correosdelbosque · October 29, 2016 06:42 · Jan 14, 2012 · Dec 28, 2011
diff --git a/private_access.cpp b/private_access.cpp
@@ -1,5 +1,11 @@
 #include <iostream>
 
+// 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
@@ -27,18 +33,18 @@ 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
 
-// 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.
+// 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

diff --git a/private_access.cpp b/private_access.cpp
@@ -0,0 +1,55 @@
+#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;
+};