抽象类的C ++继承

Question

I'm a bit new to C++ and trying to work out how to solve something with inheritance.

I have generic Open Frameworks app, BaseApp, and then MainApp, a specific implementation of the framework in BaseApp.

An app can render a fullscreen Layer and buffer another Layer for a transition. Layers also extend from a BaseLayer class, which like BaseApp does nothing until extended.

so

class BaseApp : public ofBaseApp {
    public:
        BaseLayer renderLayer;
        BaseLayer bufferedLayer;
}
// ...
class MainApp : public BaseApp {
    public:
        VideoLayer videoLayer; // Video + Image layers extend BaseLayer
        ImageLayer imageLayer;
}

And what I'd like to do is say that my renderLayer is my imageLayer, and my bufferedLayer is my videoLayer, ie: renderLayer = (some class which extends layer) so inside my BaseApp I can treat it as a BaseLayer and in my MainApp I can treat it as a VideoLayer, ImageLayer, etc.

I ended up accomplishing this with shared_ptr 's which seems dirty, but works:

class BaseApp : public ofBaseApp {
    public:
        shared_ptr<BaseLayer> renderLayer;
}

// inside MainApp.setup()
    renderLayer = make_shared<ImageLayer>();
    if (shared_ptr<ImageLayer> imageLayer = dynamic_pointer_cast<ImageLayer>(renderLayer)){
        // ImageLayer specific setup
        imageLayer->setup("assets/tiger.png",0,0,800,600);
    }

Is there a cleaner way to accomplish this? I've been trying to stick to oF style, which has as few pointers as possible, so a Layer is a POD class.

Answer 1

I suggest adding virtual functions to get the render layer and the buffered layer.

class BaseApp : public ofBaseApp {
    public:
        virtual BaseLayer& getRenderLayer() = 0;
        virtual BaseLayer& getBufferedLayer() = 0;
};

// ...
class MainApp : public BaseApp {
    public:

        virtual BaseLayer& getRenderLayer() { return imageLayer;}
        virtual BaseLayer& getBufferedLayer() { return videoLayer; }

    private:
        VideoLayer videoLayer; // Video + Image layers extend BaseLayer
        ImageLayer imageLayer;
};

Add const where appropriate.

Answer 2

I am not sure why you need shared_pointer for this, a simple unique_pointer would do nicely.

There are only two ways to handle run-time polymorphic objects in C++ - either through pointer or through reference. References do not seem to fit your design, so you are stuck with pointers. If you do not like pointers, rework the design to not use run-time polymorphism.