/*
* Copyright (c) Facebook, Inc. and its affiliates.
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
* http://www.apache.org/licenses/LICENSE-2.0
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
#pragma once
#include <type_traits>
namespace folly {
/// In functional programming, the degenerate case is often called "unit". In
/// C++, "void" is often the best analogue. However, because of the syntactic
/// special-casing required for void, it is frequently a liability for template
/// metaprogramming. So, instead of writing specializations to handle cases like
/// SomeContainer<void>, a library author may instead rule that out and simply
/// have library users use SomeContainer<Unit>. Contained values may be ignored.
/// Much easier.
///
/// "void" is the type that admits of no values at all. It is not possible to
/// construct a value of this type.
/// "unit" is the type that admits of precisely one unique value. It is
/// possible to construct a value of this type, but it is always the same value
/// every time, so it is uninteresting.
struct Unit {
constexpr bool operator==(const Unit& /*other*/) const {
return true;
}
constexpr bool operator!=(const Unit& /*other*/) const {
return false;
};
constexpr Unit unit{};
template <typename T>
struct lift_unit {
using type = T;
template <>
struct lift_unit<void> {
using type = Unit;
using lift_unit_t = typename lift_unit<T>::type;
struct drop_unit {
struct drop_unit<Unit> {
using type = void;
using drop_unit_t = typename drop_unit<T>::type;
} // namespace folly