Operator Overloading Problem. Variable is not changed

[cuoilennaocacban]

I using Operator Overloading + to add a Date object with int num. The Date will be modified. But when I test it, it's not change at all


File DateAndTime.h

// Date class declaration

#include "stdafx.h"

#ifndef DATE_H
#define DATE_H

class Date
{
private:
	int day,month,year;
public:
	Date() {};
	Date(int, int, int); //Constructor
	int leapYear();
	int valid();
	void input();
	void output();
	void nextDay(Date &);
	void previousDay(Date &);

	Date operator=(const Date &a);
	Date &operator+(int);
};

#endif 

File DateAndTime.cpp

// Date class definition

#include "stdafx.h"

#include "iostream"
using std::cout;
using std::cin;
using std::endl;

#include "Date.h"

Date::Date(int d, int m, int y)
	: day(d),
	month(m),
	year(y)
{}

int Date::leapYear()
{
	if(year%400==0 || (year%100!=0 && year%4 == 0))
		return 1;
	else
		return 0;
}

int Date::valid()
{
	if(day<0)
	{
		return 0;
	}
	else
	{
		switch(month)
		{
		case 2:
			{
				if(this->leapYear()==0)
					if(day<29)
						return 1;
					else
						return 0;
				else
					if(day<30)
						return 1;
					else
						return 0;
				break;
			}
		case 1:
		case 3:
		case 5:
		case 7:
		case 8:
		case 10:
		case 12:
			{
				if(day<32)
					return 1;
				else
					return 0;
				break;
			}
		default:
			{
				if(day<31)
					return 1;
				else
					return 0;
				break;
			}
		}
	}
}
			
void Date::input()
{
	do
	{
		cout<<"  Enter Day: ";
		cin>>day;
		cout<<"Enter Month: ";
		cin>>month;
		cout<<" Enter Year: ";
		cin>>year;
		cout<<endl;
		if(this->valid()==0)
			cout<<"Re-enter date to replace invalid date"<<endl;
	}
	while(this->valid()==0);
}

void Date::output()
{
	cout<<"You have entered: "
		<<day<<"/"
		<<month<<"/"
		<<year<<endl;
}

void Date::nextDay(Date &next)
{
	next.day=this->day;
	next.month=this->month;
	next.year=this->year;
	switch(day)
	{
	case 28:
		{
			if(this->leapYear()==0)
				next.day=next.day+1;
			else
			{
				next.day=1;
				next.month=3;
			}
			break;
		}
	case 30:
		{
			if(month==4 || month==6 || month==9 || month==11)
			{
				next.day=1;
				next.month=next.month+1;
			}
			else
				next.month=31;
		break;
		}
	case 31:
		{
			if(month=12)
			{
				next.day=1;
				next.month=1;
				next.year=next.year+1;
			}
			else
			{
				next.day=1;
				next.month=next.month+1;
			}
			break;
		}
	}
}

Date Date::operator=(const Date &a)
{
	Date b;
	b.day=a.day;
	b.month=a.month;
	b.year=a.year;
	return b;
}

Date &Date::operator+(int a)
{
	int sum=0;
	int i;
	int m[]={0,31,28,31,30,31,30,31,31,30,31,30,31};
	m[0]=leapYear();
	if(m[0]!=0)
		m[2]=29;
	for(i=1;i<month;i++)
		sum=sum+m[i];
	sum=sum+day;// The number of day in the year
	sum=sum+a;  // The number of day after plus operator

	// Change back to Date
	i=1;
	while(sum>m[i])
	{
		sum=sum-m[i];
		i++;
	}
	int d=sum;
	int mn=i;
	int y=this->year;
	Date b(d,mn,y);
	return b;
}

Main

// Main program
// Version 1.0. Testing

#include "stdafx.h"
#include "iostream"
using std::cout;
using std::cin;
using std::endl;

#include "Date.h"

int main()
{
	Date a;
	a.input();
	a.output();

	Date b=a;
	b.output();

	int num;
	cout<<"Enter number of day you want to add in: ";
	cin>>num;
	Date c;
	c=(b+num); //if I try b=b+num; b still the "old" b (=a)
	c.output();

	return 0;
}

Date c is not change (Output is just the default value of c). Why is that???? Somebody help me.

[D80DSL3A]

The = operator is assigning values to the local variable b. It should make the assignments to the members of this (the object calling the = function).
Also, the return types are reversed. The + operator should return a Date and = should return a Date&. Aren't you getting a warning about + returning a reference to a local variable?
Try:

Date& Date::operator=(const Date &a)
{
	day=a.day;
	month=a.month;
	year=a.year;
	return *this;
}

EDIT: Disch recently gave an excellent explanation for the return types of a variety of operators here: http://www.cplusplus.com/forum/beginner/53199/#msg288017

[cuoilennaocacban]

Thank you very much. The link you gave me is an excellent explanation.