Introduction to Classes in C++

Object Oriented Paradigm

Object

Class

Attribute

Method

Message

Encapsulation

Inheritance

Polymorphism

Abstract Data Type: from struct to class

Classes in C++

class Myclass // class head

{ // indicates beginning of class body

public: // public data and functions

type myfunc( );

protected: // protected data and functions

private: // private data and functions

type myvar;

}; // indicates end of class body

Levels of Data Access

Information hiding is a formal mechanism for restricting user access to the

internal representation of a class type

Members are declared as

public

protected

private

Member Functions

Any function that is declared as part of a class is called a member function

Member functions are invoked by sending messages to an instance of the cl

ass

The . (dot) operator is the “message send” operator:

instance.function(parameters)

Member functions within the same class can call each other without using t

he . (dot) operator

Rational Number Class

class Rational // implement rational numbers with addition

{ // the only operation defined

public:

void initialize(int num, int den);

void print ( );

Rational add(Rational r);

private:

int numerator;

int denominator;

int gcd(int i, int j);

void commonDenominator(Rational& r);

};

Review of .h and .C

Class declarations typically are placed in a dot h (.h file).

Member function definitions typically are placed in a .C file.

The .C file requires an “include” statement:

#include “Class_name.h”

Scope Resolution Operator

:: is used to define the code for a member function outside of its class declaration.

It can also be used to reference global variables even when the same name has been

used for a local.

For example,

int a = 15;

int f( )

{

int a = 100;

cout << ::a;

}

will print the value 15.

Rational Number Class

#include “Rational.h”

void Rational::initialize(int numerator, int denominator)

{

// initialize the numerator and denominator members

this->numerator = numerator;

this->denominator = denominator;

}

Rational Number Class (Continued)

#include “Rational.h”

#include <math.h>

int Rational::gcd(int i, int j)

{

// Find greatest common divisor of 2 integers

int divisor;

i = abs(i);

j = abs(j);

for (divisor = (i<j) ? i : j;

divisor > 1 && (i % divisor != 0 || j % divisor != 0);

divisor--);

return divisor;

}

Rational Number Class (Continued)

void Rational::commonDenominator(Rational& r)

{

// given two rational numbers, convert them

// to have a common denominator

int multiplier1, multiplier2;

int divisor = gcd(denominator, r.denominator);

multiplier1 = denominator / divisor;

multiplier2 = r.denominator/divisor;

numerator *= multiplier2;

denominator *= multiplier2;

r.numerator *= multiplier1;

r.denominator *= multiplier1;

}

Rational Number Class (Continued)

Rational Rational::add(Rational r)

{

Rational temp = *this;

Rational i;

temp.commonDenominator(r);

i.initialize(temp.numerator + r.numerator, temp.denominator);

return i;

}

void Rational::print( )

{

cout << numerator << “/” << denomiator;

}

Using the Rational Number Class

Rational a, b, c;

a.initialize(1, 2);

b.initialize(2, 3);

c.initialize(1, 1);

a.print( );

b.print( );

c = a.add(b); // Can’t do:

// print();

a.print( ); // a.commonDenominator(b);

b.print( ); // cout << a.denominator;

c.print( );

Notes About Class Definitions

Classes and structs are equivalent, except that all members are private by d

efault in a class, and public by default in a struct.

The keyword, this, references a pointer to the receiver of a message.

Data items in the receiver can be referenced directly by using the instance

variable name.

Member functions can access the private data of an instance of the class.

In other words, only member functions can “reference and dereference” pri

vate class instance variables.

Some Common Errors

Leaving off the semicolon after the class declaration gives the following er

rors (among others):

bad base type: class int

class Rational defined as return type for gcd()

(did you forget a ‘:’ after ‘}’ ?)

class Rational undefined

Leaving off the Rational::, in front of the definition of gcd gives the follow

ing error at link time:

Undefined entry, name:

(Error 28) “gcd__8RationalFiT1”

Constructors and Destructors

Rational Number Class

class Rational // implement rational numbers with addition

{ // the only operation defined

public:

void initialize(int num, int den);

void print ( );

Rational add(Rational r);

private:

int numerator;

int denominator;

int gcd(int i, int j);

void commonDenominator(Rational& r);

};

Constructor Syntax

Class Myclass

{

Public:

Myclass( ); // void constructor

Myclass(int n); // one-parameter constructor

…

};

#include “Myclass.h”

Void main( )

{

Myclass i; // invokes void constructor

Myclass j(3); // invokes the one-parameter constructor

}

Constructor

A member function with the same name as the class is called a constructor.

