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copied!<p>The standard solution for this problem, especially given your constraints regarding dependencies, is to use the <a href="http://en.wikipedia.org/wiki/Visitor_pattern" rel="nofollow noreferrer">Visitor Pattern</a>.</p> <p>Here's how Visitor Pattern would work in your case:</p> <ul> <li>You need an abstract <code>ShapeVisitor</code> class. It has an abstract <code>Visit</code> method for each concrete subclass of Shape. eg: <code>Visit(Circle*)</code>, <code>Visit(Square*)</code>, etc.</li> <li>Shape has an abstract <code>AcceptVisitor(ShapeVisitor*)</code> method.</li> <li>Each Shape subclass implements <code>AcceptVisitor</code> as just calling <code>visitor->Visit(this)</code></li> <li>Each <code>CAD</code> class is a (or has-a, up to you) a <code>ShapeVisitor</code>. The <code>Visit</code> methods do the appropriate drawing for the specific type of <code>Shape</code>. No conditional or casting required.</li> </ul> <p>Here's a modified version of your code that uses Visitor Pattern in a pretty low-impact way:</p> <pre><code>class Circle; class Square; class ShapeVisitor { virtual void Visit(Circle *circle) = 0; virtual void Visit(Square *square) = 0; } class Shape { virtual void AcceptVisitor(ShapeVisitor *visitor) = 0; } class Circle : public Shape { // some special members/methods virtual void AcceptVisitor(ShapeVisitor *visitor) { visitor->Visit(this); } } class Square : public Shape { // some special members/methods virtual void AcceptVisitor(ShapeVisitor *visitor) { visitor->Visit(this); } } class CAD : public ShapeVisitor { virtual DrawCircle(Circle *circle) = 0; virtual DrawSquare(Square *square) = 0; virtual void Visit(Circle *circle) { DrawCircle(circle); } virtual void Visit(Square *square) { DrawSquare(square); } } class SWX : public CAD { virtual DrawCircle(Circle *circle){// do some stuff that draws circle on SWX system} } class PRO : public CAD { virtual DrawCircle(Circle * circle){// do some stuff that draws circle on PRO system} } int main() { Circle * circle = new Circle(); Square * sq = new Square; vector<Shape*> shapes; shapes.push_back(circle); shapes.push_back(sq); SWX * swx = new SWX(); for( int i = 0 ; i < shapes.size() ; ++i ) { shapes[i]->AcceptVisitor(SWX); } } </code></pre> <p>In this code I've opted for making <code>CAD</code> actually a subclass of <code>ShapeVisitor</code>. Also, since you've already got virtual methods in <code>CAD</code> to do the drawing, I implemented the <code>Visit</code> methods there (once), rather than once in each subclass. Once you switch clients over to the using <code>AcceptVisitor</code> instead of calling the Draw* methods directly you could make those methods protected, and then eventually move the implementation of the <code>Visit</code> methods down to the subclasses (that is: refactor to remove the extra level of indirection caused by having <code>Visit(Foo*)</code> call <code>DrawFoo(Foo*)</code>).</p>

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