Question

Shape.h

namespace Graphics {
    class Shape {
    public:
        virtual void Render(Point point) {};
    };
}

Rect.h

namespace Graphics {
    class Rect : public Shape {
    public:
        Rect(float x, float y);
        Rect();
        void setSize(float x, float y);
        virtual void Render(Point point);

    private:
        float sizeX;
        float sizeY;
    };
}

struct ShapePointPair {
    Shape shape;
    Point location;
};

Used like this:

std::vector<Graphics::ShapePointPair> theShapes = theSurface.getList();

for(int i = 0; i < theShapes.size(); i++) {
    theShapes[i].shape.Render(theShapes[i].location);
}

This code ends up calling Shape::Render and not Rect::Render

I’m assuming this is because it is casting the Rect to a Shape, but I don’t have any idea how to stop it doing this. I’m trying to let each shape control how it is rendered by overriding the Render method.

Any ideas on how to achieve this?

Answer 1

Here’s your problem:

struct ShapePointPair {
        Shape shape;
        Point location;
};

You are storing a Shape. You should be storing a Shape *, or a shared_ptr<Shape> or something. But not a Shape; C++ is not Java.

When you assign a Rect to the Shape, only the Shape part is being copied (this is object slicing).