Is there any type trait which controls member type(not member variable)

Question

STL comes with many type traits like std::is_pointer, std::is_reference etc...

Lets say I have a class

class A 
{
   using type_member = void;     
}

Is there any type trait for controlling the type member and checking if it exists ?
Something like is_type_member_exist<typename A::type_member>();

I am both curious if a solution exist with C++17 and also curious about C++2003(at work I need this and I have vs2010 which has a bit C++11 support but not complete).

Answer 1

If type_member is public (and not private as in your question), I suppose you can do something like

#include <iostream>

template <typename X>
struct with_type_member
 { 
   template <typename Y = X>
   static constexpr bool getValue (int, typename Y::type_member * = nullptr)
    { return true; }

   static constexpr bool getValue (long)
    { return false; }

   static constexpr bool value { getValue(0) };
 };

class A 
 {
   public:
      using type_member = void;     
 };

int main ()
 {
   std::cout << with_type_member<int>::value << std::endl; // print 0
   std::cout << with_type_member<A>::value << std::endl;   // print 1
 }

Hope this helps.