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    <p>What will follow is not technically speaking a straight answer to your question as it will still make use of the "friend" functionality but it does not require modification of the tested entity itself and I think it addesses the concern of breaking the encapsulation mentioned in some of the other answers; it does though require writing some boilerplate code.</p> <p>The idea behind it is not mine and the implementation is <a href="http://bloglitb.blogspot.be/2011/12/access-to-private-members-safer.html" rel="nofollow">entirely based on a trick presented and explained by litb on his blog</a>(coupled with this <a href="http://www.gotw.ca/publications/mill02.htm" rel="nofollow">Sutter's gotw</a> for just a little bit more context, at least for me) - in short CRTP, friends, ADL and pointers to members (I must confess that to my dismay the ADL part I still don't get it entirely, but I'm relentesly working in figuring it out 100%).</p> <p>I tested it with gcc 4.6, clang 3.1 and VS2010 compilers and it works perfectly.</p> <pre><code>/* test_tag.h */ #ifndef TEST_TAG_H_INCLUDED_ #define TEST_TAG_H_INCLUDED_ template &lt;typename Tag, typename Tag::type M&gt; struct Rob { friend typename Tag::type get(Tag) { return M; } }; template &lt;typename Tag, typename Member&gt; struct TagBase { typedef Member type; friend type get(Tag); }; #endif /* TEST_TAG_H_INCLUDED_ */ /* tested_class.h */ #ifndef TESTED_CLASS_H_INCLUDED_ #define TESTED_CLASS_H_INCLUDED_ #include &lt;string&gt; struct tested_class { tested_class(int i, const char* descr) : i_(i), descr_(descr) { } private: int i_; std::string descr_; }; /* with or without the macros or even in a different file */ # ifdef TESTING_ENABLED # include "test_tag.h" struct tested_class_i : TagBase&lt;tested_class_i, int tested_class::*&gt; { }; struct tested_class_descr : TagBase&lt;tested_class_descr, const std::string tested_class::*&gt; { }; template struct Rob&lt;tested_class_i, &amp;tested_class::i_&gt;; template struct Rob&lt;tested_class_descr, &amp;tested_class::descr_&gt;; # endif #endif /* TESTED_CLASS_H_INCLUDED_ */ /* test_access.cpp */ #include "tested_class.h" #include &lt;cstdlib&gt; #include &lt;iostream&gt; #include &lt;sstream&gt; #define STRINGIZE0(text) #text #define STRINGIZE(text) STRINGIZE0(text) int assert_handler(const char* expr, const char* theFile, int theLine) { std::stringstream message; message &lt;&lt; "Assertion " &lt;&lt; expr &lt;&lt; " failed in " &lt;&lt; theFile &lt;&lt; " at line " &lt;&lt; theLine; message &lt;&lt; "." &lt;&lt; std::endl; std::cerr &lt;&lt; message.str(); return 1; } #define ASSERT_HALT() exit(__LINE__) #define ASSERT_EQUALS(lhs, rhs) ((void)(!((lhs) == (rhs)) &amp;&amp; assert_handler(STRINGIZE((lhs == rhs)), __FILE__, __LINE__) &amp;&amp; (ASSERT_HALT(), 1))) int main() { tested_class foo(35, "Some foo!"); // the bind pointer to member by object reference could // be further wrapped in some "nice" macros std::cout &lt;&lt; " Class guts: " &lt;&lt; foo.*get(tested_class_i()) &lt;&lt; " - " &lt;&lt; foo.*get(tested_class_descr()) &lt;&lt; std::endl; ASSERT_EQUALS(35, foo.*get(tested_class_i())); ASSERT_EQUALS("Some foo!", foo.*get(tested_class_descr())); ASSERT_EQUALS(80, foo.*get(tested_class_i())); return 0; } </code></pre>
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