Class Relationships And Reuse Interlude 4 Data Structures and Problem Solving with C++: Walls and...

Class Relationships And Reuse

Interlude 4

Data Structures and Problem Solving with C++: Walls and Mirrors, Frank Carrano, © 2012

Contents

• Inheritance Revisited• Containment: Has-a Relationships• Abstract Base Classes Revisited


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Inheritance Revisited

• Inheritance: Relationships among timepieces Data Structures and Problem Solving with C++: Walls and Mirrors, Frank Carrano, © 2012


• Multiple inheritance



• The class PlainBox



• Derived class MagicBox inherits members of base class PlainBox, redefines and adds members


New

Redefined


• Instance of derived class has all behaviors of its base class

• Derived class inherits private members from base class– But cannot access them directly

• Derived class’s methods can call base class’s public methods

• Clients of a derived class can invoke base class’s public methods



• Early, or static, binding: Compiler determines which version of a method to invoke



• Late binding: appropriate version of method decided at execution time

• Polymorphic method has multiple meanings

• Virtual method: can be overridden

• Method virtual in base class, virtual in any derived class


Public, Private, and Protected Sections of a Class

• Virtual method tables when cardBoxPtr points to (a) an instance of PlainBox



• Access to public, private, and protected sections of a class by a client and a derived class



• Virtual method tables when cardBoxPtr points to (b) an instance of MagicBox


Access Categories of a Class

• Public members can be used by anyone.

• Private members can be used only by methods of the class.

• Guideline: In general, class’s data members should be private


Kinds of Inheritance

• Public inheritance – Public and protected members of base class remain,

public and protected members of derived class

• Protected inheritance– Public and protected members of base class are

protected members of derived class.

• Private inheritance– Public and protected members of base class are

private members of the derived class.


Is-a and As-a Relationships

• A magic box is a plain box


Is-a and As-a Relationships

• Use public inheritance only when is-a relationship exists between two classes

• You can use instance of derived class – anywhere you can use instance of base class

• If relationship between 2 classes not is-a , – should not use public inheritance

• If class needs access to protected members of another class or if you need to redefine methods in that class, – You can form an as-a relationship


Containment: Has-a Relationships

• A pen has a or contains a ball


Execution Order of Constructors and Destructors

Constructors execute in following order:

1.Its base class constructor executes.

2.Constructors of its member objects execute in the declaration order.

3.The body of its constructor executes.


Execution Order of Constructors and Destructors

Destructors execute in the opposite order:

1.The body of its destructor executes.

2.Destructors of its member objects execute in the declaration order.

3.Its base class destructor executes.


Abstract Base Classes Revisited

• CDP and DVDP have an abstract base class Data Structures and Problem Solving with C++: Walls and Mirrors, Frank Carrano, © 2012


• A class that contains at least one pure virtual method is an abstract base class

• An abstract base class has descendants but no instance



• By definition is a class that contains at least one pure virtual method.

• Used only as the basis for derived classes – Thus defines a minimum interface for its

descendants.

• Has no instances.



• Should, in general, omit implementations – Except for the destructor methods that provide

access to private data members.

• That is, virtual methods in an abstract base class usually should be pure.

• Must implement any virtual method that is not pure, – Thereby providing a default implementation if

derived class chooses not to supply its own.


Class Relationships And Reuse Interlude 4 Data Structures and Problem Solving with C++: Walls and...

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