accessPrivateData.h

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// SPDX-FileCopyrightText: Deutsches Elektronen-Synchrotron DESY, MSK, ChimeraTK Project <chimeratk-support@desy.de>
// SPDX-License-Identifier: LGPL-3.0-or-later
#pragma once
 
// Helper classes to access private data of another class, which can be useful
// for whitebox tests This code was taken from:
// https://gist.github.com/dabrahams/1528856
 
namespace accessPrivateData {
 
  // This is a rewrite and analysis of the technique in this article:
  // http://bloglitb.blogspot.com/2010/07/access-to-private-members-thats-easy.html
 
  // ------- 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!") {}
  private:
       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;
  };
  */
 
} // namespace accessPrivateData