70 lines
2.0 KiB
C++
70 lines
2.0 KiB
C++
/*
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* Copyright 2015-present Facebook, Inc.
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*
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* Licensed under the Apache License, Version 2.0 (the "License");
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* you may not use this file except in compliance with the License.
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* You may obtain a copy of the License at
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*
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* http://www.apache.org/licenses/LICENSE-2.0
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*
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* Unless required by applicable law or agreed to in writing, software
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* distributed under the License is distributed on an "AS IS" BASIS,
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* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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* See the License for the specific language governing permissions and
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* limitations under the License.
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*/
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#pragma once
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#include <type_traits>
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namespace folly {
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/// In functional programming, the degenerate case is often called "unit". In
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/// C++, "void" is often the best analogue. However, because of the syntactic
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/// special-casing required for void, it is frequently a liability for template
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/// metaprogramming. So, instead of writing specializations to handle cases like
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/// SomeContainer<void>, a library author may instead rule that out and simply
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/// have library users use SomeContainer<Unit>. Contained values may be ignored.
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/// Much easier.
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///
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/// "void" is the type that admits of no values at all. It is not possible to
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/// construct a value of this type.
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/// "unit" is the type that admits of precisely one unique value. It is
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/// possible to construct a value of this type, but it is always the same value
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/// every time, so it is uninteresting.
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struct Unit {
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constexpr bool operator==(const Unit& /*other*/) const {
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return true;
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}
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constexpr bool operator!=(const Unit& /*other*/) const {
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return false;
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}
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};
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constexpr Unit unit{};
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template <typename T>
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struct lift_unit {
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using type = T;
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};
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template <>
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struct lift_unit<void> {
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using type = Unit;
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};
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template <typename T>
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using lift_unit_t = typename lift_unit<T>::type;
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template <typename T>
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struct drop_unit {
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using type = T;
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};
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template <>
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struct drop_unit<Unit> {
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using type = void;
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};
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template <typename T>
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using drop_unit_t = typename drop_unit<T>::type;
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} // namespace folly
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