95 lines
3.1 KiB
C
95 lines
3.1 KiB
C
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#pragma once
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/*
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* Copyright (C) 2019 The Android Open Source Project
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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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#include <utility>
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#include "android-base/macros.h"
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namespace android {
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namespace base {
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// A wrapper that makes it easy to create an object of type T with static
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// storage duration that:
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// - is only constructed on first access
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// - never invokes the destructor
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// in order to satisfy the styleguide ban on global constructors and
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// destructors.
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//
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// Runtime constant example:
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// const std::string& GetLineSeparator() {
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// // Forwards to std::string(size_t, char, const Allocator&) constructor.
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// static const base::NoDestructor<std::string> s(5, '-');
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// return *s;
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// }
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//
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// More complex initialization with a lambda:
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// const std::string& GetSessionNonce() {
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// static const base::NoDestructor<std::string> nonce([] {
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// std::string s(16);
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// crypto::RandString(s.data(), s.size());
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// return s;
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// }());
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// return *nonce;
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// }
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//
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// NoDestructor<T> stores the object inline, so it also avoids a pointer
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// indirection and a malloc. Also note that since C++11 static local variable
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// initialization is thread-safe and so is this pattern. Code should prefer to
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// use NoDestructor<T> over:
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// - A function scoped static T* or T& that is dynamically initialized.
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// - A global base::LazyInstance<T>.
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//
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// Note that since the destructor is never run, this *will* leak memory if used
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// as a stack or member variable. Furthermore, a NoDestructor<T> should never
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// have global scope as that may require a static initializer.
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template <typename T>
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class NoDestructor {
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public:
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// Not constexpr; just write static constexpr T x = ...; if the value should
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// be a constexpr.
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template <typename... Args>
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explicit NoDestructor(Args&&... args) {
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new (storage_) T(std::forward<Args>(args)...);
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}
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// Allows copy and move construction of the contained type, to allow
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// construction from an initializer list, e.g. for std::vector.
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explicit NoDestructor(const T& x) { new (storage_) T(x); }
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explicit NoDestructor(T&& x) { new (storage_) T(std::move(x)); }
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NoDestructor(const NoDestructor&) = delete;
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NoDestructor& operator=(const NoDestructor&) = delete;
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~NoDestructor() = default;
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const T& operator*() const { return *get(); }
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T& operator*() { return *get(); }
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const T* operator->() const { return get(); }
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T* operator->() { return get(); }
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const T* get() const { return reinterpret_cast<const T*>(storage_); }
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T* get() { return reinterpret_cast<T*>(storage_); }
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private:
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alignas(T) char storage_[sizeof(T)];
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};
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} // namespace base
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} // namespace android
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