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

You can use return-type SFINAE:

template
auto val_of(const T& t) -> decltype(std::declval().val())
{
    return t.val();
}

int val_of(...)
{
    return 0;
}