Constructor and inheritance in C++.

Constructor and inheritance

• The compiler automatically call a base class constructor before executing the derived class constructor. 
• The compiler’s default action is to call the default constructor in the base class. 
• If you want to specify which of several base class constructors should be called during the creation of a derived class object.
• In such situations, you must define specifically which base class constructor the compiler will call. 
• This is done by specifying the arguments to the selected base class constructor in the definition of the derived class constructor.

class Rectangle

{

private :

float width;

float length;

public:

length = 0;

Rectangle ()

 {

  width = 0;

 }

{ 

Rectangle (float len, float wid)

  length = len;

  width = wid; 

 }

class Box : public Rectangle

float area()

 {

  return length * width ;

 }

};

{

private :

Box (float len, float wid, float ht) : Rectangle(len, wid)

float height; 

public:

 Box ()

 {

  height = 0;

 }

 {

  height = ht;

 }

Box cx(4,8,5);

float volume()

 {

  return area() * height;

 }

};

int main ()

{

 Box bx;

cout << bx.volume() << endl;

 cout << cx.volume() << endl;

 return 0;

}

Expected Output :-

0

160

// Program to illustrate the concept of Constructors.

#include <iostream.h> 

#include <conio.h>   

class construct { 

public: 

    int a, b; 

    // Default Constructor 

    construct() 

    { 

        a = 10; 

        b = 20; 

    } 

};   

int main() 

{ 

    // Default constructor called automatically 

    // when the object is created 

    construct c; 

    cout << "a: " << c.a << endl 

         << "b: " << c.b; 

    return 1; 

} 

Expected Output:-

a: 10

b: 20

// C++ program to demonstrate implementation of Inheritance. 

#include <iostream.h>

#include <conio.h>

//Base class 

class Parent 

{ 

    public: 

      int id_p; 

};   

// Sub class inheriting from Base Class(Parent) 

class Child : public Parent 

{ 

    public: 

      int id_c; 

};   

//main function 

int main()  

   {   

        Child obj1;            

        // An object of class child has all data members 

        // and member functions of class parent 

        obj1.id_c = 7; 

        obj1.id_p = 91; 

        cout << "Child id is " <<  obj1.id_c << endl; 

        cout << "Parent id is " <<  obj1.id_p << endl;           

        return 0; 

   }  

Expected Output:-

Child id is 7

Parent id is 91

Overriding Base Class Functions

• A derived class can override a member function of its base class by defining a derived class member function with the same name and parameter list.
• It is often useful for a derived class to define its own version of a member function inherited from its base class. 
• This may be done to specialize the member function to the needs of the derived class. 
• When this happens, the base class member function is said to be overridden by the derived class.

class mother

{

public:

void display ()

{

cout << "mother: display function\n";

}

};

class daughter : public mother

{

cout << "daughter: display function\n\n";

public:

 void display ()

 {

 }

};

int main ()

}

{

 daughter rita;

 rita.display();

return 0;

}

Expected Output:-
daughter: display function

 Gaining Access to an Overridden Function

• It is occasionally useful to be able to call the overridden version. 
• This is done by using the scope resolution operator to specify the class of the overridden member function being accessed.

class daughter : public mother
{
public:
 void display ()
 {
cout << "daughter: display function\n\n";
mother::display();
 }
};

Expected Output:-

daughter: display function
mother: display function