Computer Programming Inheritance

8/13/2019 Computer Programming Inheritance

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CS 103

Computer Programming (CP)

Inheritance

Single Inheritance Multiple Inheritance


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Inheritance

Inheritance is an is-a relationship Example:Every Employee is a Person

Inheritance lets us create new classes from

existing classes New classes are called the derived classes

Existing classes are called thebase classes

Derived classes inherit the properties of the

Base classes

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Inheritance (cont'd.)

Inheritance can be either single

inheritance or multiple

inheritance.

Single Inheritance: derived class has a singlebase class

Multiple Inheritance: derived class has more

than one base class

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Inheritance can be viewed as atree-likeor

hierarchicalstructure wherein a base class

is shown with its derived classes.

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General syntax of a derived class:

wherememberAccessSpecifier ispublic,

protected,orprivate(default)

Theprivate members of a base class areprivate to the base class

derived class cannot directly access them

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public members of base class can be

inherited aspublicorprivatemembers

The derived class can include additional

members data and/or functions

The derived class can redefine thepublic

member functions of the base class

Applies only to the objects of the derived class

All members of the base class are also

member variables of the derived class

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Two Important Issues Related to

Inheritance

1. Redefining (Overriding) Member Functions of

the Base Class

2. Constructors of Derived and Base Classes

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1. Redefining (Overriding) Member

Functions of the Base Class

To redefine apublicmember function:

Corresponding function in derived class must have

same name/number/types of parameters

If derived class overrides a publicmember function of the base class, then to

call the base class function, specify:

Name of the base class

Scope resolution operator (: :)

Function name with appropriate parameter list

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Function Overriding vs. Function

Overloading

The name of the function being redefined in the

derived class must have the same name and thesame set of parameters (function

overriding).

If the corresponding functions in the base class

and the derived class have the same name but

different sets of parameters, then this isfunction overloading in the derived class.

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Base Class : rectangleType

//rectangleType.hclass Rectangle

{

private:

double length;

double width;

public:

Rectangle();

Rectangle(double l, double w);

void setDimension(double l, double w);

double getLength() const;

double getWidth() const;

double area() const;

double perimeter() const;

void print() const;

};

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rectangleType Class Diagram11


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//rectangleType.cpp

Rectangle::Rectangle()

{

length = 0;width = 0;

}

Rectangle::Rectangle(double l, double w)

{

setDimension(l, w);

}

void Rectangle::setDimension(double l, double w)

{

length = l;

width = w;

}double Rectangle::getLength() const

{

return length;

}

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13double Rectangle::getWidth()const

{

return width;

}

double Rectangle::area() const{

return length * width;

}

double Rectangle::perimeter() const

{

return 2 * (length + width);}

void Rectangle::print() const

{

cout


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Derived Class :boxType

//boxType.h

class Box: public Rectangle

{

private:

double height;

public:Box();

Box(double l, double w, double h);

void setDimension(double l, double w, double h);

double getHeight() const;

double area() const;double volume() const;

void print() const;

};

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boxType Class Diagram

boxTypeis derived from rectangleType,andit is apublic inheritance

Also overridesprint andarea

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boxType.cpp

Box::Box()

{

height = 0.0;}

Box::Box(double l, double w, double h):Rectangle(l, w)

{

height = h;

}

void Box::setDimension(double l, double w, double h)

{

Rectangle::setDimension(l, w);

height = h;

}

double Box::getHeight() const{

return height;

}

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double Box::area() const

{

return 2 * (getLength() * getWidth()

+ getLength() * height+ getWidth() * height);

}

double Box::volume() const

{

return Rectangle::area() * height;

}

void Box::print() const

{

Rectangle::print();

cout


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2. Constructors of Derived and

Base Classes

Derived class constructor cannot directly accessprivate members of the base class

Derived class can directly initialize onlypublic

member variables of the base class

When a derived object is declared

It must execute one of the base class constructors

Call to base class constructor is specified in

heading of derived class constructor definition

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Constructors of Derived and Base

Classes (cont'd.)

