constructors in derived class...

i was going through a program on run-time polymorphism and i encountered a program which had 1 base class and 2 classes derived from the base class..
the base class is as follows

Code:

class media
{
  protected:
    char title[50];
    float price;
  public:
    media(char *s,float a)
    {
      strcpy(title,s);
     price = a;
    }
  virtual void display() {  } // empty virtual function..
};

and one of the base class is as follows..

Code:

class book:public media
{
   int pages;
 public:
  book(char *s,float a,int p):media(s,a)
   {
     pages = p;
    }
  void display();
};

Now my doubt is with this line..in the derived class..

Code:

book(char *s,float a,int p):media(s,a)

i understand that this is a constructor with 3 parameters..
but why is this included..

Code:

:media(s,a)

is the constructor derived..?? or what..??
can someone please clarify..!!!!

It is part of the initialiser list. Here it is used to invoke the base class constructor. In fact, we could have written:

Code:

book(char* s, float a, int p) : media(s, a), pages(p) {}

hmm..i still dont understand..
are we trying to initialize the values of the memebers of the base class also..along with the members of the derived class..??

Originally Posted by narendrav

are we trying to initialize the values of the memebers of the base class also..along with the members of the derived class

Effectively, yes.

ok..thank you...very much..for the quick reply..!!!

FYI, be aware that this is dangerous practice.

>>char title[50];
This is a limited size buffer as thus you need to make sure you don't write past its limits.

>>strcpy(title,s);
This is a ticking time bomb. You don't check the length of s to make sure it fits in title.

But instead of messing with this, use std::string:

std::string title;
title = s;

Done. Safe. Easy. Quick.