Clarification of Call() in gtest EXPECT_CALL

Question

I have a class which as one of its private member has an std::function, something like this:

class MyClass
  {
  private:
    int val;
    std::function<void(int)> foo;

  public:
    MyClass(int p_val, std::function<void(int)> p_foo)
    {
      this->val = p_val;
      this->foo = p_foo;
    }

    void aMethod(int input)
    {
      this->foo(input);
    }

};

I want to test that when function aMethod is called, the callback got called anyway. For this I'm doing a test fixture in gtest that looks like this:

TEST_F(SomeFixture, MyTest)
{
  const int input;
  ::testing::MockFunction<void(int)> mockCallback;
  EXPECT_CALL(mockCallback, Call(_));

  MyClass object(0, mockCallback.AsStdFunction());  
  object.aMethod(1);
}

This seems to work because the test passes and if I comment out object.aMethod(1) in the test or this->foo(input); in the class the the test fails. What is unclear to me is the role of Call(_): I understand that _ corresponds to the input parameter but what does it mean and how does it work?

Answer 1

::testing::_ is a wildcard matcher, meaning that mock calls do not verify what is passed as argument (as long as compiler allows it).

Other matchers could be passed instead, e.g.

EXPECT_CALL(mockCallback, Call(1));
// or
EXPECT_CALL(mockCallback, Call(Eq(1)));

would verify that mockCallback was called with argument 1.