Re: Variables disappearing from scope when i don't want them to

"Jim Langston" <>
Mon, 2 Apr 2007 16:32:02 -0700
"The Cool Giraffe" <> wrote in message

Suppose there's an abstract base class and two inheriting
classes declared as follows.

class Shape {double sizeA;};
class Ellipse : Shape {double sizeB};
class Rect : Shape {double sizeC};

In our program we will have a pointer to a shape but we
don't know which one yet. So, we declare it as follows.

Shape *shapy;

Then, i'd like to do this:

shapy = new Ellipse ();
shapy->sizeA = 4;
shapy->sizeB = 5;

or this:

shapy = new Rect ();
shapy->sizeA = 4;
shapy->sizeC = 5;

but, while the first two lines work fine (i.e. the computer
finds the sizeA and can handle the pointers to Ellipse
and Rect), the implementation specific variables are not
reachable. How can i solve this without binding shapy to
Ellipse or Rect explicitly?

You have to dyanic_cast it. Fixed the many errors in your code and am
showing how it could be done. It could probably be done with dynamic
casting to a reference or something too.

#include <iostream>
#include <string>

class Shape
    double sizeA;
    virtual ~Shape() {}

class Ellipse: public Shape
    double sizeB;

class Rect: public Shape
    double sizeC;

int main()
    Shape *shapy;

    shapy = new Ellipse ();
    shapy->sizeA = 4;
    if ( dynamic_cast<Ellipse*>( shapy ) != NULL )
        dynamic_cast<Ellipse*>( shapy )->sizeB = 5;

    delete shapy;

    shapy = new Rect ();
    shapy->sizeA = 4;

    Rect* ThisRect = dynamic_cast<Rect*>( shapy );
    if ( ThisRect != NULL )
        ThisRect->sizeC = 5;


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