2.6 Introduction to classes  (Page 4/9)

{

function body

}

In order for your class to identify which functions in an implementation section belong to it, you precede the function name in the function definition header with the class name and the scope resolution operator (::).

Access functions

Access to a class’ private data should be carefully controlled by the use of member functions, called access functions . For example, to allow clients to read the value of private data, the class can provide a get function.

To enable clients to modify private data, the class can provide a set function. Such modification would seem to violate the notion of private data. But a set member function can provide data validation capabilities (such as range checking) to ensure that the value is set properly. A set function can also translate between the form of data used in the interface and the form used in the implementation.

A get function need not expose the data in “raw” format; rather, the get function can edit data and limit the view of the data the client will see.

Example

// time1.h

#if !defined(TIME1_H)

#define TIME1_H

class Time {

public:

Time(); // constructor

void setTime( int, int, int ); // set hour, minute, second

void printMilitary(); // print military time format

void printStandard(); // print standard time format

private:

int hour;

int minute;

int second;

};

#endif;

// time1.cpp

#include “time1.h”

#include<iostream.h>

// Time constructor initializes each data member to zero.

// Ensures all Time objects start in a consistent state.

Time::Time() {

hour = minute = second = 0;

}

void Time::setTime( int h, int m, int s )

{

hour = ( h>= 0&&h<24 ) ? h : 0;

minute = ( m>= 0&&m<60 ) ? m : 0;

second = ( s>= 0&&s<60 ) ? s : 0;

}

void Time::printMilitary()

{

cout<<( hour<10 ? "0" : "" )<<hour<<":"

<<( minute<10 ? "0" : "" )<<minute;

}

void Time::printStandard()

{

cout<<( ( hour == 0 || hour == 12 ) ? 12 : hour % 12 )

<<":"<<( minute<10 ? "0" : "" )<<minute

<<":"<<( second<10 ? "0" : "" )<<second

<<( hour<12 ? " AM" : " PM" );

}

// Driver to test simple class Time

int main()

{

Time t; // instantiate object t of class Time

cout<<"The initial military time is ";

t.printMilitary();

cout<<"\nThe initial standard time is ";

t.printStandard();

t.setTime( 13, 27, 6 );

cout<<"\n\nMilitary time after setTime is ";

t.printMilitary();

cout<<"\nStandard time after setTime is ";

t.printStandard();

t.setTime( 99, 99, 99 ); // attempt invalid settings

cout<<"\n\nAfter attempting invalid settings:"

<<"\nMilitary time: ";

t.printMilitary();

cout<<"\nStandard time: ";

t.printStandard();

cout<<endl;

return 0;

}

The output of the above program:

The initial military time is 00:00

The initial standard time is 12:00:00 AM

Military time after setTime is 13:37

Standard time after setTime is 1:27:06 PM

After attempting invalid settings:

Military time: 00:00

Standard time: 12:00:00 AM

In the above example, for the class Time we can see that member functions printStandard and printMilitary are two get function and member function setTime is a set function.

Constructor functions

A constructor function is a special function with the same name as its class. This function is called automatically when an object from a class is instantiated.