Know Your Test Doubles

A test double is an object that can stand in for a real object in test. The most common types of test doubles are:

• Stub - Returns a specific values to promote a specific state.

  AccessManager access_manager(kStubAuthenticationService);
  
  ON_CALL(kStubAuthenticationService, IsAuthenticated(kUserId))
      .WillByDefault(Return(false));
  EXPECT_FALSE(access_manager.UserHasAccess(kUserId));
  
  ON_CALL(kStubAuthenticationService, IsAuthenticated(kUserId))
      .WillByDefault(Return(true));
  EXPECT_TRUE(access_manager.UserHasAccess(kUserId));
  

• Mock - Sets expectations about how other objects should interact with it.

  AccessManager access_manager(mockAuthenticationService);
  
  EXPECT_CALL(mockAuthenticationService, IsAuthenticated(kUserId));
  access_manager.UserHasAccess(kUserId);
  

• Fake - A lightweight implementation when the real implementation is unsuitable for test.

  FakeAuthenticationService fake_authentication_service;
  AccessManager access_manager(fake_authentication_service);
  
  EXPECT_FALSE(access_manager.UserHasAccess(kUserId));
  
  fake_authentication_service.AddAuthenticatedUser(kUser);
  EXPECT_TRUE(access_manager.UserHasAccess(kUserId));