70 lines
2.0 KiB
C
70 lines
2.0 KiB
C
|
/*
|
||
|
|
* Copyright 2015-present Facebook, Inc.
|
||
|
|
*
|
||
|
|
* 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;
|
||
|
|
};
|
||
|
|
template <typename T>
|
||
|
|
using lift_unit_t = typename lift_unit<T>::type;
|
||
|
|
|
||
|
|
template <typename T>
|
||
|
|
struct drop_unit {
|
||
|
|
using type = T;
|
||
|
|
};
|
||
|
|
template <>
|
||
|
|
struct drop_unit<Unit> {
|
||
|
|
using type = void;
|
||
|
|
};
|
||
|
|
template <typename T>
|
||
|
|
using drop_unit_t = typename drop_unit<T>::type;
|
||
|
|
|
||
|
|
} // namespace folly
|