| // Copyright 2018 The Chromium Authors. All rights reserved. |
| // Use of this source code is governed by a BSD-style license that can be |
| // found in the LICENSE file. |
| |
| #ifndef BASE_NO_DESTRUCTOR_H_ |
| #define BASE_NO_DESTRUCTOR_H_ |
| |
| #include <utility> |
| |
| 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. Code should prefer to use NoDestructor<T> over: |
| // - The CR_DEFINE_STATIC_LOCAL() helper macro. |
| // - 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 (get()) T(std::forward<Args>(args)...); |
| } |
| |
| ~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 |
| |
| #endif // BASE_NO_DESTRUCTOR_H_ |