utility.hpp

#ifndef ENTT_CORE_UTILITY_HPP
#define ENTT_CORE_UTILITY_HPP
 
#include <type_traits>
#include <utility>
 
namespace entt {

struct identity {
    using is_transparent = void;

    template<typename Type>
    [[nodiscard]] constexpr Type &&operator()(Type &&value) const noexcept {
        return std::forward<Type>(value);
    }
};

template<typename Type, typename Class>
[[nodiscard]] constexpr auto overload(Type Class::*member) noexcept {
    return member;
}

template<typename Func>
[[nodiscard]] constexpr auto overload(Func *func) noexcept {
    return func;
}

template<typename... Func>
struct overloaded: Func... {
    using Func::operator()...;
};

template<typename... Func>
overloaded(Func...) -> overloaded<Func...>;

template<typename Func>
struct y_combinator {
    constexpr y_combinator(Func recursive) noexcept(std::is_nothrow_move_constructible_v<Func>)
        : func{std::move(recursive)} {}

    template<typename... Args>
    constexpr decltype(auto) operator()(Args &&...args) const noexcept(std::is_nothrow_invocable_v<Func, const y_combinator &, Args...>) {
        return func(*this, std::forward<Args>(args)...);
    }

    template<typename... Args>
    constexpr decltype(auto) operator()(Args &&...args) noexcept(std::is_nothrow_invocable_v<Func, y_combinator &, Args...>) {
        return func(*this, std::forward<Args>(args)...);
    }
 
private:
    Func func;
};
 
} // namespace entt
 
#endif