gistfile1.cpp

class Subroutine : public Value {
public:
    template<typename F>
    explicit Subroutine(F slow)
        : slow(std::move(slow)), fast(), fastAvailable(false) { }

    boost::intrusive_ptr<Value> operator()(VM& vm, std::size_t argc, Value** argv) {
        if (fastAvailable) {
            return fast(vm, argc, argv);
        } else {
            return slow(vm, argc, argv);
        }
    }

    std::function<boost::intrusive_ptr<Value>(VM&, std::size_t, Value**)> slow;
    std::function<boost::intrusive_ptr<Value>(VM&, std::size_t, Value**)> fast;
    std::atomic<bool> fastAvailable;
};