7());` may general a message like this:
```
Value of: x
Expected: is divisible by 7
  Actual: 23 (the remainder is 2)
```

## Writing New Polymorphic Matchers ##

You've learned how to write your own matchers in the previous
recipe. Just one problem: a matcher created using `MakeMatcher()` only
works for one particular type of arguments. If you want a
_polymorphic_ matcher that works with arguments of several types (for
instance, `Eq(x)` can be used to match a `value` as long as `value` ==
`x` compiles -- `value` and `x` don't have to share the same type),
you can learn the trick from `<gmock/gmock-matchers.h>` but it's a bit
involved.

Fortunately, most of the time you can define a polymorphic matcher
easily with the help of `MakePolymorphicMatcher()`. Here's how you can
define `NotNull()` as an example:

```
using ::testing::MakePolymorphicMatcher;
using ::testing::MatchResultListener;
using ::testing::NotNull;
using ::testing::PolymorphicMatcher;

class NotNullMatcher {
 public:
  // To implement a polymorphic matcher, first define a COPYABLE class
  // that has three members MatchAndExplain(), DescribeTo(), and
  // DescribeNegationTo(), like the following.

  // In this example, we want to use NotNull() with any pointer, so
  // MatchAndExplain() accepts a pointer of any type as its first argument.
  // In general, you can define MatchAndExplain() as an ordinary method or
  // a method template, or even overload it.
  template <typename T>
  bool MatchAndExplain(T* p,
                       MatchResultListener* /* listener */) const {
    return p != NULL;
  }

  // Describes the property of a value matching this matcher.
  void DescribeTo(::std::ostream* os) const { *os << "is not NULL"; }

  // Describes the property of a value NOT matching this matcher.
  void DescribeNegationTo(::std::ostream* os) const { *os << "is NULL"; }
};

// To construct a polymorphic matcher, pass an instance of the class
// to MakePolymorphicMatcher().  Note the return type.
inline PolymorphicMatcher<NotNullMatcher> NotNull() {
  return MakePolymorphicMatcher(NotNullMatcher());
}
...

  EXPECT_CALL(foo, Bar(NotNull()));  // The argument must be a non-NULL pointer.
```

**Note:** Your polymorphic matcher class does **not** need to inherit from
`MatcherInterface` or any other class, and its methods do **not** need
to be virtual.

Like in a monomorphic matcher, you may explain the match result by
streaming additional information to the `listener` argument in
`MatchAndExplain()`.

## Writing New Cardinalities ##

A cardinality is used in `Times()` to tell Google Mock how many times
you expect a call to occur. It doesn't have to be exact. For example,
you can say `AtLeast(5)` or `Between(2, 4)`.

If the built-in set of cardinalities doesn't suit you, you are free to
define your own by implementing the following interface (in namespace
`testing`):

```
class CardinalityInterface {
 public:
  virtual ~CardinalityInterface();

  // Returns true iff call_count calls will satisfy this cardinality.
  virtual bool IsSatisfiedByCallCount(int call_count) const = 0;

  // Returns true iff call_count calls will saturate this cardinality.
  virtual bool IsSaturatedByCallCount(int call_count) const = 0;

  // Describes self to an ostream.
  virtual void DescribeTo(::std::ostream* os) const = 0;
};
```

For example, to specify that a call must occur even number of times,
you can write

```
using ::testing::Cardinality;
using ::testing::CardinalityInterface;
using ::testing::MakeCardinality;

class EvenNumberCardinality : public CardinalityInterface {
 public:
  virtual bool IsSatisfiedByCallCount(int call_count) const {
    return (call_count % 2) == 0;
  }

  virtual bool IsSaturatedByCallCount(int call_count) const {
    return false;
  }

  virtual void DescribeTo(::std::ostream* os) const {
    *os << "called even number of times";
  }
};

Cardinality EvenNumber() {
  return MakeCardinality(new EvenNumberCardinality);
}
...

  EXPECT_CALL(foo, Bar(3))
      .Times(EvenNumber());
```

## Writing New Actions Quickly ##

If the built-in actions don't work for you, and you find it
inconvenient to use `Invoke()`, you can use a macro from the `ACTION*`
family to quickly define a new action that can be used in your code as
if it's a built-in action.

