Detect existence of private member

Question

I want to write a type trait to check if some type has a member member. If member were public, there are any number of ways to do this (e.

Answer 1

You can create another class MemberBase that does have that member, and then subclass the two classes (the class to check T and BaseMember) and try to access the member of the subclass. If T also has a member member, then you will get an ambiguity problem.

Code:

#include 

// Yakk's can_apply

templatestruct voider{using type=void;};
templateusing void_t=typename voider::type;

templatestruct types{using type=types;};
namespace details {
  templateclass Z, class types, class=void>
  struct can_apply : std::false_type {};
  templateclass Z, class...Ts>
  struct can_apply< Z, types, void_t< Z > >:
    std::true_type
  {};
}
templateclass Z, class...Ts>
using can_apply = details::can_apply>;

// Main code

class MemberBase {
    public:
        int member;
};

template
class MemberCheck: public ToCheck, public MemberBase {
};

template 
using member_type = decltype(&T::member);

template 
using hasnot_member = can_apply>;

template  
using static_not = std::integral_constant;

template 
using has_member = static_not>;

// Tests

class A {
    int member;
};

class Ap {
    public:
    int member;
};

class B {
    float member;
};

class C {
    int member();
};

class D {
};

static_assert(has_member{}, "!"); // ok
static_assert(has_member{}, "!"); // ok
static_assert(has_member{}, "!"); // ok
static_assert(has_member{}, "!"); // ok
static_assert(has_member{}, "!"); // fail

However, this definitely smells like a dirty hack to me.