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#ifndef ENTT_CORE_UTILITY_HPP
#define ENTT_CORE_UTILITY_HPP
#include <type_traits>
#include <utility>
namespace entt {
/*! @brief Identity function object (waiting for C++20). */
struct identity {
/*! @brief Indicates that this is a transparent function object. */
using is_transparent = void;
/**
* @brief Returns its argument unchanged.
* @tparam Type Type of the argument.
* @param value The actual argument.
* @return The submitted value as-is.
*/
template<typename Type>
[[nodiscard]] constexpr Type &&operator()(Type &&value) const noexcept {
return std::forward<Type>(value);
}
};
/**
* @brief Constant utility to disambiguate overloaded members of a class.
* @tparam Type Type of the desired overload.
* @tparam Class Type of class to which the member belongs.
* @param member A valid pointer to a member.
* @return Pointer to the member.
*/
template<typename Type, typename Class>
[[nodiscard]] constexpr auto overload(Type Class::*member) noexcept {
return member;
}
/**
* @brief Constant utility to disambiguate overloaded functions.
* @tparam Func Function type of the desired overload.
* @param func A valid pointer to a function.
* @return Pointer to the function.
*/
template<typename Func>
[[nodiscard]] constexpr auto overload(Func *func) noexcept {
return func;
}
/**
* @brief Helper type for visitors.
* @tparam Func Types of function objects.
*/
template<typename... Func>
struct overloaded: Func... {
using Func::operator()...;
};
/**
* @brief Deduction guide.
* @tparam Func Types of function objects.
*/
template<typename... Func>
overloaded(Func...) -> overloaded<Func...>;
/**
* @brief Basic implementation of a y-combinator.
* @tparam Func Type of a potentially recursive function.
*/
template<typename Func>
struct y_combinator {
/**
* @brief Constructs a y-combinator from a given function.
* @param recursive A potentially recursive function.
*/
constexpr y_combinator(Func recursive) noexcept(std::is_nothrow_move_constructible_v<Func>)
: func{std::move(recursive)} {}
/**
* @brief Invokes a y-combinator and therefore its underlying function.
* @tparam Args Types of arguments to use to invoke the underlying function.
* @param args Parameters to use to invoke the underlying function.
* @return Return value of the underlying function, if any.
*/
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)...);
}
/*! @copydoc operator()() */
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
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