te_draw.cpp

/*
* from discussion https://twitter.com/krisjusiak/status/981505271796256768
*/

#include <memory>
#include <type_traits>
#include <utility>

volatile int state = 0;

class Drawable {
    struct concept {
        virtual ~concept() = default;
        virtual void draw() const = 0;
    };

    template <class T>
    struct model final : concept {
        T _instance;
        template <class U>
        model(U&& x) : _instance{std::forward<U>(x)} {}
        void draw() const override { _instance.draw(); }
    };

    std::unique_ptr<concept> _self;

public:
    template <class T>
    Drawable(T&& x) : _self{std::make_unique<model<std::decay_t<T>>>(std::forward<T>(x))} {}
    void draw() const { _self->draw(); }
};

struct Square {
    void draw() const { state = 1; }
};

struct Circle {
    void draw() const { state = 2; }
};

const auto draw = [](auto const& drawable) { drawable.draw(); };

int main() {
    Drawable d0{Circle{}};
    Drawable d1{Square{}};

    d0.draw();
    d1.draw();
}