By writing
```
ACTION(name) { statements; }
```
in a namespace scope (i.e. not inside a class or function), you will
define an action with the given name that executes the statements.
The value returned by `statements` will be used as the return value of
the action.  Inside the statements, you can refer to the K-th
(0-based) argument of the mock function as `argK`.  For example:
```
ACTION(IncrementArg1) { return ++(*arg1); }
```
allows you to write
```
... WillOnce(IncrementArg1());
```

Note that you don't need to specify the types of the mock function
arguments.  Rest assured that your code is type-safe though:
you'll get a compiler error if `*arg1` doesn't support the `++`
operator, or if the type of `++(*arg1)` isn't compatible with the mock
function's return type.

Another example:
```
ACTION(Foo) {
  (*arg2)(5);
  Blah();
  *arg1 = 0;
  return arg0;
}
```
defines an action `Foo()` that invokes argument #2 (a function pointer)
with 5, calls function `Blah()`, sets the value pointed to by argument
#1 to 0, and returns argument #0.

For more convenience and flexibility, you can also use the following
pre-defined symbols in the body of `ACTION`:

| `argK_type` | The type of the K-th (0-based) argument of the mock function |
|:------------|:-------------------------------------------------------------|
| `args`      | All arguments of the mock function as a tuple                |
| `args_type` | The type of all arguments of the mock function as a tuple    |
| `return_type` | The return type of the mock function                         |
| `function_type` | The type of the mock function                                |

For example, when using an `ACTION` as a stub action for mock function:
```
int DoSomething(bool flag, int* ptr);
```
we have:
| **Pre-defined Symbol** | **Is Bound To** |
|:-----------------------|:----------------|
| `arg0`                 | the value of `flag` |
| `arg0_type`            | the type `bool` |
| `arg1`                 | the value of `ptr` |
| `arg1_type`            | the type `int*` |
| `args`                 | the tuple `(flag, ptr)` |
| `args_type`            | the type `std::tr1::tuple<bool, int*>` |
| `return_type`          | the type `int`  |
| `function_type`        | the type `int(bool, int*)` |

## Writing New Parameterized Actions Quickly ##

Sometimes you'll want to parameterize an action you define.  For that
we have another macro
```
ACTION_P(name, param) { statements; }
```

For example,
```
ACTION_P(Add, n) { return arg0 + n; }
```
will allow you to write
```
// Returns argument #0 + 5.
... WillOnce(Add(5));
```

For convenience, we use the term _arguments_ for the values used to
invoke the mock function, and the term _parameters_ for the values
used to instantiate an action.

Note that you don't need to provide the type of the parameter either.
Suppose the parameter is named `param`, you can also use the
Google-Mock-defined symbol `param_type` to refer to the type of the
parameter as inferred by the compiler.  For example, in the body of
`ACTION_P(Add, n)` above, you can write `n_type` for the type of `n`.

Google Mock also provides `ACTION_P2`, `ACTION_P3`, and etc to support
multi-parameter actions.  For example,
```
ACTION_P2(ReturnDistanceTo, x, y) {
  double dx = arg0 - x;
  double dy = arg1 - y;
  return sqrt(dx*dx + dy*dy);
}
```
lets you write
```
... WillOnce(ReturnDistanceTo(5.0, 26.5));
```

You can view `ACTION` as a degenerated parameterized action where the
number of parameters is 0.

You can also easily define actions overloaded on the number of parameters:
```
ACTION_P(Plus, a) { ... }
ACTION_P2(Plus, a, b) { ... }
```

## Restricting the Type of an Argument or Parameter in an ACTION ##

For maximum brevity and reusability, the `ACTION*` macros don't ask
you to provide the types of the mock function arguments and the action
parameters.  Instead, we let the compiler infer the types for us.

