starting out with c++, 3 rd edition 1 chapter 15 what is inheritance? inheritance allows a new class...

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Starting Out with C++, 3rd Edition

Chapter 15 What is Chapter 15 What is Inheritance?Inheritance?

• Inheritance allows a new class to be based on an existing class.

• The new class inherits all the member variables and functions of the class it is based on.

• Note difference between is-a and has-a.

• A station-wagon is-a car, but a car has-a wheel.

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Inherited ClassInherited Class

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Inheriting from Base ClassInheriting from Base Class

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test.htest.h - - Program Program #ifndef TEST_H#define TEST_H#include "grade.h"

class Test : public Grade{ private:

int numQuestions;float pointsEach;int numMissed;

public:Test ( int, int );

}; // Test#endif

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Why would you want?Why would you want?• In our set example, we could derive sets with

various differences– traits, prints differently– new methods, isKind– keep track of number of traits

• Animals, add/replace features. Logical categorization

• Cars, add common characteristics, options• Allows customization, reuse of base class.• Keeps design consistent. All cars have

dealerRetail. All cars have taxAmount.

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Protected Members and Protected Members and Class AccessClass Access

• Protected members of a base class are like private members, but they may be accessed by derived classes.

• The base class access specification determines how private, protected, and public base class members may be accessed by derived classes.

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Base Class AccessBase Class Access

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Constructors and Constructors and DestructorsDestructors

• The base class’s constructor is called before the derived class’s constructor.

• Since constructors have arguments, we are often not content with an argument-less call (the default).

• The destructors are called in reverse order, with the derived class’s destructor being called first.

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Passing Arguments to Passing Arguments to Base Class ConstructorsBase Class Constructors

• Assume a class called Cube is derived from a class called Rect. Here is how the constructor looks in Rect:Rect::Rect ( float w, float l ){ width = w; length = l; area = length * width;} // Rect::Rect

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Cube ConstructorCube Constructor

Cube::Cube( float wide, float long, float high ) : Rect(wide, long)

{ height = high;volume = area * high;

} // Cube::Cube

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Overriding Base Class Overriding Base Class FunctionsFunctions

• A member function of a derived class may have the same name as a member function of a base class.

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Polymorphism and Virtual Polymorphism and Virtual Member FunctionsMember Functions

• The term polymorphism means the ability to take many form.

• A virtual member function in a base class expects to be overridden in a derived class

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Need for Virtual Member Need for Virtual Member FunctionsFunctions

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Abstract Base Classes and Abstract Base Classes and Pure Virtual FunctionsPure Virtual Functions

• An abstract base class is not instantiated, but other classes are derived from it.

• A pure virtual function is a virtual member function of a base class that must be overridden.

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Abstract Base Classes and Abstract Base Classes and Pure Virtual Functions Pure Virtual Functions (cont)(cont)

• A class becomes an abstract base class when it contains one or more pure virtual functions.

• A pure virtual function is a virtual member function declared in a manner similar to the following:virtual void showInfo(void) = 0;

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Base Class PointersBase Class Pointers• Pointers to a base class may be

assigned the address of a derived class object.

• The pointer, however, ignores any overrides the derived class performs

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Classes Derived from Classes Derived from Derived ClassesDerived Classes

• A base class can also be derived from another class.

• This gives us a “chain” of inheritance where one class is derived from a second, which in turn is derived from a third.

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Chain of InheritanceChain of Inheritance

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Multiple InheritanceMultiple Inheritance• Multiple inheritance is when a

derived class has two or more base classes.

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Multiple Inheritance Multiple Inheritance (cont)(cont)

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ExamplesExamples• Could have a fruit which falls under two

categories: both fruit and vegetable. A tomato is sometimes referred to as a fruit and sometimes as a vegetable.

• fruit - the ripened reproductive body of a seed plant

• vegetable - edible seeds or roots or stems or leaves or bulbs or tubers or nonsweet fruits of any of numerous herbaceous plant

• Cross-categorization. An apple is a fruit and an apple is a breakfast food.

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Given the following, organize Given the following, organize them into inheritance or them into inheritance or

compositioncomposition• A test for 1720, a student in 1720• An electronic device, a computer, a

calculating tool• A woman, an employee, a person• A course, a course evaluation• Clothing, scarf, winter clothes, boots• Sesame Street Character, Big Bird,

Oscar the Grouch

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Concerns on multiple Concerns on multiple inheritanceinheritance

• Multiple inheritance is generally thought to be a bad idea because a common base class (B and C inherit from A, and D inherits from both B and C) cause problems with both B and C using the common variables differently.

• Java handles with interface – function use is specified, but no implementation.

starting out with c++, 3 rd edition 1 chapter 15 what is inheritance? inheritance allows a new class...

Documents

base class slide

derived class slide

base class access slide

base class constructors

rd edition

reuse of base class

inherited class slide

new class