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Box::Box()

{

height = 0.0;

}

Box::Box(double l, double w, double h):Rectangle(l, w)

{ height = h;

}


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20int main()

{

Rectangle Rectangle1;

Rectangle Rectangle2(8, 6);

cout


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21Box Box1;

Box Box2(10, 7, 3);

cout


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Constructors of Derived and Base

Classes (cont'd.)Rect angl e Rect angl e1( 5. 0, 3. 0) ;Box Box1( 6. 0, 5. 0, 4. 0) ;

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Destructors in a Derived Class

Destructors

Used to deallocate dynamic memory allocated by the objects

of a class

When a derived class object goes out of scope

Automatically invokes its destructor

When the destructor of the derived class executes

Automatically invokes the destructor of the base class

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Header File of a Derived Class

To define new classes

Create new header files

To create new classes based on previously defined

classesHeader files of the new classes contain commands that specify

where to look for the definitions of the base classes

The definitions of the member functions can be

placed in a separate file

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Multiple Inclusions of a Header

File

Use the preprocessor command (#include) to

include a header file in a program

The preprocessor processes the program before it

is compiled To avoid multiple inclusion of a file in a program

Use certain preprocessor commands in the header file

(file guard)

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Problem: Solution:

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Multiple Inclusions of a Header File

(contd.)


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Inheritance and Access Specifiers

// Inherit from Base publicly

class Der: public Base

{

};

// Inherit from Base privately

class Der: private Base{

};

// Inherit from Base protectedly

class Der: protected Base

{

};

class Der: Base // Defaults to private inheritance

{

};

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public Inheritance

IfmemberAccessSpecifierispublic:

publicmembers ofAarepublicmembers

ofB and can be directly accessed in class Bprotectedmembers ofAareprotected

members ofBand can be directly accessed bymember functions (and friend functions) ofB

privatemembers ofAare hidden in Bandcan be accessed by member functions of Bthroughpublicorprotectedmembers ofA

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public Inheritance (contd.)

publicInheritance

Base Access

Specifier

Derived Access

Specifier

Derived Class

Access?public public Yes

private private No

protected protected Yes

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protected Inheritance

If memberAccessSpecifier isprotected:

public members ofA are protectedmembers of B and can be accessed by themember functions (and friend functions) ofB

protectedmembers ofAareprotectedmembers of B and can be accessed by the

member functions (and friend functions) ofBprivatemembers ofAare hidden in Band

can be accessed by member functions of Bthroughpublicorprotectedmembers ofA

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protected Inheritance (contd.)

protected Inheritance

Base Access

Specifier

Derived Access

Specifier

Derived Class

Access?public protected Yes

private private No

protected protected Yes

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private Inheritance

If memberAccessSpecifier isprivate:

publicmembers ofAareprivatemembersofBand can be accessed by member functionsofB

protected members ofA areprivatemembers ofBand can be accessed by member

functions (and friend functions) ofBprivatemembers ofAare hidden in Band

can be accessed by member functions of Bthroughpublic/protectedmembers ofA

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private Inheritance (contd.)

private Inheritance

Base Access

Specifier

Derived Access

Specifier

Derived Class

Access?public private Yes

private private No

protected private Yes

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Multiple Inheritance

Multiple Inheritance enables a derived class to

inherit members from more than one base class.

Lets say we wanted to write a program to keep

track of a group of teachers. A teacher is a person.

However, a teacher is also an employee.

Multiple inheritance can be used to create a

Teacher class that inherits properties from both

Person and Employee.

To use multiple inheritance, simply specify each

base class (just like in single inheritance),

separated by a comma.

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MultipleInheritance.cpp

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Problems with Multiple Inheritance

While multiple inheritance seems like a simpleextension of single inheritance, multiple

inheritance introduces a lot of issues that can

markedly increase the complexity of programs and

make them a maintenance terrible.

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DiamondProblem.cpp

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virtual Base Class

To share a base class, simply insert thevirtualkeyword in the inheritance list of the

derived class.

This creates what is called a virtual base

class, which means there is only one base object

that is shared.

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class PoweredDevice{

};

class Scanner: virtual public PoweredDevice

{};

class Printer: virtual public PoweredDevice

{

};

class Copier: public Scanner, public Printer

{

};

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Now, when you create a Copier class, you will getonly one copy ofPoweredDevicethat will be

shared by both Scanner andPrinter.

However, this leads to one more problem: if

Scanner and Printer share aPoweredDevicebase class, who is responsible

for creating it?

The answer, as it turns out, is Copier. The

Copier constructor is responsible for creating

PoweredDevice.

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Computer Programming Inheritance