Sometimes, however, we may want to be more explicit about the types.
There are several tricks to do that.  For example:
```
ACTION(Foo) {
  // Makes sure arg0 can be converted to int.
  int n = arg0;
  ... use n instead of arg0 here ...
}

ACTION_P(Bar, param) {
  // Makes sure the type of arg1 is const char*.
  ::testing::StaticAssertTypeEq<const char*, arg1_type>();

  // Makes sure param can be converted to bool.
  bool flag = param;
}
```
where `StaticAssertTypeEq` is a compile-time assertion in Google Test
that verifies two types are the same.

## Writing New Action Templates Quickly ##

Sometimes you want to give an action explicit template parameters that
cannot be inferred from its value parameters.  `ACTION_TEMPLATE()`
supports that and can be viewed as an extension to `ACTION()` and
`ACTION_P*()`.

The syntax:
```
ACTION_TEMPLATE(ActionName,
                HAS_m_TEMPLATE_PARAMS(kind1, name1, ..., kind_m, name_m),
                AND_n_VALUE_PARAMS(p1, ..., p_n)) { statements; }
```

defines an action template that takes _m_ explicit template parameters
and _n_ value parameters, where _m_ is between 1 and 10, and _n_ is
between 0 and 10.  `name_i` is the name of the i-th template
parameter, and `kind_i` specifies whether it's a `typename`, an
integral constant, or a template.  `p_i` is the name of the i-th value
parameter.

Example:
```
// DuplicateArg<k, T>(output) converts the k-th argument of the mock
// function to type T and copies it to *output.
ACTION_TEMPLATE(DuplicateArg,
                // Note the comma between int and k:
                HAS_2_TEMPLATE_PARAMS(int, k, typename, T),
                AND_1_VALUE_PARAMS(output)) {
  *output = T(std::tr1::get<k>(args));
}
```

To create an instance of an action template, write:
```
  ActionName<t1, ..., t_m>(v1, ..., v_n)
```
where the `t`s are the template arguments and the
`v`s are the value arguments.  The value argument
types are inferred by the compiler.  For example:
```
using ::testing::_;
...
  int n;
  EXPECT_CALL(mock, Foo(_, _))
      .WillOnce(DuplicateArg<1, unsigned char>(&n));
```

If you want to explicitly specify the value argument types, you can
provide additional template arguments:
```
  ActionName<t1, ..., t_m, u1, ..., u_k>(v1, ..., v_n)
```
where `u_i` is the desired type of `v_i`.

`ACTION_TEMPLATE` and `ACTION`/`ACTION_P*` can be overloaded on the
number of value parameters, but not on the number of template
parameters.  Without the restriction, the meaning of the following is
unclear:

```
  OverloadedAction<int, bool>(x);
```

Are we using a single-template-parameter action where `bool` refers to
the type of `x`, or a two-template-parameter action where the compiler
is asked to infer the type of `x`?

## Using the ACTION Object's Type ##

If you are writing a function that returns an `ACTION` object, you'll
need to know its type.  The type depends on the macro used to define
the action and the parameter types.  The rule is relatively simple:
| **Given Definition** | **Expression** | **Has Type** |
|:---------------------|:---------------|:-------------|
| `ACTION(Foo)`        | `Foo()`        | `FooAction`  |
| `ACTION_TEMPLATE(Foo, HAS_m_TEMPLATE_PARAMS(...), AND_0_VALUE_PARAMS())` |	`Foo<t1, ..., t_m>()` | `FooAction<t1, ..., t_m>` |
| `ACTION_P(Bar, param)` | `Bar(int_value)` | `BarActionP<int>` |
| `ACTION_TEMPLATE(Bar, HAS_m_TEMPLATE_PARAMS(...), AND_1_VALUE_PARAMS(p1))` | `Bar<t1, ..., t_m>(int_value)` | `FooActionP<t1, ..., t_m, int>` |
| `ACTION_P2(Baz, p1, p2)` | `Baz(bool_value, int_value)` | `BazActionP2<bool, int>` |
| `ACTION_TEMPLATE(Baz, HAS_m_TEMPLATE_PARAMS(...), AND_2_VALUE_PARAMS(p1, p2))` | `Baz<t1, ..., t_m>(bool_value, int_value)` | `FooActionP2<t1, ..., t_m, bool, int>` |
| ...                  | ...            | ...          |

