constructors in derived class...

**narendrav** · 12-20-2011

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..!!!!

**laserlight** · 12-20-2011

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) {}

**narendrav** · 12-20-2011

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..??

**laserlight** · 12-20-2011

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.

**narendrav** · 12-20-2011

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

**Elysia** · 12-20-2011

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.

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