sum op overloading w/o return C++

/*program to overload bianry operator +*/
#include<iostream.h>
#include<conio.h>

class Sum       //class declaration
{
	private : int a;
	public : void read();
			 void display();
			 void operator+(Sum);
};
void Sum :: read()
{
	cout<<"Enter value : ";
	cin>>a;
}
void Sum :: operator+(Sum x)
{
	a=a+x.a;
}
void Sum :: display()
{
	cout<<"Sum= "<<a<<endl;
}
int main()
{
	Sum s1,s2,s3;
	clrscr();
	s1.read();
	s2.read();
	s1+s2;
	cout<<"After Addition:\n";
	s1.display();
	getch();
	return 0;
}