How to get object type of pointer to non-static data member at compile time?

user_5082773 0
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Suppose we have a simple data class like this:

struct DataObj
{ 
  char member[32];
}

And the type of pointer to the member in the data object:

typedef decltype(&DataObj::member) memberObjPtr;

How can I deduce the type of the member variable the pointer points to? Specifically, how do I get: 

typedef myExpression<memberObjPtr>::type myType;
std::is_same<char[32],myType>::value == true

What I tried so far:

std::remove_pointer
std::remove_reference
std::decay

Without success. Is there something like remove_member_object_pointer somewhere hidden in the standard? Which is what I would need but can't seem to find..

typetraits member-pointers templates c++
2 Answers

Member pointers and regular pointers are completely different types. There is nothing you can add or remove to go from a member pointer to a regular object pointer. You need a dedicated type trait.

// General case
// If a type isn't supported by a partial specialization
//  it will use this case and fail to compile
template<class T>
struct mbrptr_to_type;

// Partial specialization that matches any data member pointer type
// T C::* means "pointer to member of type `T` in class `C`"
template<class T, class C>
struct mbrptr_to_type<T C::*> {
    // Save the member type so it can be retrieved
    using type = T;
};

// Helper alias
template<class T>
using mbrptr_to_type_t = typename mbrptr_to_type<T>::type;

Using your test case : 

struct DataObj
{
    char member[32];
};

// myType will be char[32]
using myType = mbrptr_to_type_t<decltype(&DataObj::member)>;

// Verification
#include <type_traits>
static_assert(std::is_same<char[32], myType>::value);

Live example : Godbolt

user_73590940

If you don't care about references, you can do the following in-place:

using member_t = std::remove_reference_t<decltype(std::declval<DataObj>().member)>;

It is less general but a bit shorter then the other answer.

user_123456560
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