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 <type_traits>

// Yakk's can_apply

template<class...>struct voider{using type=void;};
template<class...Ts>using void_t=typename voider<Ts...>::type;

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

// Main code

class MemberBase {
    public:
        int member;
};

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

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

template <typename T>
using hasnot_member = can_apply<member_type, MemberCheck<T>>;

template <typename T> 
using static_not = std::integral_constant<bool, !T::value>;

template <typename T>
using has_member = static_not<hasnot_member<T>>;

// Tests

class A {
    int member;
};

class Ap {
    public:
    int member;
};

class B {
    float member;
};

class C {
    int member();
};

class D {
};

static_assert(has_member<A>{}, "!"); // ok
static_assert(has_member<Ap>{}, "!"); // ok
static_assert(has_member<B>{}, "!"); // ok
static_assert(has_member<C>{}, "!"); // ok
static_assert(has_member<D>{}, "!"); // fail

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

Answer 2

There is indeed a way for non-final non-union class types:

namespace detail {
    struct P {typedef int member;};
    template <typename U>
    struct test_for_member : U, P
    {
        template <typename T=test_for_member, typename = typename T::member>
        static std::false_type test(int);
        static std::true_type test(float);
    };
}
template <typename T>
using test_for_member =
  std::integral_constant<bool, decltype(detail::test_for_member<T>::test(0)){}>;

Demo. The trick is to check whether lookup into different base classes will yield an ambiguity. [class.member.lookup]/2:

Member name lookup determines the meaning of a name (id-expression) in a class scope (3.3.7). Name lookup can result in an ambiguity, in which case the program is ill-formed. […] Name lookup takes place before access control (3.4, Clause 11).

Note that GCCs lookup is broken insofar as it ignores non-type names for lookup in typename-specifiers.