2.6 Introduction to classes  (Page 9/9)

; // doing nothing

else if (newfloor>currentFloor) // move elevator up

{

cout<<“\nStarting at floor “<<currentFloor<<endl;

while (newfloor>currentFloor)

{

currentFloor++;

cout<<“ Going up – now at floor “<<currentFloor<<endl;

}

cout<<“ Stopping at floor “<<currentFloor<<endl;

}

else // move elevator down

{

cout<<“\nStarting at floor “<<currentFloor<<endl;

while (newfloor<currentFloor)

{

currentFloor--;

cout<<“ Going down – now at floor “<<currentFloor<<endl;

}

cout<<“ Stopping at floor “<<currentFloor<<endl;

}

return;

}

The constructor function is straightforward. When an Elevator object is declared, it is initialized to the floor specified; if no floor is explicitly given, the default value of 1 is used. For example, the declaration

Elevator a(7);

Initializes the variable a.currentFloor to 7, whereas the declaration

Elevator a;

uses the default argument value and initializes the variable a.currentFloor to 1.

The request() function defined in the implementation section is more complicated and provides the class’s primary service. Essentially, this function consists of an if-else statement having three parts: If an incorrect service is requested, no action is taken; if a floor above the current position is selected, the elevator is moved up; and if a floor below the current position is selected, the elevator is moved down. For movement up or down, the function uses a while loop to increment or decrement the position one floor at a time and report the elevator’s movement using a cout statement. The following program includes this class in a working program.

#include<iostream.h>

const int MAXFLOOR = 15;

//class declaration

class Elevator

{

private:

int currentFloor;

public:

Elevator(int = 1); //constructor

void request(int);

};

// implementation section

Elevator::Elevator(int cfloor)

{

currentFloor = cfloor;

}

void Elevator::request(int newfloor)

{

if (newfloor<1 || newfloor>MAXFLOOR || newfloor == currentFloor)

; // doing nothing

else if (newfloor>currentFloor) // move elevator up

{

cout<<“\nStarting at floor “<<currentFloor<<endl;

while (newfloor>currentFloor)

{

currentFloor++;

cout<<“ Going up – now at floor “<<currentFloor<<endl;

}

cout<<“ Stopping at floor “<<currentFloor<<endl;

}

else // move elevator down

{

cout<<“\nStarting at floor “<<currentFloor<<endl;

while (newfloor<currentFloor)

{

currentFloor--;

cout<<“ Going down – now at floor “<<currentFloor<<endl;

}

cout<<“ Stopping at floor “<<currentFloor<<endl;

}

return;

}

int main()

{

Elevator a; // declare 1 object of type Elevator

a.request(6);

a.request(3);

return 0;

}

Test and correct the program

Within the main() function, three class function calls are included. The first statement creates an object name a of type Elevator. Since no explicit floor has been given, the elevator begins at floor 1, which is the default constructor argument.

A request is then made to move the elevator to floor 6, which is followed by a request to move to floor 3. The output produced by the program is:

Note that control is provided by the main() function. This control is sequential, with two calls made to the same object operation, using different argument values. This control is perfectly correct for testing purposes. However, by incorporating calls to request() within a while loop and using the random number function rand() to generate random floor requests, a continuous simulation of the elevator’s operation is possible.

Design techniques of object-oriented programs

The basic requirements of object-oriented programming are evident in even as simple a program as the above program. Before the main() function can be written, a useful class must be constructed. For programs that used objects, the design process is loaded with the requirement that careful consideration of the class – its declaration and implementation – be given. Code contained in the implementation section effectively removes code that would otherwise be part of main()’s responsibility. Thus, any program that uses the object does not have to repeat the implementation details within its main() function. Rather, the main() function is only concerned with sending messages and how the state of the object is retained are not main()’s concern; these details are hidden within the class construction.