private_access.cpp

#include <iostream>

// Define the template function `GetMember`.
// This function will be the friend non-member function of the class `ClassType`.
template <typename ClassType, typename ClassType::MemberType member>
struct AccessPrivate
{
    friend typename ClassType::MemberType GetMember(ClassType)
    {
        return member;
    }
};

struct A
{
    A() : x(100) {}

private:
    int x;
};

// Declare a tag struct.
struct A_x_AccessTag
{
    // typedef the `MemberType` for `GetMember(A_x_AccessTag)` function.
    // In this case, `MemberType` will be a pointer that point to "an int member variable of class A".
    typedef int A::*MemberType;

    // Declare GetMember(A_x_AccessTag) as a friend function.
    friend MemberType GetMember(A_x_AccessTag);
};

// Specialize the function `GetMember<A_x_AccessTag>` to access the member `A::x`.
// In this case, will declare a varable:
//     A_x_AccessTag::MemberType member = &A::x;
// Is the same as:
//     int A::*member = &A::x;
template struct AccessPrivate<A_x_AccessTag, &A::x>;

int main()
{
    A a;

    // GetMember(A_x_AccessTag()) will return the pointer to member `&A::x`.
    // And use .* operator to access `A::x` of a.
    std::cout << "Hello private memeber: " << a.*GetMember(A_x_AccessTag()) << std::endl;

    return 0;
}