1

Classes and Objects in C++

Data Types

• Recall that C++ has predefined data types, such as int

• int x; // Creates a specific instance of an // integer named x

• C++ also predefines operations that can be used on integers, such as + and *

Classes

• Sometimes, a programmer will want to define a custom "thing" and the operations that can be performed on that "thing"

• A class is the definition • An object is a particular instance of a class• Classes contain

– data, called members– functions, called methods

2

Class Declaration

class Class_name{

public:member (data) definitions method (function) definitions

private:member (data) definitionsmethod (function) definitions

};// order of public and private can be reversed// data is seldom public// note semicolon after }

Public and Private

• Public members and methods can be accessed from outside the class and provide the interface

• Private members and methods cannot be accessed from outside the class

Data Hiding

• Recall that many programmers can each write a small piece of a large program

• Need to have some way to define how other programmers can use your stuff – Public methods are the only way for any other code to

access the class

• Need to have some way to keep other programmers from messing up your stuff– Private methods and members cannot be accessed

from outside the class

3

Reusability and Changeability

• Writing programs is expensive, so organizations try to reuse code and avoid changing it

• If classes are well written, they can be reused in several programs

• Also, the internals of a class can be rewritten - as long as the interface does not change, programs that use that class do not need to be changed

Example 1

Create a counter. Other parts of the program should be able to increment the counter and read the counter

class Counter {public:

// constructor to initialize the object - note no function typeCounter ( ) {

currentcount = 0;};// increment the countervoid count( ) {

currentcount++;};// get the current value of the counterint readcounter( ) {

return currentcount;};

private:int currentcount;

};

4

Constructors

• Constructors are methods that initialize an object– Must have same name as the class– Declaration is odd - no function type

Counter ( ) { ..... }– Not required, but should be used when data

needs to be initialized– Never explicitly called - automatically called

when an object of this class is declared

Destructors• Destructors are methods that clean up

when an object is destroyed– Must have the same name as the class, but

with a ~ in front– No function type in declaration

~ Counter ( ) { ..... }– Also not required, but should be provided if

necessary to release memory, for example– Never explicitly called - automatically called

when an object of this class is destroyed

Creating an Object

• To create an instance of class counter (called an object) declare it as you would any other variableClass_name object_name(s);– This automatically calls the constructor

method to initialize the object

Counter my_counter;

5

Using an Object

• Using a public method is similar to a function callobject_name.method_name (arguments)

my_counter.count( );int current = my_counter.readcounter( );

Using an Object

• Common error - using the class name instead of the object name

Counter.count ( ); // WRONG!my_counter.count ( ); // RIGHT!

• Common error - trying to use private members or methods outside the class

cout << currentcount ; // WRONG!cout << my_counter.readcounter ( ); // RIGHT!

Putting It All Together

#include <iostream>using namespace std;

6

class Counter {public:

// constructor to initialize the object Counter ( ) {

currentcount = 0;};// increment the countervoid count( ) {

currentcount++;};// get the current value of the counterint readcounter( ) {

return currentcount;};

private:int currentcount;

};

int main ( ){// declare two objectsCounter first_counter, second_counter;

// increment countersfirst_counter.count( );second_counter.count( );second_counter.count( );

//display countscout << "first counter is " << first_counter.readcounter( ) << endl;cout << "second counter is " << second_counter.readcounter( ) << endl;

return 0;}

Output

first counter is 1second counter is 2

7

Global Scope

• Anything declared outside of a function, such as the class in this example or a variable, can be used by any function in the program and is global

• Anything declared inside a function can only be used in that function

• Usual to declare classes to be global• Global variables are bad programming

practice and should be avoided

Function Prototypes in Class Declarations

• In the previous example, the functions (methods) were completely declared within the class declaration

• Often more readable to put only function prototypes in the class declaration and put the method implementations later

• use class_name::method_name when declaring the methods

• This is the usual convention

class Counter {public:

Counter ( ); void count( );int readcounter( );

private:int currentcount;

}

Counter::Counter ( ) {

currentcount = 0;}

void Counter::count ( ){

currentcount++;}

int Counter::readcounter ( ){

return currentcount ;}

8

Identifying Classes

• Often, it is not immediately obvious what the classes should be to solve a particular problem

• One hint is to consider some of the nouns in the problem statement to be the classes. The verbs in the problem statement will then be the methods.

