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