Type trait: Check if class have specific function (maybe inherit)

Question

I know that there are many possible ways to detect if a class has a specific function but non of them really work for my exact case. My current implementation to check for t

Answer 1

Here is a way to do it (work for your 4 test cases, did not test it intensively though), thanks @Jarod42 for the improvement (see initial answer at the end):

template 
int call_foo (int (T::*)(float));

template 
std::true_type has_foo(decltype(call_foo(&C::foo)));

template 
std::false_type has_foo (...);    

template
using HasFoo = decltype(has_foo(0));

The problem with your code was that you were expecting U::* whereas &B::foo is A::* (not B::*). Here I let the compiler choose the value of T by using implicit type deduction so I don't run into such issue.

The code works as follow:

• If T does not have a foo member, then the compiler will choose the second overload of has_foo.
• If T does have a foo member, the compiler will try the first overload but will fail since there is no matching call_foo function so it will again choose the second one and make a std::false_type.

Working code on ideone: http://ideone.com/erh93I.

You can put everything in a class if you want:

template 
class HasFoo {

    template 
    static int call_foo (int (C::*)(float));

    template 
    static std::true_type has_foo (decltype(call_foo(&C::foo)));

    template 
    static std::false_type has_foo (...);

public:
    static constexpr bool value = decltype(has_foo(0)){};
};