Data Structures - CSCI 102

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Data Structures - CSCI 102

CS102C++ OperatorOverloading

Prof Tejada

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Operator OverloadingC++ has tons of built-in operators (e.g. +,-,<<, etc.)

They work on built-in types (e.g. int, string)They don’t work on your own classes!

string a = "hello ";string b = " world!";

string c = a + b; //additioncout << c << endl; //print to console

String is a class, so why does this work?

Since C++ doesn’t know how to do this stuff by default, wehave to tell it how

Rules

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Operator Overloading

You can only overload an operator that has at least oneuser defined type (i.e. you can’t overload "int + int")You can only overload existing operators, you can’tcreate your own new ones

All operators keep their normal associativity &precedenceYou can’t overload any of these

::.->.*?:sizeof

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The Point Classclass Point{

private:int x;int y;

public:Point() { x=0; y=0; }

Point(int newx, int newy) { x=newx; y=newy; }

int getX() const { return x; }int getY() const { return y; }void setX(int newx) { x=newx; }void setY(int newy) { y=newy; }

};

You can implement all the operators in C++ as a functioncall somewhere behind the scenes

Point p1(10,10), p2(50,50);Point p3 = p1 + p2;

When you see something like this:

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The + Operator

You should really picture it like this:

Should this function be a member of the Point class?It can be, but it doesn’t really need to be (no changesare made to p1 or p2)Could we get access to the private data of "a" and"b" without being part of class "Point"?

Point operator+(const Point &a, const Point &b)

The function isn’t a class member functionWhat does it mean to tag a function as a friend of a class?

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The friend Keyword

The function can still access to the private data of theclass

Most of the time it is, but it’s useful for operators thatdon’t modify class data

That sounds like a horrible idea!

friend functions should only read "private" classdata, they shouldn’t modify itDon’t break encapsulation

Avoid using friend in general is probably not a badpolicy

Remember, C++ is built for speed

class Point{

...friend Point operator+(const Point &a,

const Point &b);...

};

//this is NOT a member function (no Point::needed)Point operator+(const Point &a, const Point &b){

...}

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The friend Keyword

p1 += p2; //we’re changing p1 hereInstead of defining it as a friend function, we just define itas a normal class function

class Point{

...void operator+=(const Point &b);...

};

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The += OperatorIf the operator actually changes one of its operands, it’sbetter off as a member function

Point p1(10,10), p2(50,50);

Point p1(5,10);cout << p1 << endl;

Isn’t it annoying that this code doesn’t normally work?

It can work! Overload the insertion operator (<<)!ostream& operator<<(ostream& out,const Point &b)

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The Insertion Operator (<<)

This function looks a lot different than the othersWhat is ostream &out?Why is it not const?Why does it return ostream&?NOTE: It cannot be a class member function

NOTE: ostream is std::ostream from <iostream>NOTE: cout << p1 << endl means

(cout << p1) << endl

Point p1;cin >> p1;

How can we read in something then?

Overload the extraction operator (>>)istream& operator>>(istream& in, Point &b)

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The Extraction Operator (>>)

What is istream &in?This function looks a lot different than the others

Why does it return istream&?Why is Point &b not const?NOTE: It cannot be a class member function

NOTE: cin >> p1 >> p2 means (cin >> p1) >> p2

Comparison operators can be reused to define each othersince many of them act as logical opposites

Point p1(10,10), p2(10,10);if(p1 == p2) cout << "They’re equal!" << endl;

What about comparing two instances of a class?

It still looks like what we’re used to:bool operator==(const Point &a, const Point &b)

Comparison always return bool

== vs. !=>= vs. <<= vs. >

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Comparison Operators

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Unary Operators

Some other things to consider

Point p1(10,10);Point p2 = -p1;

Some operators only take in a single argument

This looks a bit different too:Point operator-(const Point &a)

Why does it return a Point?Why does it only have one argument?How does C++ know we’re not trying to redefinesubtraction instead?Other unary operators include ! and +

Can also be:Point operator-()

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What Other Stuff Can You Overload?

Point p1(10,10);cout << p1[0];

Subscript ([])

Prefix/Postfix Increment/Decrement (++, --)

Equality (==)

And many, many more


point.h#ifndef POINT_H_#define POINT_H_

class Point{

private:int x;int y;

public:Point() { x = y = 0; }Point(int newx,int newy) { x = newx; y = newy; }

x; }y; }newx; }newy; }

int getX() const { returnint getY() const { returnvoid setX(int newx) { x =void setY(int newy) { y =

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point.h (Cont...)void operator+=(const Point& a);void operator+=(const int value);

int& operator[](const int index);

void operator++(); //prefix (e.g. ++point)void operator++(int dummy); //postfix (e.g. point++)

Point operator-();

bool operator==(const Point& another_point);bool operator!=(const Point& another_point);

};

Point operator+(const Point& a,const Point& b);Point operator+(const Point& a,const int b);Point operator+(const int a,const Point& b);

std::ostream& operator<<(std::ostream& out, const Point& b);std::istream& operator>>(std::istream& in, Point& b);

#endif /* POINT_H_ */

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point.cpp#include <iostream>#include <stdexcept>

using namespace std;

#include "point.h"

Point operator+(const Point& a,const Point& b)

{Point p;p.setX(a.getX()+b.getX());p.setY(a.getY()+b.getY());return p;

Point operator+(const Point& a,const int b)

{Point p;p.setX(a.getX()+b);p.setY(a.getY()+b);return p;

}

Point operator+(const int a,const Point& b)

{return b + a;

}

ostream& operator<<(ostream& out,const Point& b)

{out << "(" << b.getX() << ","

<< b.getY() << ")";return out;

}}

istream& operator>>(istream& in,Point& b)

int x, y;in >> x >> y;b.setX(x);b.setY(y);return in;

{

}

bool Point::operator==(const Point& b)

{return x == b.getX() &&

y == b.getY();

bool Point::operator!=(const Point& b)

{return !((*this) == b);

}

void Point::operator+=(const Point& b)

{this->x += b.getX();this->y += b.getY();

}

Point Point::operator-(){Point p;p.setX(-(this->x));p.setY(-(this->y));return p;

}

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point.cpp (Cont...)void Point::operator++(){

this->x++;this->y++;

}}

void Point::operator++(int dummy){

++(*this);}

int& Point::operator[](const int index)

{if(index == 0) {

return this->x;} else if(index == 1) {

return this->y;}throw out_of_range(

"Point index was out of bounds");}

Data Structures - CSCI 102

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