How to mock method with optional parameter in Google Mock?

Change implementation of your method set_enable to use a helper method, like this:

void set_enable( bool enabled = true ) { set_enable_impl(enabled); }

Now, in class MockA, create a mock method for set_enable_impl:

MOCK_METHOD1( set_enable_impl, void( bool ) );

Then, in your production code you simply use set_enable as you would in the first place, while in tests you can set expectations on method set_enable_impl:

MockA mockA;
EXPECT_CALL(mockA, set_enable_impl(_))...;

An alternative would be to overload the method by having versions with one and zero parameters. It is up to you to determine which way works better for your case.

This is an alternative of Marko's answer: If you don't want to change your original code, just implement the helper in the mock class:

class A
{ 
public:
    virtual void set_enable( bool enabled = true );
};

class MockA : public A
{
    MOCK_METHOD1( set_enable_impl, void( bool ) );
    virtual void set_enable( bool enabled = true )
    {
        set_enable_impl( enabled );
    }
};

You still have to expect calls of set_enable_impl in your tests, for example

MockA mockA;
EXPECT_CALL(mockA, set_enable_impl(true)).Times(Exactly(1));
EXPECT_CALL(mockA, set_enable_impl(false)).Times(Exactly(1));