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Accessing Private Data
#include <iostream>
// This is a rewrite and analysis of the technique in this article:
// ------- Framework -------
// The little library required to work this magic
// 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 -------
// A demonstration of how to use the library, with explanation
struct A
A() : x("proof!") {}
char const* x;
// A tag type for A::x. Each distinct private member you need to
// access 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;

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come-raczy commented Aug 27, 2013

why having several layers of classes and static members instead of a single class with static method like this:

template <class Type, Type value>
struct accessor
    static Type getValue() {return value;}

It seems to work (at least with gcc 4.8.1 using c++11)


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polkovnikov-ph commented Aug 29, 2014

@come-raczy That's an incorrect behaviour. You cannot pass pointer to private member anywhere except the arguments for explicit instantiation.


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garethsb commented May 14, 2015

(Revised 13-Oct-2016!)

This technique can also be used to access private static member variables.


struct B
    static char const* x;

char const* B::x = "proof!";

struct B_x { typedef char const** type; };
template class stow_private<B_x,&B::x>;

int main()
        // Use the stowed private static member pointer
        std::cout << *stowed<B_x>::value << std::endl;

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mdadams commented Feb 2, 2017

@dabrahams Just for fun, I tried a variation on the ideas from your code example using variable templates:
The code in my example works okay with GCC 6.3.0, but Clang 3.9.1 yields undefined references to saved_private_v<Widget_i_> and saved_private_v<Widget_f_> when linking. I am not sure if this is a bug in Clang or I am making some non-portable assumptions about templates in my code.

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