Question

Suppose I have a class template template <typename T> class X

Is it somehow possible to use type traits or a similar technique to call a (static) method of T but only if the type T declares such method, e.g. something like this:

template <typename T> 
class X {
    static void foo(){
       if(has_method(T,bar)) //Something like this      
         T::bar(); //If T has no bar() method, then foo does nothing
    }

};

Answer 1

template <typename T>
class X {
public:
    static void foo() {
        foo_impl(static_cast<T*>(nullptr));
    }
private:
    // foo_impl #1
    template <typename U>
    static auto foo_impl(U*) -> decltype(U::bar(), void()) {
        U::bar();
    }

    // foo_impl #2
    static void foo_impl(...) {}
};

Because of the SFINAE rule, foo_impl #1 is not in the overload set when U::bar() is not a valid expression, and foo_impl #2 gets called instead. If type deduction does succeed for foo_impl #1, it will always be a better conversion than the ellipsis.

Ideone demo: http://ideone.com/UKVmIB