unique_function.h

/*
This file is part of Telegram Desktop,
the official desktop application for the Telegram messaging service.

For license and copyright information please follow this link:
https://github.com/telegramdesktop/tdesktop/blob/master/LEGAL
*/
#pragma once

#include <functional>

#ifndef Unexpected
#define Unexpected(message) std::abort()
#define UniqueFunctionUnexpected
#endif // Unexpected

namespace base {
namespace details {

template <typename Callable>
class moveable_callable_wrap {
public:
	static_assert(
		std::is_move_constructible_v<Callable>,
		"Should be at least moveable.");

	moveable_callable_wrap(Callable &&other)
	: _value(std::move(other)) {
	}
	moveable_callable_wrap &operator=(Callable &&other) {
		_value = std::move(other);
		return *this;
	}
	moveable_callable_wrap(moveable_callable_wrap &&other)
	: _value(std::move(other._value)) {
	}
	moveable_callable_wrap(
			const moveable_callable_wrap &other)
	: _value(fail_construct()) {
	}
	moveable_callable_wrap &operator=(
			moveable_callable_wrap &&other) {
		_value = std::move(other._value);
		return *this;
	}
	moveable_callable_wrap &operator=(
			const moveable_callable_wrap &other) {
		return fail_assign();
	}

	template <typename ...Args>
	decltype(auto) operator()(Args &&...args) {
		return _value(std::forward<Args>(args)...);
	}

private:
	[[noreturn]] Callable fail_construct() {
		Unexpected("Attempt to copy-construct a move-only type.");
	}
	[[noreturn]] moveable_callable_wrap &fail_assign() {
		Unexpected("Attempt to copy-assign a move-only type.");
	}

	Callable _value;

};

} // namespace details

template <typename Function>
class unique_function;

template <typename Return, typename ...Args>
class unique_function<Return(Args...)> final {
public:
	unique_function(std::nullptr_t = nullptr) noexcept {
	}
	unique_function(const unique_function &other) = delete;
	unique_function &operator=(const unique_function &other) = delete;

	// Move construct / assign from the same type.
	unique_function(unique_function &&other)
	: _impl(std::move(other._impl)) {
	}
	unique_function &operator=(unique_function &&other) {
		_impl = std::move(other._impl);
		return *this;
	}

	template <
		typename Callable,
		typename = std::enable_if_t<
			std::is_convertible_v<
				decltype(std::declval<Callable>()(
					std::declval<Args>()...)),
				Return>>>
	unique_function(Callable &&other)
	: unique_function(
		std::forward<Callable>(other),
		std::is_copy_constructible<std::decay_t<Callable>>{}) {
	}

	template <
		typename Callable,
		typename = std::enable_if_t<
			std::is_convertible_v<
				decltype(std::declval<Callable>()(
					std::declval<Args>()...)),
				Return>>>
	unique_function &operator=(Callable &&other) {
		using Decayed = std::decay_t<Callable>;
		if constexpr (std::is_copy_constructible_v<Decayed>) {
			_impl = std::forward<Callable>(other);
		} else if constexpr (std::is_move_constructible_v<Decayed>) {
			_impl = details::moveable_callable_wrap<Decayed>(
				std::forward<Callable>(other));
		} else {
			static_assert(false_t(other), "Should be moveable.");
		}
		return *this;
	}

	void swap(unique_function &other) {
		_impl.swap(other._impl);
	}

	Return operator()(Args ...args) {
		return _impl(std::forward<Args>(args)...);
	}

	explicit operator bool() const {
		return _impl.operator bool();
	}

	friend inline bool operator==(
			const unique_function &value,
			std::nullptr_t) noexcept {
		return value._impl == nullptr;
	}
	friend inline bool operator==(
			std::nullptr_t,
			const unique_function &value) noexcept {
		return value._impl == nullptr;
	}
	friend inline bool operator!=(
			const unique_function &value,
			std::nullptr_t) noexcept {
		return value._impl != nullptr;
	}
	friend inline bool operator!=(
			std::nullptr_t,
			const unique_function &value) noexcept {
		return value._impl != nullptr;
	}

private:
	template <typename Callable>
	unique_function(Callable &&other, std::true_type)
	: _impl(std::forward<Callable>(other)) {
	}

	template <typename Callable>
	unique_function(Callable &&other, std::false_type)
	: _impl(details::moveable_callable_wrap<std::decay_t<Callable>>(
		std::forward<Callable>(other))) {
	}

	std::function<Return(Args...)> _impl;

};

} // namespace base

#ifdef UniqueFunctionUnexpected
#undef UniqueFunctionUnexpected
#undef Unexpected
#endif // UniqueFunctionUnexpectedb