Tip on debugging tests that use mocks

[Zhanyong Wan (λx.x x)]

Hi,

I just added a recipe to the Google Mock cookbook on debugging tests that use mocks:

http://code.google.com/p/googlemock/wiki/CookBook#Gaining_Super_Vision_into_Mock_Calls

Gaining Super Vision into Mock Calls

You have a test using Google Mock. It fails: Google Mock tells you that some expectations aren't satisfied. However, you aren't sure why: Is there a typo somewhere in the matchers? Did you mess up the order of the EXPECT_CALLs? Or is the code under test doing something wrong? How can you find out the cause?

Won't it be nice if you have X-ray vision and can actually see the trace of all EXPECT_CALLs and mock method calls as they are made? For each call, would you like to see its actual argument values and which EXPECT_CALL Google Mock thinks it matches?

You can unlock this power by running your test with the --gmock_verbose=info flag. For example, given the test program:

using testing::_;
using testing::HasSubstr;
using testing::Return;

class MockFoo {
 public:
  MOCK_METHOD2(F, void(const string& x, const string& y));
};

TEST(Foo, Bar) {
  MockFoo mock;
  EXPECT_CALL(mock, F(_, _)).WillRepeatedly(Return());
  EXPECT_CALL(mock, F("a", "b"));
  EXPECT_CALL(mock, F("c", HasSubstr("d")));

  mock.F("a", "good");
  mock.F("a", "b");
}

if you run it with --gmock_verbose=info, you will see this output:

[ RUN      ] Foo.Bar

foo_test.cc:14: EXPECT_CALL(mock, F(_, _)) invoked
foo_test.cc:15: EXPECT_CALL(mock, F("a", "b")) invoked
foo_test.cc:16: EXPECT_CALL(mock, F("c", HasSubstr("d"))) invoked
foo_test.cc:14: Mock function call matches EXPECT_CALL(mock, F(_, _))...
    Function call: F(@0x7fff7c8dad40"a", @0x7fff7c8dad10"good")
foo_test.cc:15: Mock function call matches EXPECT_CALL(mock, F("a", "b"))...
    Function call: F(@0x7fff7c8dada0"a", @0x7fff7c8dad70"b")
foo_test.cc:16: Failure
Actual function call count doesn't match EXPECT_CALL(mock, F("c", HasSubstr("d")))...
         Expected: to be called once
           Actual: never called - unsatisfied and active
[  FAILED  ] Foo.Bar

Suppose the bug is that the "c" in the third EXPECT_CALL is a typo and should actually be "a". With the above message, you should see that the actual F("a", "good") call is matched by the first EXPECT_CALL, not the third as you thought. From that it should be obvious that the third EXPECT_CALL is written wrong. Case solved.

Cheers,

--
Zhanyong