Returning a reference to the derived class from a base class method

Question

I have a task to implement a simple SVG generator. I need to support Circle, Polyline and Text. All three have at least 4 common methods: - SetStrokeColor - SetFillColor - SetStrokeWidth - ToString One of the main requirements is to support chaining, e.g.: Polyline{}.SetStrokeColor("white").SetFillColor("black")...

I decided to implement a base class Element, which all the other classes inherit from. The idea is to have a class Document that holds a vector of all the elements added to the document. A sample signature for a base method:

// source of trouble
Element &SetStrokeColor(const Color &color) {
    ...
    return *this;
}

My derived classes do call these methods, but the trouble is that the methods return a reference to the base class Element, not the derived class.

My question is whether it is all together possible to implement in c++???

Further discussion here

Answer 1

如果要共享实现并保留类型信息，则需要CRTP：

struct ElementBase { };

template <class Concrete>
struct Element : ElementBase {

    Concrete &setStrokeWidth(int width) {
        // Actual implementation...
        (void) width;

        return cthis();
    }

private:
    friend Concrete;
    Element() = default;

    Concrete &cthis() { return static_cast<Concrete &>(*this); }
    Concrete &cthis() const { return static_cast<Concrete const &>(*this); }
};

struct Circle : Element<Circle> {
    Circle &SetCircleCenter(int x, int y) {
        // Actual implementation...
        (void) x;
        (void) y;

        return *this;
    }
};

int main() {
    Circle c;
    c.setStrokeWidth(4).SetCircleCenter(0, 0);
}

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