Note that we have to pick different suffixes (`Action`, `ActionP`,
`ActionP2`, and etc) for actions with different numbers of value
parameters, or the action definitions cannot be overloaded on the
number of them.

## Writing New Monomorphic Actions ##

While the `ACTION*` macros are very convenient, sometimes they are
inappropriate.  For example, despite the tricks shown in the previous
recipes, they don't let you directly specify the types of the mock
function arguments and the action parameters, which in general leads
to unoptimized compiler error messages that can baffle unfamiliar
users.  They also don't allow overloading actions based on parameter
types without jumping through some hoops.

An alternative to the `ACTION*` macros is to implement
`::testing::ActionInterface<F>`, where `F` is the type of the mock
function in which the action will be used. For example:

```
template <typename F>class ActionInterface {
 public:
  virtual ~ActionInterface();

  // Performs the action.  Result is the return type of function type
  // F, and ArgumentTuple is the tuple of arguments of F.
  //
  // For example, if F is int(bool, const string&), then Result would
  // be int, and ArgumentTuple would be tr1::tuple<bool, const string&>.
  virtual Result Perform(const ArgumentTuple& args) = 0;
};

using ::testing::_;
using ::testing::Action;
using ::testing::ActionInterface;
using ::testing::MakeAction;

typedef int IncrementMethod(int*);

class IncrementArgumentAction : public ActionInterface<IncrementMethod> {
 public:
  virtual int Perform(const tr1::tuple<int*>& args) {
    int* p = tr1::get<0>(args);  // Grabs the first argument.
    return *p++;
  }
};

Action<IncrementMethod> IncrementArgument() {
  return MakeAction(new IncrementArgumentAction);
}
...

  EXPECT_CALL(foo, Baz(_))
      .WillOnce(IncrementArgument());

  int n = 5;
  foo.Baz(&n);  // Should return 5 and change n to 6.
```

## Writing New Polymorphic Actions ##

The previous recipe showed you how to define your own action. This is
all good, except that you need to know the type of the function in
which the action will be used. Sometimes that can be a problem. For
example, if you want to use the action in functions with _different_
types (e.g. like `Return()` and `SetArgumentPointee()`).

If an action can be used in several types of mock functions, we say
it's _polymorphic_. The `MakePolymorphicAction()` function template
makes it easy to define such an action:

```
namespace testing {

template <typename Impl>
PolymorphicAction<Impl> MakePolymorphicAction(const Impl& impl);

}  // namespace testing
```

As an example, let's define an action that returns the second argument
in the mock function's argument list. The first step is to define an
implementation class:

```
class ReturnSecondArgumentAction {
 public:
  template <typename Result, typename ArgumentTuple>
  Result Perform(const ArgumentTuple& args) const {
    // To get the i-th (0-based) argument, use tr1::get<i>(args).
    return tr1::get<1>(args);
  }
};
```

This implementation class does _not_ need to inherit from any
particular class. What matters is that it must have a `Perform()`
method template. This method template takes the mock function's
arguments as a tuple in a **single** argument, and returns the result of
the action. It can be either `const` or not, but must be invokable
with exactly one template argument, which is the result type. In other
words, you must be able to call `Perform<R>(args)` where `R` is the
mock function's return type and `args` is its arguments in a tuple.

