week14 (1).ppt

Inheritance
Adapted from Textbook (SAVITCH)

Inheritance
• We may need a process by which a new class
can be created from another class.
• Inheritance gives an ability to define new classes
using existing classes as a basis; those are
called “derived” and “base” classes, respectively
(a.k.a. parent/ancestor-child/descendant
classes).
• Inheritance provides code reuse
– Allows one class to "derive" from another,
adding features

Inheritance
• Derived class inherits data members and
member functions from base class(es).
• Derived class objects inherit members of
base class
–And may add members

Inheritance
• Private member variables in base class cannot
be accessed "by name" in derived
• Protected members in base class:
– Can be accessed "by name" in derived class
member functions
• Constructors are not inherited
– Are invoked from derived class’ constructor

Single and Multiple
• When a class is derived from a base class
this inheritance is called “single
inheritance”.
• A class may inherit from more than one
class; this type of inheritance is “multiple
inheritance”.

Access Restrictions
• Inheritance does not occur without
limitations.
• Based on access restrictions there are
three forms:
– Public
– Protected
– Private

Syntax
class X{
…
};
class Y : public X{
…
};
Class Y is derived from class X with “public”
derivation

Protected Members
• Members that can be accessed within the
same class and by classes derived from
that class, also by friends of the class.
#include <iostream>
using namespace std;
class A{
protected:
int number;
};
class B:public A{
public:
void f(){number=0; cout << ++number;}
};
main(){
B myb;
myb.f();
}

Example
class A{
public:
int a,b;
};
class B : public A{
public:
int b,c;
};
main(){
B b; // b as object name of type B
b.a=1; // a as inherited from the base class A
b.b=3; // 2nd b is of derived class B
b.A::b=2; // 2nd b is of base class A
b.c=4; // c is of derived class B
}

Multiple Inheritance
• You may need a class to inherit more than one
base class.
• C++ provides such flexibility with multiple
inheritance
• For example, a police car and an ambulance are
both emergency vehicles.
– Both classes may inherit from vehicle and
emergency parent classes although vehicle and
emergency classes are not linked to each other

Example
car
emergency truck
vehicle
police car ambulance fire engine
class vehicle
{...};
class emergency
{...};
class car:public vehicle
{...};
class truck:public vehicle
{...};
class policeCar:public car, public emergency
{...};
class ambulance:public car, public emergency
{...};
class fireEngine:public truck, public emergency
{...};

week14 (1).ppt

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week14 (1).ppt