If the intended modified Button will continue to be a Ctrl derivative so that it works seamlessly with existing code, the minimum size it will have is sizeof(Ctrl). In this particular situation, the Ctrl member variable LogPos(and maybe more candidates) can be done without, we can reuse it to put a Style * st; and probably WhenPush event variable somewhere else. In any case, the minimum size will be sizeof(Ctrl). The savings won't be quite as good (significantly less than the case when buttons or even frames are ridden of), even though it indeed involves least work and least chance of surprise.

Another way is to define it something like

class FakeButton 
{
public:
   void Paint(Draw& w, Rect& where, Button::Style& style, int state/*normal,hot,pressed,disabled*/);

   Event<> WhenPush;
};

This way sizeof(FakeButton)==sizeof(Event<>)(==sizeof(void*) ), but the effort involved in revising containing Ctrl (ScrollBar, EditWithSpin, DropList, etc)'s Paint,LeftDown,LeftRepeat, etc, will be quite similar to hardcoding each of the Ctrl's method that we need to change to allow the savings to take place.

class Buttons : Ctrl {
   virtual int  GetButtonCount() = 0;
   virtual Rect GetButtonRectint i) = 0;
   virtual int  ButtonMouseDown(Point p);
   virtual int  ButtonMouseUp(Point p);
   virtual int  ButtonMouseMove(Point p);
}