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Operator Overload Question

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Join Date: May 2008

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hmaich

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Operator Overload Question

May 10th, 2008

Hello all,

I have a doubt concerning to overloaded operators.
Does the compiler create new temporary object to the elements involved on the overloaded operation?

E.g. Operator + overloaded.

a = b + c;

Does the compiler creat temporary object for b and c?

See the following code and the print outs. When the + operator is called, 3 new objects are destroyed and I cant see the construction message.
I guess the default copy constructor is called by the compiler, but I am not sure.

Does any one knows the answer for this question???

Thank you,
Helbert

 Help with Code Tags 
 C++ Syntax (Toggle Plain Text) 
// Class1.cpp : Defines the entry point for the console application.
//
 
#include "stdafx.h"
#include <iostream>
 
using namespace std;
class Rectangle{
	int x, y;
 
	public:
		Rectangle(int x, int y);
		Rectangle(){cout << "Created:  " << this << endl;};
		void set_values(int x, int y);
		int getX() {return x;};
		int getY() {return y;};
		int area() {return (x*y); }
		Rectangle operator +(Rectangle);
		~Rectangle();
};
 
Rectangle::Rectangle(int x, int y){
	this->x = x;
	this->y = y;
	cout << "Created:  " << this << endl;
}
 
Rectangle::~Rectangle(){
	cout << "Retangle has been destroyed  " << this << endl;
}
 
Rectangle Rectangle::operator +(Rectangle add){
	Rectangle tmp;
	tmp.set_values(add.getX(), add.getY());
	return tmp;
}
void Rectangle::set_values(int x, int y){
	this->x = x;
	this->y = y;
}
int main(){
 
	Rectangle rect(2,3);
	Rectangle rectb(1,2);
	Rectangle rectc;
 
	cout << "AREA: " << rect.area() << endl;
 
	cout << "AREA: " << rectb.area() << endl;
 
	rectc = rect + rectb;
 
	cout << "AREA: " << rectc.area() << endl;
	return 0;
}
// Class1.cpp : Defines the entry point for the console application.
//

#include "stdafx.h"
#include <iostream>

using namespace std;
class Rectangle{
	int x, y;
	
	public:
		Rectangle(int x, int y);
		Rectangle(){cout << "Created:  " << this << endl;};
		void set_values(int x, int y);
		int getX() {return x;};
		int getY() {return y;};
		int area() {return (x*y); }
		Rectangle operator +(Rectangle);
		~Rectangle();
};

Rectangle::Rectangle(int x, int y){
	this->x = x;
	this->y = y;
	cout << "Created:  " << this << endl;
}

Rectangle::~Rectangle(){
	cout << "Retangle has been destroyed  " << this << endl;
}

Rectangle Rectangle::operator +(Rectangle add){
	Rectangle tmp;
	tmp.set_values(add.getX(), add.getY());
	return tmp;
}
void Rectangle::set_values(int x, int y){
	this->x = x;
	this->y = y;
}
int main(){
	
	Rectangle rect(2,3);
	Rectangle rectb(1,2);
	Rectangle rectc;
	
	cout << "AREA: " << rect.area() << endl;

	cout << "AREA: " << rectb.area() << endl;

	rectc = rect + rectb;

	cout << "AREA: " << rectc.area() << endl;
	return 0;
}

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Join Date: Dec 2006

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vijayan121 vijayan121 is offline

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Re: Operator Overload Question

May 10th, 2008

with

 Help with Code Tags 
 C++ Syntax (Toggle Plain Text) 
class Rectangle
{
  //...
  public:
  //...
     Rectangle operator +(Rectangle);
  //...
};
class Rectangle
{
  //...
  public:
  //...
     Rectangle operator +(Rectangle);
  //...
};

// a, b, c are Rectangles
a = b + c ; evaluates to a = b.operator+(c) ; since c is passed by value, a copy of c is made (and destroyed later).
the return value is an anonymous temporary, which is constructed on return (and destroyed).
if you got a third construct/destroy, the (perhaps old?) compiler you are using is not doing any NRV optimization (in the implementation, temporary Rectangle tmp is constructed and destroyed on return).

to avoid the copy, and to make the code const-correct, write

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(Toggle Plain Text)

class Rectangle
{
  //...
  public:
  //...
     Rectangle operator+ ( const Rectangle& ) const ;
  //...
};

note: adding rectangles does not seem to be intuitive.

Last edited by vijayan121; May 10th, 2008 at 11:54 pm.

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Join Date: May 2008

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hmaich

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Newbie Poster

Re: Operator Overload Question

May 11th, 2008

Thanks for the explanation.

About the solution ( const Rectangle& ) const ; , if I do so, I get a compiling error ": error C2662: 'Rectangle::getX' : cannot convert 'this' pointer from 'const Rectangle' to 'Rectangle &' Conversion loses qualifiers
. Any clue??

About the "adding rectangles" stuff, it is just an exercice. I started with the rectangles and then adding functionalities :-)

Thanks again!

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Join Date: Oct 2007

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Duoas

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Re: Operator Overload Question

May 11th, 2008

You can't overload the + operator to be const unless you change the operation to not modify *this. In other words, if you change *this, get rid of the second const.

The + operator typically is expected to not modify the b object (as vijayan121 explained with a = b.operator+(c) ;).

Hence, the typical layout of such a function is:

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 C++ Syntax (Toggle Plain Text) 
Rectangle Rectangle::operator + (const Rectangle& rect) const
  {
  Rectangle result;
  result.x = this->x + rect.x;
  ...
  return result;
  }
Rectangle Rectangle::operator + (const Rectangle& rect) const
  {
  Rectangle result;
  result.x = this->x + rect.x;
  ...
  return result;
  }

Unless your object uses a lot of memory or other resources or has existential side-effects, I wouldn't really worry too much about temporaries. For something like your Rectangle, the difference is negligible: copying is fast and often optimized away.

Hope this helps.

[edit]
Oh yeah, often, to avoid duplicating code, the + operator is often written in terms of the += operator:

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 C++ Syntax (Toggle Plain Text) 
Rectangle Rectangle::operator + (const Rectangle& rect) const
  {
  return (Rectangle( *this ) += rect);
  }
Rectangle Rectangle::operator + (const Rectangle& rect) const
  {
  return (Rectangle( *this ) += rect);
  }