Method chaining + inheritance don't play well together?

Question

Consider:

// member data omitted for brevity

// assume that \"setAngle\" needs to be implemented separately
// in Label and Image, and that Button does need         


        

Answer 1

This compiles on gcc 4.3.2 and is sort of a mixin pattern.

#include 

using namespace std;

struct Point {
    Point() : x(0), y(0) {}
    Point(int x, int y) : x(x), y(y) {}

    int x, y;
};

template 
struct Widget {
    T& move(Point newPos) {
        pos = newPos;
        return *reinterpret_cast (this);
    }

    Point pos;
};

template 
struct Label : Widget > {
    T& setText(string const& newText) {
        text = newText;
        return *reinterpret_cast (this);
    }
    T& setAngle(double newAngle) {
        angle = newAngle;
        return *reinterpret_cast (this);
    }

    string text;
    double angle;
};

struct Button : Label {
    Button& setAngle(double newAngle) {
        backgroundImage.setAngle(newAngle);
        Label::setAngle(newAngle);
        return *this;
    }

    Label backgroundImage;
};

int main() {
    Button btn;

    // oops: Widget::setText doesn't exist
    btn.move(Point(0,0)).setText("Hey");

    // oops: calling Label::setAngle rather than Button::setAngle
    btn.setText("Boo").setAngle(0.0); 
}