Accessing Private Data

private_access.cpp

#include <iostream>

// Generate a static data member of type Tag::type in which to store
// the address of a private member.  It is crucial that Tag does not
// depend on the /value/ of the the stored address in any way so that
// we can access it from ordinary code without directly touching
// private data.
template <class Tag>
struct stowed
{
     static typename Tag::type value;
}; 
template <class Tag> 
typename Tag::type stowed<Tag>::value;

// Generate a static data member whose constructor initializes
// stowed<Tag>::value.  This type will only be named in an explicit
// instantiation, where it is legal to pass the address of a private
// member.
template <class Tag, typename Tag::type x>
struct stow_private
{
     stow_private() { stowed<Tag>::value = x; }
     static stow_private instance;
};
template <class Tag, typename Tag::type x> 
stow_private<Tag,x> stow_private<Tag,x>::instance;

// ------- Usage -------

// Demonstration type with private data
struct A
{
     A() : x("proof!") {}
private:
     char const* x;
};

// A tag type for A::x.  Each distinct private member should have its
// own tag.  Each tag should contain a nested ::type that is the
// corresponding pointer-to-member type.
struct A_x { typedef char const*(A::*type); };

// Explicit instantiation; the only place where it is legal to pass
// the address of a private member.  Generates the static ::instance
// that in turn initializes stowed<Tag>::value.
template class stow_private<A_x,&A::x>;

int main()
{
        A a;

        // Use the stowed private member pointer
        std::cout << a.*stowed<A_x>::value << std::endl;
};