Next, we use `MakePolymorphicAction()` to turn an instance of the
implementation class into the polymorphic action we need. It will be
convenient to have a wrapper for this:

```
using ::testing::MakePolymorphicAction;
using ::testing::PolymorphicAction;

PolymorphicAction<ReturnSecondArgumentAction> ReturnSecondArgument() {
  return MakePolymorphicAction(ReturnSecondArgumentAction());
}
```

Now, you can use this polymorphic action the same way you use the
built-in ones:

```
using ::testing::_;

class MockFoo : public Foo {
 public:
  MOCK_METHOD2(DoThis, int(bool flag, int n));
  MOCK_METHOD3(DoThat, string(int x, const char* str1, const char* str2));
};
...

  MockFoo foo;
  EXPECT_CALL(foo, DoThis(_, _))
      .WillOnce(ReturnSecondArgument());
  EXPECT_CALL(foo, DoThat(_, _, _))
      .WillOnce(ReturnSecondArgument());
  ...
  foo.DoThis(true, 5);         // Will return 5.
  foo.DoThat(1, "Hi", "Bye");  // Will return "Hi".
```

## Teaching Google Mock How to Print Your Values ##

When an uninteresting or unexpected call occurs, Google Mock prints
the argument values to help you debug.  The `EXPECT_THAT` and
`ASSERT_THAT` assertions also print the value being validated when the
test fails.  Google Mock does this using the user-extensible value
printer defined in `<gmock/gmock-printers.h>`.

This printer knows how to print the built-in C++ types, native arrays,
STL containers, and any type that supports the `<<` operator. For
other types, it prints the raw bytes in the value and hope you the
user can figure it out.

Did I say that the printer is `extensible`? That means you can teach
it to do a better job at printing your particular type than to dump
the bytes. To do that, you just need to define `<<` for your type:

```
#include <iostream>

namespace foo {

class Foo { ... };

// It's important that the << operator is defined in the SAME
// namespace that defines Foo.  C++'s look-up rules rely on that.
::std::ostream& operator<<(::std::ostream& os, const Foo& foo) {
  return os << foo.DebugString();  // Whatever needed to print foo to os.
}

}  // namespace foo
```

Sometimes, this might not be an option. For example, your team may
consider it dangerous or bad style to have a `<<` operator for `Foo`,
or `Foo` may already have a `<<` operator that doesn't do what you
want (and you cannot change it). Don't despair though - Google Mock
gives you a second chance to get it right. Namely, you can define a
`PrintTo()` function like this:

```
#include <iostream>

namespace foo {

class Foo { ... };

// It's important that PrintTo() is defined in the SAME
// namespace that defines Foo.  C++'s look-up rules rely on that.
void PrintTo(const Foo& foo, ::std::ostream* os) {
  *os << foo.DebugString();  // Whatever needed to print foo to os.
}

}  // namespace foo
```

What if you have both `<<` and `PrintTo()`? In this case, the latter
will override the former when Google Mock is concerned. This allows
you to customize how the value should appear in Google Mock's output
without affecting code that relies on the behavior of its `<<`
operator.

**Note:** When printing a pointer of type `T*`, Google Mock calls
`PrintTo(T*, std::ostream* os)` instead of `operator<<(std::ostream&, T*)`.
Therefore the only way to affect how a pointer is printed by Google
Mock is to define `PrintTo()` for it. Also note that `T*` and `const T*`
are different types, so you may need to define `PrintTo()` for both.

Why does Google Mock treat pointers specially? There are several reasons:

  * We cannot use `operator<<` to print a `signed char*` or `unsigned char*`, since it will print the pointer as a NUL-terminated C string, which likely will cause an access violation.
  * We want `NULL` pointers to be printed as `"NULL"`, but `operator<<` prints it as `"0"`, `"nullptr"`, or something else, depending on the compiler.
  * With some compilers, printing a `NULL` `char*` using `operator<<` will segfault.
  * `operator<<` prints a function pointer as a `bool` (hence it always prints `"1"`), which is not very useful.