Calling a free function instead of a method if it doesn't exist

Question

Suppose you have a family of type-unrelated classes implementing a common concept by means of a given method returning a value:

class A { public: int val() const         


        

Answer 1

Use this type trait class to determine whether a type has got val member function.

template
struct has_val {
    typedef char yes;
    typedef struct { char c[2]; } no;

    template
    static constexpr auto test(int) -> decltype( std::declval().val(), yes() );

    template
    static constexpr no test(...);

    static constexpr bool value = sizeof(test(0)) == sizeof(yes);
};

You can use it as a condition to enable_if.