Constructors are called whenever a member of the class is created

(declarations, parameter passing, function returns).

Constructor declarations can appear only inside of class declarations.

Any number of constructors are allowed.

Constructors cannot have return types.

Rational Number Class with Constructors-1

class Rational // implement rational numbers with addition

{ // the only operation defined

public:

Rational( ); // void constructor

Rational(int n); // one-parameter constructor

Rational(int n, int d); // two-parameter constructor

void print ( );

Rational add(Rational r);

private:

int numerator;

int denominator;

int gcd(int i, int j);

void commonDenominator(Rational& r);

};

Rational Number Class with Constructors-2

class Rational // implement rational numbers with addition

{ // the only operation defined

public:

Rational(){numerator = 0;

denominator = 1;

}

Rational(int n){

numerator = n;

denominator = 1;

}

Rational(int n, int d){

numrator = n;

denominator = d;

}

…

Rational Number Class with Constructors-3

The original add function

Rational Rational::add(Rational r)

{

Rational temp = *this;

Rational i;

temp.commonDenominator(r);

i.initialize(temp.numerator + r.numerator, temp.denominator);

return i;

}

The new add function using constructors

Rational Rational::add(Rational r)

{

Rational temp = *this;

temp.commonDenominator(r);

return Rational(temp.numerator + r.numerator, temp.denominator);

}

Default Constructor Arguments

With default arguments and a single constructor, it is possible to declare:

Rational a, b(3), c(1,4)

calss Rational

{

Rational (int num=0, int den=1)

{

if(den < 0){

num = -num;

den = -den;

}

numerator = num;

denominator = den;

}

…

};

Inline Code

Inline causes the actual code for a function to be substituted for a call to

the function.

Member functions can be made inline by putting their definition (that is,

their code) within the class declaration or by preceding their definition,

outside of the class, by the keyword inline.

Any other function is made inline by preceding its definition with the

keyword inline.

Inline is a suggestion to the translator. For example, functions containing

loops will not be made inline.

Inline Code (Continued)

calss Rational // With in-line constructor

{

Rational (int num=0, int den=1)

{

if(den < 0){

num = -num;

den = -den;

}

numerator = num;

denominator = den;

}

…

};

Inline Code (Continued)

An alternative

inline Rational::Rational (int num=0, int den=1)

{

if(den < 0){

num = -num;

den = -den;

}

numerator = num;

denominator = den;

}

Vectors of Class Instances

A class must have a constructor with no arguments (a void constructor), in

order to declare a vector of instances of it, without explicitly initializing the

entire vector.

A constructor with all of its arguments having default values is equivalent t

o a void constructor.

Arrays of class instances can be initialized using any combination of constr

uctors. There must be enough initializers for the entire vector.

Vectors of Class Instances (Continued)

main()

{

Rational fractions1[25];

Rational fractions2[3]={Rational(1,2), Rational( ), 4};

fractions1[0] = Rational(1,4);

fractions2[1] = Rational(1,2);

fractions2[1].print( );

}

Destructor

A destructor is a function named by ~className.

A destructor is invoked whenever an element of the class is destroyed (leav

ing scope, dynamic freeing, deallocation of temporary)

Destructor declarations can appear only inside of class declarations.

There can be, at most, one destructor.

Destructors cannot have parameters or return types.

