struct drawable {};

struct texture {};

template<typename...>
struct void_ { using type = void; };

template<typename T, typename = void>
struct has_callback : std::false_type {};
template<typename T>
struct has_callback<T, typename void_<decltype(std::declval<T>().callback())>::type> : std::true_type {};

struct test {
    template<typename Drawable, typename std::enable_if<has_callback<Drawable>::value, int>::type = 0>
    void do_register(const Drawable& t) {
        // register stuff's texture
        // ...
        // after registering, do a callback since it exists
        t.callback();
    }

    template<typename Drawable, typename std::enable_if<!has_callback<Drawable>::value, int>::type = 0>
    void do_register(const Drawable& t) {
        // register the stuff's texture
        // ...
        // after registering, the callback doesn't
        // exist so don't use it
    }

    void do_register(const texture& t) {
        // register texture
        // no callback
    }
};