19. Reference to a member

// point.h
class Point
{
public:
    int& refx();
private:
    int m_x;
    int m_y;
};

// point.cpp
#include "point.h"
int& Point::refx()
{
    return m_x;
}

// main.cpp
#include "point.h"
int main()
{
    Point pt;
    pt.refx() = 14;
    return 0;
}

Returning a reference to a class member creates a "trapdoor" or "backdoor" that allows the data to be modified outside of the class regardless whether it is public or private.

Trapdoor is a secret way of gaining access to a program, online service, or other resource.