Destructor (Continued)

class Rational // With Destructor

{

public:

…

~Rational( )

{

cout << “Instance of Rational destroyed” << endl;

}

};

Some Common Errors

If you have provided one or more constructors, but not a void constructor, t

he following errors are possible:

The declaration Rational r[25]; causes the following error message:

array of class Rational that does not have a

constructor taking no arguments

The declaration Rational r; gives the following error message:

argument 1 of type int expected for Rational::Rational( )

Static Members

Members declared to be static are shared among all instances of a class

If a static member is public, it can be referred to outside of the class:

className::staticMember;

Static members cannot be initialized when declared. Therefore, if they are i

nstances of a class, the class must have a void constructor, if it has any con

structors.

A static member, whether private or public, must be initialized, exactly onc

e, at file scope:

type className::staticMember = initialValue;

Static Members (Continued)

class Rational

{

public:

…

int howMany( );

private:

…

static int numRationals;

};

Static Members (Continued)

In each constructor, add the following statement:

numRationals++;

In the destructor, add the follwing statement:

numRational--;

Initialize the static somewhere at file scope:

int Rational::numRationals = 0;

Implement the howMany( ) function:

int Rational::howMany( )

{

return numRationals;

}

Static Member Functions

Member functions accessing only static members can be declared static.

Static member functions can be called though a member or by

ClassName::ftn( ).

For example,

Rational::howMany( )

Static member functions have no this pointer. Any explicit reference to thi

s, or to a nonstatic member, causes a compile-time error.

Static Member Functions

class Rational

{

public:

…

static int howMany( ); // Return the number of Rationals at any ti

me

private:

…

static int numRationals;

};

Overloading Functions

Overloading Functions

The following are allowed in the same program:

int f(int a);

float f(float a);

int f(int a, int b);

int f(int a, float b);

int f(char a);

int f(int a, int b, int c, …)

Overloading Functions (Continued)

Given the declarations on the previous slide, the function declaration

float f(int a)

generates the error message

two different return value types for f( ): int and float

Given the declarations on the previous slide, the function declaration

int f(int a, int b, int c=1);

generates the error message

two exact matches for f( ): int (int, int, int) and int (int, int)

Overloading Functions (Continued)

Overloaded functions must be distinguishable by the number and type of

arguments.

Return type cannot be used to distinguish overloaded functions.

The number, type, and order of the arguments establishes the function

signature

Default Arguments

When default arguments are used, the resulting function is equivalent to a s

et of functions whose members are that function with each possible numbe

r of arguments.

For example, the function declaration

void f (int a = 1, int b = 2, int c = 3);

is equivalent to the four declarations

void f( );

void f(int);

void f(int, int);

void f(int, int, int);

Argument Matching

Each argument in a call to an overloaded function is compared to the corre

sponding arguments in the declaration.

The compiler will choose the function for which each argument resolution

is the same or better than for the other functions.

If there is either no match or an ambiguous match, a compilation error is ge

nerated.

The argument matching algorithm distinguishes between constant and nonc

onstant pointer and reference arguments.

Argument Matching (Continued)

The comparison takes place using following steps:

1. Look for an exact match

2. Look for a match using promotions

3. Look for a match based on standard conversions

(for example, SomeClass* to void*)

4. Look for a match using user-defined conversions. Only one level of user-d

efined conversion will be used

Argument Matching

Given the declarations

void f (int, float);

void f(float, int);

the call

f(1.5, 1.5);

Gives the error message

two standard conversions possible for f():

void (int, float) and void (float, int)

Constant Member Functions

A member function can be declared as constant by adding the keyword con

st between the argument list and the body of the function.

Only constant member functions can be sent to class instances declared as

constant.

It is illegal to declare a member function as constant if it modifies an instan

ce variable.

There can be constant and nonconstant version of a member function. The

function is selected, based on whether or not the receiver is a constant.

Constant Member Functions

class Rational

{

…

void print( ) const;

};

void Rational::print( ) const

{

…

}

main()

{

const Rational r(3,5);

r.print();

}

Overloading Operators

Rational Number Class

Rational Rational::add(Rational r)

{

Rational temp = *this;

temp.commonDenominator(r);

return Rational(temp.numerator + r.numerator, temp.denominator);

}

Overloading an Operator

Operator overloading is redefining and operator that has a built-in function.

