How to detect whether there is a specific PRIVATE member variable in class?

Question

This question is based on this post.

Goal: I would like to know if a class has the member variable x. I would like to receive true

Answer 1

Well... not sure about correctness and limits of this solution... but...

If you define an helper struct with an x element accessible

struct check_x_helper
 { int x; };

you can write a template struct that inherit from both check_x_helper and the class you want to see if contain a x member

template 
struct check_x : public T, check_x_helper

Inside check_x you can declare (declare only: are used inside a decltype()) as follows

template 
static constexpr std::false_type check (int);

static constexpr std::true_type check (long);

Observe the first one, the template one: when the checked class (T) contains an x member, the decltype(U::x) is ambiguous because x is inherited from both T and check_x_helper, so this function is SFINAE discarded.

On contrary, when T doesn't contains an x member, there isn't an ambiguity, the decltype(U::x) is the type of check_x_helper::x (int) and the first check() function remain enabled.

Now you need something as

using type = decltype(check(0));

static constexpr auto value = type::value;

to call check(0) (the int parameter express the preference to the template version) and save the detected value in a static constexpr variable.

The following is a full compiling example

#include 
#include 

class foo
 { int x; };

struct bar
 { };

struct check_x_helper
 { int x; };

template 
struct check_x : public T, check_x_helper
 {
   template 
   static constexpr std::false_type check (int);

   static constexpr std::true_type check (long);

   using type = decltype(check(0));

   static constexpr auto value = type::value;
 };

int main()
 {
   std::cout << check_x::value << std::endl;
   std::cout << check_x::value << std::endl;
 }

Drawback of this solution: decltype(U::x) fail (ambiguity) also when T declare x as a method or as a using type. So given

class foo
 { int x () { return 0;} ; };

or

class foo
 { using x = int; };

from check_x::value you obtain 1.