C++ polymorphism: Checking data type of sub class [duplicate]

Question

Possible Duplicate:
Finding the type of an object in C++

Hello,
I am sorry if it's a duplicate but I was not able to find answer to my question here.
Let's assume we have following class structure in c++:

class CPolygon {
  protected:
    int width, height;
  public:
    void set_values (int a, int b)
      { width=a; height=b; }
  };

class CRectangle: public CPolygon {
  public:
    int area ()
      { return (width * height); }
  };

Now I have got a pointer to CPolygon object. How do I check if it's actually a pointer to the object of class CRectangle?

Answer 1

You can do this by checking if dynamic_cast<CRectangle*>(ptr) return non-null, where ptr is a pointer to CPolygon. However this requires the base class (CPolygon) to have at least one virtual member function which you probably need anyway (at least a virtual destructor).

Answer 2

Ideally, you don't. You use polymorphism to just do the right thing:

class CPolygon {
  protected:
    int width, height;
  public:
    void set_values (int a, int b)
      { width=a; height=b; }

    virtual int area() const = 0;
  };

class CRectangle: public CPolygon {
  public:
    int area () const
      { return (width * height); }
  };

Call area() on your CPolygon pointer, and you'll get the area for a CRectangle if that's what it is. Everything derived from CPolygon will have to implement area() or you won't be able to instantiate it.