Unit tests: How to access private members in C++

In order to write unit tests sometimes one needs to access private class members or methods.
There are a couple of ways to do this in C++:

1. Use the friend keyword for the test classes or methods:

class ToTest
{
public:
    void foo();
    int bar();
private:
    void private_foobar();
    int m_private_value;

    friend void testToTest();
};

void testToTest(){
    ToTest testObj;
//  test testObj.m_private_value
}

This method is usable but it means modifying production code in order to add tests.

2. Use preprocessor defines to change the access level

class ToTest
{
public:
    void foo();
    int bar();
#ifdef UNIT_TESTS
public:
#else
private:
#endif
    void private_foobar();
    int m_private_value;
};

This method is still intrusive and adds modifications to production code. Also, you need separate builds for testing and deployment.

3. Define private as public for the tested class

#define private public
#include <class_to_test.h>
#define private private

This method is non-intrusive, easiest and it allows to test everything in one single build. It will yield some compiler warnings, but they can be ignored, for the sake o testing.