Question

I have the following C++ code (simplified version):

class Shape
{
    bool isCircle = false;
    bool isSquare = false;
}

class Circle : public Shape
{
    // some special members/methods
}

class Square : public Shape
{
    // some special members/methods
}

class CAD
{
    virtual DrawCircle(Circle * circle) = 0;
}

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();
    circle->isCircle = true;

    Square * sq = new Square;
    sq->isSquare = true;

    vector<Shape*> shapes;
    shapes.push_back(circle);
    shapes.push_back(sq);

    SWX * swx = new SWX();

    for( int i = 0 ; i < shapes.size() ; ++i )
    {
        if( shapes[i]->isCircle )
    {
        SWX->DrawCircle((Circle*)(shapes[i]));
    }
    else if( shapes[i]->isSquare )
    {
        SWX->DrawSquare((Square*)(shapes[i]));
    }
}

I wish to remove the need for if…else (if at all possible within the constraints stated below).

My constraints right now are:

• The CAD and derived classes are ginormous classes with various external dependencies.
• The CAD classes cannot be merged with the Shape and derived classes (that would have been ideal, since then I can use polymorphism to solve my problem), since other projects/classes depend on the Shape classes and cannot depend on the CAD classes.
• There are more than a dozen Shape-derived classes with a half dozen CAD-derived classes and this if…else is happening in numerous locations – so it would help if any solution is simple to understand (easier to convince my teammates to change legacy code).

Any suggestions/comments/solution you have would be most welcome.

Answer 1

The standard solution for this problem, especially given your constraints regarding dependencies, is to use the Visitor Pattern.

Here’s how Visitor Pattern would work in your case:

• You need an abstract ShapeVisitor class. It has an abstract Visit method for each concrete subclass of Shape. eg: Visit(Circle*), Visit(Square*), etc.
• Shape has an abstract AcceptVisitor(ShapeVisitor*) method.
• Each Shape subclass implements AcceptVisitor as just calling visitor->Visit(this)
• Each CAD class is a (or has-a, up to you) a ShapeVisitor. The Visit methods do the appropriate drawing for the specific type of Shape. No conditional or casting required.

Here’s a modified version of your code that uses Visitor Pattern in a pretty low-impact way:

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);
    }
}

In this code I’ve opted for making CAD actually a subclass of ShapeVisitor. Also, since you’ve already got virtual methods in CAD to do the drawing, I implemented the Visit methods there (once), rather than once in each subclass. Once you switch clients over to the using AcceptVisitor instead of calling the Draw* methods directly you could make those methods protected, and then eventually move the implementation of the Visit methods down to the subclasses (that is: refactor to remove the extra level of indirection caused by having Visit(Foo*) call DrawFoo(Foo*)).