| /* |
| * Copyright 2017 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 { |
| // These are structs rather than type aliases because MSVC 2017 RC has |
| // trouble correctly resolving dependent expressions in type aliases |
| // in certain very specific contexts, including a couple where this is |
| // used. See the known issues section here for more info: |
| // https://blogs.msdn.microsoft.com/vcblog/2016/06/07/expression-sfinae-improvements-in-vs-2015-update-3/ |
| |
| template <typename T> |
| struct Lift : std::conditional<std::is_same<T, void>::value, Unit, T> {}; |
| template <typename T> |
| struct Drop : std::conditional<std::is_same<T, Unit>::value, void, T> {}; |
| |
| constexpr bool operator==(const Unit& /*other*/) const { |
| return true; |
| } |
| constexpr bool operator!=(const Unit& /*other*/) const { |
| return false; |
| } |
| }; |
| |
| constexpr Unit unit {}; |
| |
| } |