CMSC 202, Version 2/02 1 Operator Overloading Strong Suggestion: Go over the Array class example in Section 8.8 of your text. (You may ignore the Array.

CMSC 202, Version 2/02 1

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.)


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.


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);


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.


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;

};


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


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);


Declaring operator+ As a Member Function

class Complex {

public:

const Complex

operator+ (const Complex &rhs) const;

…

};

• Note all of the const’s!


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

Complex sum;// accessor and mutators not required

sum.imagine = imagine + rhs.imagine;

// but preferred

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

}


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?)


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


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);…

};


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

const Complex operator+ (const Complex& lhs, const Complex& rhs) {Complex sum;// accessors and mutators not required

sum.imagine = lhs.imagine + rhs.imagine;

// but preferred

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

} // violates encapsulation! Non-friend better.


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;


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.


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


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.


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.


Unary operator-

const Complex Complex :: operator- ( ) const

{

Complex x;

x.real = -real;

x.imagine = imagine;

return x;

}


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.


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.


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.

CMSC 202, Version 2/02 1 Operator Overloading Strong Suggestion: Go over the Array class example in Section 8.8 of your text. (You may ignore the Array.

Documents

complex complex

complex operator

complex rhs

complex c1

complex class

complex lhs

complex b

complex objects