visual c++ programming: concepts and projects chapter 13a: object-oriented programming (concepts)

Visual C++ Programming: Concepts and Projects

Chapter 13A:Object-Oriented Programming

(Concepts)

Objectives

In this chapter, you will:• Design class definitions• Implement data hiding and encapsulation• Use accessor and mutator methods• Implement an initializing constructor• Use public and private access modes• Create and reference a two-dimensional array

2Programming with Visual C++

Introduction

• An abstraction is an idea with varying amounts of definition

• In computer science, abstractions are class definitions– Examples of system-defined class definitions: Button,

TextBox, Label, Form

• Objects are created from class definitions (abstractions)

• Objects are implementations of the abstraction


Introduction (continued)


OOP Example

• Consider a program in which Frog objects are created and made to hop across the screen

• The Frog class definition is an abstraction• The Frogs seen on the interface are objects

(instances of the class)• Objects exhibit all the properties and methods

defined by the class they are derived from


OOP Example (continued)


The Frog Class Definition

• A Frog class definition needs:– Class variables (the Frog icons)– Instance variables

• x and y coordinates of the Frog• The assigned icon

– Constructor


The Frog Class Definition (continued)

• A Frog class definition needs: (continued)– Instance methods

• Operations that Frog objects can perform• Example: showIcon()

– Returns the Frog’s assigned icon to the client program so that it can be displayed


The Frog Class Definition (continued)


Instantiation and Use

• Frog objects can be instantiated using gcnew


Instantiation and Use (continued)


Initializing Constructors

• Constructors are used to instantiate objects• Initializing constructors use parameters to

construct objects with specific properties that can be passed in as actual arguments


Initializing Constructors (continued)

• The code for an initializing constructor (from Frog.h)

• Parameters xcoord and ycoord are assigned to instance variables x and y


Initializing Constructors (continued)

• The default constructor assigns every Frog object it makes the same x and y coordinates

• Initializing constructors may be preferable to default constructors because the client can specify the location for each Frog object created

• To prevent the client from using the default constructor, it should be placed in the private portion of the Frog class definition


Data Hiding

• Data hiding is the practice of making data members inaccessible by designating them as private

• Data members that are private can only be accessed by class methods and not by the client


Data Hiding (continued)

• In this example, the client constructs a Frog with x and y locations -50, -75 and later assigns other negative coordinate values

• This could lead to problems, since negative coordinates are off of the interface



• To make it impossible for the client to place invalid values into instance variables like x and y, these data members can be “hidden” by placing them in the private portion of the class definition– Invalid assignments by the client are no longer allowed– Unfortunately, valid assignments are not allowed either

• public methods are provided to the client to allow it to change values stored in private data members



• Hidden data members are not visible to the client

• The good news– Invalid assignments are no longer possible

• The bad news– Valid assignments are not allowed either

• public methods are provided to the client to allow it to change values stored in private data members


Accessor and Mutator Methods

• Accessor and mutator methods are public methods that provide access to private data members

• An accessor method is a public method that returns the value stored in a private data member


Accessor and Mutator Methods (continued)

• In this example, public methods getX() and getY() are available to the client, allowing it to retrieve the data stored in private data members x and y



• The definitions of public methods getX() and getY() are contained in the class definition



• A mutator method is a public method that changes the value stored in a private data member

• In this example, setX() is a public mutator method used to change the private x coordinates of each of four Frog objects



• Mutator methods can be used to screen data before it is assigned to a private data member, as shown in the class definition of setX()


Utility Methods

• Utility methods are instance methods that perform operations other than those performed by constructors, destructors, accessors, and mutators (Example: verifyX() )


Utility Methods (continued)

• A Frog class definition might want a public method that the client can call when he or she wants a frog to hop


Complete Frog Class Definition

• private data members– x, y, icon

• private methods– Default constructor (Frog())– Utility methods

• verifyX()• verifyY()


Complete Frog Class Definition (continued)

• public methods– Initializing constructor (Frog(int, int))– Accessor methods

• getX()• getY()• showIcon()



• public methods– Mutator methods

• setX(): used to assign a value to x• setY(): used to assign a value to y• setLeaping(): used to assign the leaping icon• setSitting(): used to assign the sitting icon

– Utility method• hop(): used to add 25 to the x coordinate


Client Code

• The client needs to instantiate four Frog objects and position them vertically along a starting line

• Constant STARTX stores the starting x coordinate position

• Constants START1, START2, START3 , and START4 store the starting y coordinate positions


Client Code (continued)



• Using the initializing constructor, each new Frog can be positioned in a different location



• To draw a single Frog, you first construct a Rectangle object– Upper-left corner x coordinate is Frog->getX()– Upper-left corner y coordinate is Frog->getY()– Width and height are both 25 pixels

• The DrawIcon() method displays an icon in a Rectangle– The icon is Frog1->showIcon()



• To reset the interface, all frogs are repositioned back to their starting X location and all frog icons are set to the sitting frog icon



• A random number is used to select one of the four frogs; the chosen frog’s icon is set to the leaping icon


Summary

• Class definitions are abstractions• Objects are instances of the class• Initializing constructors are useful if the

objects they construct need to have unique values assigned to their attributes

• public data members and methods are visible to the client

• private data members and methods are not visible to the client


Summary (continued)

• Data hiding is used to restrict client access to data members

• Accessor methods are public methods that can be used to retrieve a value stored in a private data member

• Mutator methods are public methods that can be used to assign new values to a private data member


Summary (continued)

• Utility methods carry out operations that constructors, destructors, accessors, and mutators do not perform


visual c++ programming: concepts and projects chapter 13a: object-oriented programming (concepts)

Documents

visual c programming

visual c slide

class objects

frog class definition

use frog objects

class methods

class variables

defined class definitions