Re: Confusion about inheritance and the allocating memory via new
<tharringtonan@netscape.net> wrote in message...
On Jul 29, 2:41 pm, Robert Bauck Hamar wrote:
[snip]
If ppoly1 had been changed to point to a CPolygon object, what should
have
happened then?
This is the explanation that I was looking for.
Here is my problem in general.
I am writing a molecular dyanmics code. The user of the program will
set in the input file the type of boundary conditions. The boundary
conditions are represented by objects in the code. The important fact
are that there are several types of boundary conditions and they all
share many similar methods. In addition each boundary condition has a
few distinct methods.
So I figured I would write a base class so I could reuse the methods
that are shared by the numerous boundary conditions. However, I seem
to have to declare the methods that aren't shared by the boundaruy
conditions as virtual ...=0 in the base class.
Is there any way around of having to declare the nonshared methods as
virtual ... = 0 in the base class?
// ref: class CRectangle: public CPolygon {};
int main (){
CPolygon *ppoly1 = new CRectangle;
ppoly1->set_values( 4, 5 );
// - downcast -
CRectangle *pCR( dynamic_cast<CRectangle*>( ppoly1 ) );
if( pCR ){
cout << pCR->area() << endl;
} // if(*)
delete ppoly1; // see 'virtual ~CPolygon(){}' below
return 0;
} // main()
// output: 20
Are you aware that you can 'define' a 'pure virtual' (virtual ... = 0;)?
class CPolygon{ public:
// CPolygon( int a = 0, int b = 0 ) : width(a), height(b){}
virtual void set_values (int a, int b) { width=a; height=b; };
virtual int area () = 0;
virtual ~CPolygon(){} // it's a base class
protected:
int width, height;
};
// - definition must be outside the class declaration. -
int CPolygon::area(){ return 0; };
// you don't want to 'return width * height;' here, unless you insure
// they are initialised ( proper Ctor).
HTH.
--
Bob R
POVrookie