Example 2

• Write a program that manages a checkbook. The user should be able to set the original account balance, deposit money in the account, remove money when a check is written, and query the current balance.

Example 2

class CheckBookpublic methods are init, deposit, check, and query

Pseudocode for main programdisplay menu and get user choicewhile user does not choose quit

Set starting balance: get the amount, call initDeposit: get amount, call depositWrite a check: get amount, call checkBalance: call query, display balancedisplay menu and get user choice

9

Example 2 Program

#include <iostream>#include <iomanip>using namespace std;

Class Declaration

class CheckBook{private:

float balance;public:

CheckBook ( ); //constructorvoid init (float); // set balancevoid deposit (float); //add depositvoid check (float); //subtract checkfloat query ( ); //get balance

};

Class Method Declarations

CheckBook::CheckBook ( ) {balance = 0;

}void CheckBook::init (float money) {

balance = money;}void CheckBook::deposit (float money) {

balance = balance + money;}void CheckBook:: check (float money){

balance = balance - money;}float CheckBook:: query ( ){

return balance;}

10

Menu Function

int menu ( ) {int choice;cout << "0: Quit" << endl;cout << "1: Set initial balance" << endl;cout << "2: Deposit" << endl;cout << "3: Deduct a check" << endl;cout << "4: Find current balance" << endl;cout << "Please enter your selection: ";cin >> choice;return choice;

}

int main ( ){

int sel = menu ( ); // get initial user inputfloat amount;CheckBook my_checks; // declare object// loop until user enters 0 to quitwhile (sel != 0) {

// set initial balanceif (sel == 1) {

cout << "Please enter initial balance: ";cin >> amount;my_checks.init(amount );

}// depositelse if (sel == 2) {

cout << "Please enter deposit amount: ";cin >> amount;my_checks.deposit (amount):

}

// checkselse if (sel == 3) {

cout << "Please enter amount of check: ";cin >> amount;my_checks.check (amount);

}// balance inquiryelse if (sel == 4) {

cout << fixed << setprecision(2);cout << "The balance is " <<

my_checks.query ( ) << endl;}// get next user choicesel = menu ( );

} // end whilereturn 0;

}

11

Example 3

• Write a class Can that calculates the surface area, volume, and weight of a cansurface area = 2p r(r+h)volume = p r2hweight (aluminum) = 0.1oz/in2 of surface areaweight (steel) = 0.5 oz/in2 of surface

Class Can

class Can {private:

float radius, height;char material; // S for steel, A for aluminum

public:Can (float, float, char);float volume ( );float surface_area( );float weight ( );

};

Methods

// constructor has argumentsCan::Can(float r, float h, char m){

radius = r;height = h;material = m;

} float Can::volume( ) {

return (3.14 * radius * radius * height);}

12

Methods

float Can::surface_area ( ){return ( 2 * 3.14* radius * (radius + height));

}

float Can::weight ( ) {if (material == 'S')

return ( 0.5 * surface_area( ));else

return (0.1 * surface_area( ) );}

Mainint main ( ) {

Can popcan(1, 5, 'A');cout << "Aluminum popcan is about 5 inches high and 1

inch in diameter." << endl;cout << "Volume is " << popcan.volume( ) << " cubic

inches" << endl;cout << "Surface area is " << popcan.surface_area ( )

<<" square inches" << endl;cout << "Weight is " << popcan.weight ( ) << " ounces"

<< endl;return 0;

}

Output

Aluminum popcan is about 5 inches high and 1 inch in diameter.

Volume is 15.7 cubic inchesSurface area is 37.68 square inchesWeight is 3.768 ounces

13

C++ has built-in classes

• Recall that to create an input file, use // class definition in fstream#include <fstream>// class is ifstream, object is input_fileifstream input_file;//close is a method of ifstreaminput_file.close ( );

String Class

#include <string> //class definition here// class is string, object is my_namestring my_name; // Can set the object directlymy_name = "Joan"; // methodscout << my_name.length( ); //4//substring of length 2 starting from character 0cout << my_name.substr(0, 2); //Jo