The function name consists of the keyword operator, followed by the

operator symbol

For example, given a String class, we can write the following function to

concatenate two Strings:

String operator+ (String x);

The we can concatenate the Strings:

String a, b, c;

a = b + c;

Overloading Operators

class Rational

{

public:

…

Rational operator+ (Rational);Rational operator+ (Rational);

protected:

private:

int numerator;

int denominator;

int gcd(int i, int j);

void commonDenominator(Rational& r);

};

Overloading Operators (Continued)

Rational Rational::operator + (Rational r)

{

Rational temp = *this;

temp.commonDenominator(r);

return Rational(temp.numerator+r.numerator, temp.denominator);

}

Using + operator:

Rational a(1,4), b(1,3), c(1,1);

c = a + b; // c = a.operator+(b);

Overloading Operators (Continued)

Functions can be named with standard C++ operators.

The types of the operands are used to distinguish between the various

declarations

The operator does not imply any particular semantics.

Precedence follows standard C++ precedence rules.

It is possible to overload the prefix and postfix increment (++) and

decrement (--) operators.

Overloading Operators (Continued)

op1 binaryOp op2 is interpreted as op1 receiving the message binaryOp wi

th the argument op2.

The expression can also be written as follows:

op1. oprator binaryOp(op2)

For example

r1 + r2 or r1.operator+(r2)

Friends

Classes or functions can be declared to be a friend of a class.

Friends have access to the private members of a class.

Friend functions are not received by class instances and thus do not have

access to this pointer.

Friend functions are necessary when the left operand is not an instance of

the class being defined.

Friends versus Members

class Rational

{

public:

friend Rational operator+(Rational, Rational);friend Rational operator+(Rational, Rational);

}

Versus the member function

class Rational

{

public:

Rational operator+(Rational);Rational operator+(Rational);

};

Friends versus Members (Continued)

Binary operators that are friend functions have two arguments; those that

are member functions have one. Unary operators that are friend functions

have one argument; those that are member functions have none.

Constructors, destructors, and virtual functions must be members.

The overloaded operators = (assignment), ( ) (function call), [ ]

(subscripting), and -> (class member access) must be member functions,

not friends.

Operations modifying state should be members.

Choose members when possible.

A Special Friend

class Rational

{

friend ostream& operator << (ostream&, Rational&);

…

};

ostream& operator << (ostream& s, Rational& r)

{

return (s << r.numerator << “/” << r.denominator << “\n”);

}

Constructors for Type Conversion

class Rational

{

public:

Rational operator+(Rational);

Rational operator+(int);

friend Rational operator+(int, Rational);

};

Constructors for Type Conversion (Continued)

Using implicit type conversion is easier:

class Rational

{

public:

Rational (int, int=1); // or any constructor that takes one int

friend Rational operator+(Rational, Rational);

…

};

Constructors for Type Conversion (Continued)

Both of these allow:

Rational a(1, 2), b(1, 1);

b = a + a;

b = a + 1;

b = 1 + a;

Using implicit conversion allows the addtion function to be written once in

stead of three times.

Constructors for Type Conversion (Continued)

A constructor with one argument is a user-defined conversion operator that

can be implicitly applied

Only one level of conversion will be implicitly applied.

Conversion can be explicitly requested by casting:

typeName(expression)

This notation can also be used for basic types as an alternative to the traditi

onal C cast notation.

Member functions are invoked only for real objects; friend functions can b

e called for an object created by implicit conversion.

Conversion Operators

class Rational

{

…

public:

operatior int( ) //convert from Rational to int

{

return int (float (numerator) / denominator + 0.5);

}

…

}

Constructors for Type Conversion

There are several limitations in using constructors for type conversion:

They may not allow implicit conversion from a user-defined type to a b

asic type

They must modify the declaration for an old type when specifying a co

nversion from a new type to it.

When both forms of type conversion are in a program, the compiler flags a

mbiguities. For example, if both the one-parameter constructor for Rational

and the int( ) operator exist, given

Rational a(1, 2);

a + 1 generates a compile-time error, because a can be converted to an int,

or 1 can be converted to a Rational.