Class-Filter

---

by: hazurl & Cxx

#include <iostream>

#include <tuple>
#include <utility>
#include <type_traits>

struct base {
    int value;

    // Value is transformed in some way and returned.
    auto get() const {
        return value * 2;
    }
};

struct getter : base {
    using tag = struct getter_tag{};
};

struct setter : base {
    using tag = struct setter_tag{};
};

template <class... types>
class request
{
    private:
        std::tuple <types...> t;

        template <class Tag, class Type>
        static decltype(auto) get_element(Type&& s) {
            if constexpr (std::is_same <Tag, getter::tag>::value) {
                return std::tuple(std::forward <Type>(s));
            }

            else {
                return std::tuple <>();
            }
        }

    public:
        request(types&&... args) : t(std::forward <types>(args)...) {
        }

        decltype(auto) get() const && {
            return std::apply([](types... args) {
                return std::tuple_cat(get_element <typename types::tag> (std::move(args.get()))...);
            }, t);
        }
};

int main(int argc, const char* argv[]) {
    auto &&[a, b] = request{
        getter{2},
        setter{1}, // This should not be returned.
        getter{5}
    }.get();

    std::cout << a << '\n'; // Should output 4 .
    std::cout << b << '\n'; // Should output 10.
    return 0;
}

1: hazurl - https://godbolt.org/z/l9qP83
2: Cxx - https://godbolt.org/z/w5rZ0e
3: Cxx - https://godbolt.org/z/VKVvHl

License

This project is licensed under the MIT License. See the LICENSE file for more details.