Testing private class member in C++ without friend [duplicate]

Answer 1

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.

The idea behind it is not mine and the implementation is entirely based on a trick presented and explained by litb on his blog(coupled with this Sutter's gotw 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%).

I tested it with gcc 4.6, clang 3.1 and VS2010 compilers and it works perfectly.

/* test_tag.h */
#ifndef TEST_TAG_H_INCLUDED_
#define TEST_TAG_H_INCLUDED_

template 
struct Rob
{
    friend typename Tag::type get(Tag)
    {
        return M;
    }
};

template  
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 

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 { };
    struct tested_class_descr : TagBase { };

    template struct Rob;
    template struct Rob;

#   endif

#endif /* TESTED_CLASS_H_INCLUDED_ */

/* test_access.cpp */
#include "tested_class.h"

#include 
#include 
#include 

#define STRINGIZE0(text) #text
#define STRINGIZE(text) STRINGIZE0(text)

int assert_handler(const char* expr, const char* theFile, int theLine)
{
    std::stringstream message;
    message << "Assertion " << expr << " failed in " << theFile << " at line " << theLine;
    message << "." << std::endl;
    std::cerr << message.str();

    return 1;
}

#define ASSERT_HALT() exit(__LINE__)

#define ASSERT_EQUALS(lhs, rhs) ((void)(!((lhs) == (rhs)) && assert_handler(STRINGIZE((lhs == rhs)), __FILE__, __LINE__) && (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 << " Class guts: " << foo.*get(tested_class_i()) << " - " << foo.*get(tested_class_descr()) << 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; 
}