Original, non-working sketch of virtual classes base on examples in CaesarJ http://caesarj.org/index.php/ProgrammingGuide/VirtualClasses

gistfile1.cpp

#include <iostream>

using namespace std;

class Button
{
protected:
    class Border {
    public:
        using Color = int;
        virtual void draw() = 0;
        Color getBaseColor();
    };
    class FlatBorder : public Border {
    public:
        void draw() override {
            cout << "Button::FlatBorder::draw()" << endl;
        }
    };
    class PushedBorder : public Border {
    public:
        void draw() override {
            cout << "Button::PushedBorder::draw()" << endl;
        }
    };

private:
    unique_ptr<Border> border_;

    void setBorder(unique_ptr<Border> border) {
        border_ = std::move(border);
    }

public:
    void push() {
        setBorder(unique_ptr<Border>(new PushedBorder()));
    }
    void release() {
        setBorder(unique_ptr<Border>(new FlatBorder()));
    }
    void draw() {
        border_->draw();
    }
};

class MyButton : public Button
{
    class PushedBorder : public Border
    {
    public:
        void draw() override {
            cout << "MyButton::PushedBorder::draw()" << endl;
        }
    };    
};

int main()
{
    MyButton button;
    button.push();
    button.draw();// Should output MyButton::PushedBorder::draw()
}