Demonstrating the dangerous effect of returning a reference from a class. Gotten from the book C how to program.

main.cpp

#include <iostream>
using std::cout;
using std::endl;

#include "Time.h"

int main(){
    Time t; // create Time object

    // initialize hourRef with the reference returned by badSetHour
    int &hourRef = t.badSetHour( 20 ); // 20 is a valid

    cout << "Valid hour before modification: " << hourRef;
    hourRef = 30;
    cout << "\nInvalid hour after modification: " << t.getHour();

    // Dangerous function call that returns
    // a reference caan be used as a lvalue!
    t.badSetHour( 12 ) = 74;

    cout << "\n\n**********************************************\n"
            << "POOR PROGRAMMING PRACTICE!!!!!!!!!\n"
            << "t.badSetHour( 12 ) as lvalue, invalid hour: "
            << t.getHour()
         << "\n**********************************************" << endl;

    return 0;
}

Time.cpp

#include "Time.h"

Time::Time(int hr, int min, int sec){
    setTime(hr, min, sec);
}

void Time::setTime(int h, int m, int s){
    hour = ( h >= 0 && h < 24 ) ? h : 0;
    minute = ( m >= 0 && m < 60 ) ? m : 0;
    second = ( s >= 0 && s < 60 ) ? s : 0;
}

int Time::getHour(){
    return hour;
}

// Poor programming practice:
// Returning a returning to a private data member
int &Time::badSetHour( int hr ){
    hour = ( hr >= 0 && hr < 24 ) ? hr : 0;
    return hour; // It is dangerous to return reference
}

Time.h

#ifndef TIME_H
#define TIME_H

class Time{
    public:
        Time( int = 0, int = 0, int = 0);
        void setTime(int, int, int);
        int getHour();
        int &badSetHour( int ); // It is dangerous to return a reference from a class
    private:
        int hour;
        int minute;
        int second;
};

#endif