Operator Overloading in C++

Operator Overloading in C++
An Introduction

Computer Science Department

bernstdh@jmu.edu

Motivation

The Situation:
- You are writing a digital cookbook that uses "English" units
- You write a Weight class
An Inconvenient Syntax:
- Weight *flourForCookies, *flourForCake, *totalFlour; . . . totalFlour = new Weight(0,0); totalFlour.increase(flourForCake); totalFlour.increase(flourForCookies);

Motivation (cont.)

A Nicer Syntax:
- Weight *flourForCookies, *flourForCake, *totalFlour; . . . totalFlour = new Weight(0,0); totalFlour = flourForCake + flourForCookies;
Can We Do It?
- Not in Java
- In C++ we can overload the + and = operators

Ignoring Some Details to Start

What We Will Ignore Initially:
- The assignment operator (though it is necessary)
- Return by value/reference
What We Will Focus On Initially:
- Two different ways of implementing the arithmetic operators

Operator "Functions"

Some Details:
- Declared outside of the class specification because they are not part of the class
- Identified using the operator keyword
An Example:
- Weight operator +(Weight a, Weight b);

Operator "Functions" (cont.)

Wherever Possible:
- The operands should be passed by reference
- The operands should be declared const (the += operator is an exception)
An Example Revisited:
- Weight operator +(const Weight &a, const Weight &b);

Operator "Functions" (cont.)

An Example - The Specification cppexamples/operatoroverloading/functions/Weight.h

Operator "Functions" (cont.)

An Example - The Implementation cppexamples/operatoroverloading/functions/Weight.cpp

Operator "Functions" (cont.)

An Example - Use cppexamples/operatoroverloading/functions/Driver.cpp

Operator "Methods"

Some Details:
- Declared in the specification of the class that they operate on
- Identified using the operator keyword
An Example:
- Weight operator +(const Weight &a) const;
A Difficulty:
- While they are more in keeping with the OOP paradigm they can be somewhat confusing because binary operators look like unary operators, etc...

Operator "Methods" (cont.)

An Example - The Specification cppexamples/operatoroverloading/methods/Weight.h

Operator "Methods" (cont.)

An Example - The Implementation cppexamples/operatoroverloading/methods/Weight.cpp

Operator "Methods" (cont.)

An Example - Use cppexamples/operatoroverloading/methods/Driver.cpp

Return by Value/Reference

Two Observations:
- The arithmetic operators return an object by value
- The object that is returned is local
Consequences:
- A copy of the local variable is made and returned
- The local variable goes out of scope after the return
Implications:
- You must write an explicit copy constructor unless the class is very simple (e.g., has only values as attributes)
- Your destructor must be correct or memory will leak each time a copy is made and deleted

The Assignment Operator

Which Approach to Use:
- The operator "method" approach (see both examples above)
What Needs to be Returned:
- The obvious answer is that nothing needs to be returned (i.e., it should be void) since it is a "setter"
An Interesting Situation:
- Suppose you want to write w = u = v;?

The Assignment Operator (cont.)

Fixing this Problem:
- Return the calling object
The Specification:
- Weight operator=(const Weight &rhs);
The Implementation:
- Weight Weight::operator =(const Weight &rhs) { pounds = rhs.pounds; ounces = rhs.ounces; // We must return an object so must dereference this return *this; }

The Assignment Operator (cont.)

An Observation:
- A copy is being made for every assignment
A Fix:
- Return by reference
The Implementation:
- Weight& Weight::operator =(const Weight &rhs) { pounds = rhs.pounds; ounces = rhs.ounces; return *this; }
Convince Yourself:
- The return by reference will not cause problems in this situation (though it will in many other situations)

There's Always More to Learn