Operator Overloading

1CMSC 202, Version 2/02

Operator Overloading

Strong Suggestion: Go over the Array class example in Section 8.8 of your text. (You may ignore the Array copy constructor for now.)

2CMSC 202, Version 2/02

Review -- Function Signatures• A function signature is what the compiler

and linker use to identify a function.• In C, functions are identified only by their

name.• In C++, a function’s signature includes its

name, parameters, and (for member functions) const. It does NOT include the return type.

3CMSC 202, Version 2/02

A C++ swap( ) Function• We still need separate functions, but

they can all have the same name.

– void swap (int& a, int& b);– void swap (double& a, double& b);– void swap (struct bob& a, struct bob& b);

4CMSC 202, Version 2/02

Operator Overloading Overview

• Many C++ operator are already overloaded for primitive types. Examples:

+ - * / << >>• It is often convenient for our classes to imitate

the operations available on primitive types (e.g., + or - ).

• Then we can use the same concise notation for manipulating our objects.

5CMSC 202, Version 2/02

A Complex Number Classclass Complex {

public:Complex (int real = 0, int imagine = 0);int getReal ( ) const;int getImagine ( ) const;void setReal (int n);void setImagine (int d);

private:int real;int imagine;

};

6CMSC 202, Version 2/02

Using Complex Class

• It makes sense to want to perform mathematical operations with Complex objects.

Complex C1 (3, 5), C2 (5, 9), C3;C3 = C1 + C2; // additionC2 = C3 * C1; // subtractionC1 = -C2; // negation

7CMSC 202, Version 2/02

Operators Are Really Functions• For user-defined types, when you use an

operator, you are making a function call.• Consider the expression: C2 + C1

– This is translated into a function call.– The name of the function is “operator+”– The call is:

C2.operator+(C1);

8CMSC 202, Version 2/02

Declaring operator+ As a Member Function

class Complex {public: const Complex

operator+ (const Complex &rhs) const;…

};

• Note all of the const’s!

9CMSC 202, Version 2/02

operator+ Implementationconst Complex Complex :: operator+ (const Complex &rhs) const {

Complex sum;// accessor and mutators not requiredsum.imagine = imagine + rhs.imagine;

// but preferredsum.setReal( getReal( ) + rhs.getReal ( ) ); return sum;

}

10CMSC 202, Version 2/02

Using operator+ • We can now write

C3 = C2 + C1;

• We can also use cascading operators.

C4 = C3 + C2 + C1;

• And we can write

C3 = C2 + 7;

• But C3 = 7 + C2 is a compiler error. (Why?)

11CMSC 202, Version 2/02

operator+ As aNon-member, Non-friend

const Complex operator+ (const Complex &lhs, // extra parameter const Complex &rhs) // not const{ // must use accessors and mutators

Complex sum;sum.setImagine (lhs.getImagine( )

+ rhs.getImagine( ) );sum.setReal (lhs.getReal ( ) + rhs.getReal( ) );return sum;

} // is now commutative

12CMSC 202, Version 2/02

Declaring operator+As a Non-member Friend

• Declare operator+ as a friend in the class definition.

class Complex {public:

friend const Complex operator+ (const Complex& a,

const Complex& b);…

};

13CMSC 202, Version 2/02

Operator+ As aNon-member Friend (con’t)

const Complex operator+ (const Complex& lhs, const Complex& rhs) {

Complex sum;// accessors and mutators not requiredsum.imagine = lhs.imagine + rhs.imagine;

// but preferredsum.setReal( lhs.getReal( ) + rhs.getReal( )) ;return sum;

} // violates encapsulation! Non-friend better.

14CMSC 202, Version 2/02

Printing Objects• Each object should be responsible for

printing itself.• This guarantees objects are always printed

the same way.• It allows us to write intuitive output code:

Complex C5 (5, 3);

cout << C5 << endl;

15CMSC 202, Version 2/02

Operator<<• The insertion operator << is a function and

can (and should) be overloaded. We can do operator>>, too.

• << is a binary operator.• The left-hand operand is of type ostream&• Therefore, operator<< cannot be a

member function. It must be a non-member.

16CMSC 202, Version 2/02

operator<<ostream& operator<< (ostream& out, const Complex& c) {

out << c.getReal( );int imagine = c.getImagine( );out << (imagine < 0 ? “ - ” : “ + ” ) out << imagine << “i”;return out;

}• Could be, and often, is a friend• Note: no endl

17CMSC 202, Version 2/02

Operator<< Returns Type ‘ostream &’

• Why? So we can write statements such as

cout << C5 << “is a complex number”OR

cout << C3 << endl << C2 << endl;

• << associates from left to right.

18CMSC 202, Version 2/02

Overloading Unary Operators

Complex C1(4, 5), C2;C2 = -C1;

is an example of a unary operator (minus).

• We can and should overload this operator as a member function.

19CMSC 202, Version 2/02

Unary operator-

const Complex Complex :: operator- ( ) const{

Complex x;x.real = -real;x.imagine = imagine;return x;

}

20CMSC 202, Version 2/02

Overloading =• Remember that assignment performs a

memberwise (shallow) copy by default.• This is not sufficient when a data member

is dynamically allocated.• = must be overloaded to do a deep copy.

21CMSC 202, Version 2/02

Restrictions

• Not all operators can be overloaded.• You can’t make up your own operators.• You can’t overload operators for

primitive types (like int).• You can’t change the precedence of an

operator.• You can’t change the associativity of an

operator.

22CMSC 202, Version 2/02

Good Programming Practices• Overload operators so that they mimic the

behavior of primitive data types.• Overloaded binary arithmetic operators should

– return const objects by value– be written as non-member functions when appropriate

to allow commutativity– be written as non-friend functions (if data member

accessors are available)• Overload unary operators as member functions.• Always overload <<• Always overload = for objects with dynamic

data members.