access-object-private-members.cpp

// library

#pragma GCC diagnostic push
#pragma GCC diagnostic ignored "-Wnon-template-friend"

template<typename Tag, typename Tag::type MemberPointer>
struct Rob {
    friend decltype(MemberPointer) access(Tag) { return MemberPointer; }
};

template<typename Tag, typename Type>
struct TagBase
{
    using type = Type;
    friend type access(Tag);
};

#pragma GCC diagnostic pop

// example

#include <cstdio>

class A
{
private:
    int m_i0;
    int m_i1;
    bool m_b;
};

struct A_m_i0 : TagBase<A_m_i0, int A::*> {};
struct A_m_i1 : TagBase<A_m_i1, int A::*> {};
struct A_m_b : TagBase<A_m_b, bool A::*> {};
template struct Rob<A_m_i0, &A::m_i0>; // KEY!!! must be template instantiation
template struct Rob<A_m_i1, &A::m_i1>; // KEY!!! must be template instantiation
template struct Rob<A_m_b, &A::m_b>; // KEY!!! must be template instantiation

int main() {
    A a;
    a.*access(A_m_i0()) = 42;
    fprintf(stdout, "%d\n", a.*access(A_m_i0()));
    return 0;
}