DaniWeb Forum Index > Software Development > C

downcasting and dynamic_cast

Please support our C advertiser: Programming Forums - DaniWeb Sister Site

•

Join Date: Jun 2006

Posts: 147

Reputation: Laiq Ahmed will become famous soon enough

Solved Threads: 20

Laiq Ahmed Laiq Ahmed is offline

Offline

Junior Poster

downcasting and dynamic_cast

Jul 8th, 2006

I've gone through several books they explain Upcasting as
Base class pointer pointing to Drived class Object

this definition is correct.
Downcasting:

•

Drived class pointer pointing to base class object..

i got a bit confused.....not having crystal clear image in my mind.....
if the above definition is correct... whats wrong with the below code

 Help with Code Tags 
 C Syntax (Toggle Plain Text) 
class Shape {
public:
   virtual ~Shape() {}
};
class Circle: public Shape {
public: 
   ~Circle() {} 
};
 
int main() 
{
      //why this can't work ...........according to definition....
    //  Circle* c = dynamic_cast<Shape*>(new Shape);
 
     //why this
     Shape* s = new Shape;
     Circle* c = dynamic_cast<Circle*>(s);
     return 0;
}
class Shape {
public:
   virtual ~Shape() {}
};
class Circle: public Shape {
public: 
   ~Circle() {} 
};
 
int main() 
{
      //why this can't work ...........according to definition....
    //  Circle* c = dynamic_cast<Shape*>(new Shape);
 
     //why this
     Shape* s = new Shape;
     Circle* c = dynamic_cast<Circle*>(s);
     return 0;
}

I know that downcasting is performed between polymorphic types

. I m bit confused with its definition and implementation... please give me the solid concept

•

Join Date: Apr 2004

Posts: 4,461

Reputation: Dave Sinkula has a brilliant future

Solved Threads: 254

Dave Sinkula Dave Sinkula is offline

Offline

long time no c

Re: downcasting and dynamic_cast

Jul 8th, 2006

Hmm. Clues here: http://www.gotw.ca/gotw/017.htm ?

"One of the methods used by statists to destroy capitalism consists in establishing controls that tie a given industry hand and foot, making it unable to solve its problems, then declaring that freedom has failed and stronger controls are necessary." --Ayn Rand