C++ Tutorial
C++ Inheritance
One of the
most important concepts in object-oriented programming is that of inheritance.
Inheritance allows us to define a class in terms of another class, which makes
it easier to create and maintain an application. This also provides an
opportunity to reuse the code functionality and fast implementation time.
When
creating a class, instead of writing completely new data members and member
functions, the programmer can designate that the new class should inherit the
members of an existing class. This existing class is called the base class, and the new class is referred
to as the derived class.
The idea
of inheritance implements the is
a relationship. For example,
mammal IS-A animal, dog IS-A mammal hence dog IS-A animal as well and so on.
Base & Derived
Classes:
A class
can be derived from more than one classes, which means it can inherit data and
functions from multiple base classes. To define a derived class, we use a class
derivation list to specify the base class(es). A class derivation list names
one or more base classes and has the form:
class derived-class: access-specifier base-class
Where
access-specifier is one of public,
protected, or private, and base-class is the
name of a previously defined class. If the access-specifier is not used, then
it is private by default.
Consider a
base class Shape and its derived class Rectangle as follows:
#include <iostream>
using namespace std;
// Base class
class Shape
{
public:
void setWidth(int w)
{
width = w;
}
void setHeight(int h)
{
height = h;
}
protected:
int width;
int height;
};
// Derived class
class Rectangle: public Shape
{
public:
int getArea()
{
return (width * height);
}
};
int main(void)
{
Rectangle Rect;
Rect.setWidth(5);
Rect.setHeight(7);
// Print the area of the object.
cout << "Total area: " << Rect.getArea() << endl;
return 0;
}
When the
above code is compiled and executed, it produces the following result:
Total area: 35
Access Control and
Inheritance:
A derived
class can access all the non-private members of its base class. Thus base-class
members that should not be accessible to the member functions of derived
classes should be declared private in the base class.
We can
summarize the different access types according to who can access them in the
following way:
|
Access
|
public
|
protected
|
private
|
|
Same class
|
yes
|
yes
|
yes
|
|
Derived
classes
|
yes
|
yes
|
no
|
|
Outside
classes
|
yes
|
no
|
no
|
A derived
class inherits all base class methods with the following exceptions:
·
Constructors, destructors and copy constructors of the base class.
·
Overloaded operators of the base class.
·
The friend functions of the base class.
Type of Inheritance:
When
deriving a class from a base class, the base class may be inherited through public, protected or private inheritance. The type of inheritance
is specified by the access-specifier as explained above.
We hardly
use protected or private inheritance, but public inheritance is commonly used. While
using different type of inheritance, following rules are applied:
·
Public Inheritance: When deriving a class from a public base class, publicmembers of the base class
become public members of the derived class andprotected members of the base class become protected members of the derived class. A base
class's private members are never accessible directly
from a derived class, but can be accessed through calls to the public and protected members of the base class.
·
Protected Inheritance: When deriving from a protected base class, public andprotected members of the base class become protected members of the derived class.
·
Private Inheritance: When deriving from a private base class, public andprotected members of the base class become private members of the derived class.
Multiple Inheritances:
A C++
class can inherit members from more than one class and here is the extended
syntax:
class derived-class: access baseA, access baseB....
Where
access is one of public,
protected, or private and would be given for every base
class and they will be separated by comma as shown above. Let us try the
following example:
#include <iostream>
using namespace std;
// Base class Shape
class Shape
{
public:
void setWidth(int w)
{
width = w;
}
void setHeight(int h)
{
height = h;
}
protected:
int width;
int height;
};
// Base class PaintCost
class PaintCost
{
public:
int getCost(int area)
{
return area * 70;
}
};
// Derived class
class Rectangle: public Shape, public PaintCost
{
public:
int getArea()
{
return (width * height);
}
};
int main(void)
{
Rectangle Rect;
int area;
Rect.setWidth(5);
Rect.setHeight(7);
area = Rect.getArea();
// Print the area of the object.
cout << "Total area: " << Rect.getArea() << endl;
// Print the total cost of painting
cout << "Total paint cost: $" << Rect.getCost(area) << endl;
return 0;
}
When the
above code is compiled and executed, it produces the following result:
Total area: 35
Total paint cost: $2450
