You can not select more than 25 topics
Topics must start with a letter or number, can include dashes ('-') and can be up to 35 characters long.
95 lines
3.1 KiB
95 lines
3.1 KiB
4 months ago
|
#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
|