verdnatura-chat/ios/Pods/Folly/folly/Indestructible.h

178 lines
5.5 KiB
C++

/*
* 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 <cassert>
#include <type_traits>
#include <utility>
#include <folly/Traits.h>
namespace folly {
/***
* Indestructible
*
* When you need a Meyers singleton that will not get destructed, even at
* shutdown, and you also want the object stored inline.
*
* Use like:
*
* void doSomethingWithExpensiveData();
*
* void doSomethingWithExpensiveData() {
* static const Indestructible<map<string, int>> data{
* map<string, int>{{"key1", 17}, {"key2", 19}, {"key3", 23}},
* };
* callSomethingTakingAMapByRef(*data);
* }
*
* This should be used only for Meyers singletons, and, even then, only when
* the instance does not need to be destructed ever.
*
* This should not be used more generally, e.g., as member fields, etc.
*
* This is designed as an alternative, but with one fewer allocation at
* construction time and one fewer pointer dereference at access time, to the
* Meyers singleton pattern of:
*
* void doSomethingWithExpensiveData() {
* static const auto data = // never `delete`d
* new map<string, int>{{"key1", 17}, {"key2", 19}, {"key3", 23}};
* callSomethingTakingAMapByRef(*data);
* }
*/
template <typename T>
class Indestructible final {
public:
template <typename S = T, typename = decltype(S())>
constexpr Indestructible() noexcept(noexcept(T())) {}
/**
* Constructor accepting a single argument by forwarding reference, this
* allows using list initialzation without the overhead of things like
* in_place, etc and also works with std::initializer_list constructors
* which can't be deduced, the default parameter helps there.
*
* auto i = folly::Indestructible<std::map<int, int>>{{{1, 2}}};
*
* This provides convenience
*
* There are two versions of this constructor - one for when the element is
* implicitly constructible from the given argument and one for when the
* type is explicitly but not implicitly constructible from the given
* argument.
*/
template <
typename U = T,
std::enable_if_t<std::is_constructible<T, U&&>::value>* = nullptr,
std::enable_if_t<
!std::is_same<Indestructible<T>, remove_cvref_t<U>>::value>* =
nullptr,
std::enable_if_t<!std::is_convertible<U&&, T>::value>* = nullptr>
explicit constexpr Indestructible(U&& u) noexcept(
noexcept(T(std::declval<U>())))
: storage_(std::forward<U>(u)) {}
template <
typename U = T,
std::enable_if_t<std::is_constructible<T, U&&>::value>* = nullptr,
std::enable_if_t<
!std::is_same<Indestructible<T>, remove_cvref_t<U>>::value>* =
nullptr,
std::enable_if_t<std::is_convertible<U&&, T>::value>* = nullptr>
/* implicit */ constexpr Indestructible(U&& u) noexcept(
noexcept(T(std::declval<U>())))
: storage_(std::forward<U>(u)) {}
template <typename... Args, typename = decltype(T(std::declval<Args>()...))>
explicit constexpr Indestructible(Args&&... args) noexcept(
noexcept(T(std::declval<Args>()...)))
: storage_(std::forward<Args>(args)...) {}
template <
typename U,
typename... Args,
typename = decltype(
T(std::declval<std::initializer_list<U>&>(),
std::declval<Args>()...))>
explicit constexpr Indestructible(std::initializer_list<U> il, Args... args) noexcept(
noexcept(
T(std::declval<std::initializer_list<U>&>(),
std::declval<Args>()...)))
: storage_(il, std::forward<Args>(args)...) {}
~Indestructible() = default;
Indestructible(Indestructible const&) = delete;
Indestructible& operator=(Indestructible const&) = delete;
Indestructible(Indestructible&& other) noexcept(
noexcept(T(std::declval<T>())))
: storage_(std::move(other.storage_.value)) {
other.erased_ = true;
}
Indestructible& operator=(Indestructible&& other) noexcept(
noexcept(T(std::declval<T>()))) {
storage_.value = std::move(other.storage_.value);
other.erased_ = true;
}
T* get() noexcept {
check();
return &storage_.value;
}
T const* get() const noexcept {
check();
return &storage_.value;
}
T& operator*() noexcept {
return *get();
}
T const& operator*() const noexcept {
return *get();
}
T* operator->() noexcept {
return get();
}
T const* operator->() const noexcept {
return get();
}
private:
void check() const noexcept {
assert(!erased_);
}
union Storage {
T value;
template <typename S = T, typename = decltype(S())>
constexpr Storage() noexcept(noexcept(T())) : value() {}
template <typename... Args, typename = decltype(T(std::declval<Args>()...))>
explicit constexpr Storage(Args&&... args) noexcept(
noexcept(T(std::declval<Args>()...)))
: value(std::forward<Args>(args)...) {}
~Storage() {}
};
Storage storage_{};
bool erased_{false};
};
} // namespace folly