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67 lines
2.4 KiB
C++
67 lines
2.4 KiB
C++
/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
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/* vim: set ts=8 sts=2 et sw=2 tw=80: */
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/* This Source Code Form is subject to the terms of the Mozilla Public
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* License, v. 2.0. If a copy of the MPL was not distributed with this file,
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* You can obtain one at http://mozilla.org/MPL/2.0/. */
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#ifndef mozilla_NeverDestroyed_h
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#define mozilla_NeverDestroyed_h
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#include <new>
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#include <type_traits>
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#include <utility>
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#include "mozilla/Attributes.h"
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namespace mozilla {
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// Helper type for creating a local static member of type `T` when `T` has a
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// non-trivial static destructor. When used for the local static value, this
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// type will avoid introducing a static destructor for these types, as they
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// will survive until shutdown.
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//
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// This can be very useful to avoid static destructors, which are heavily
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// discouraged. Using this type is unnecessary if `T` already has a trivial
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// destructor, and may introduce unnecessary extra overhead.
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//
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// This type must only be used with static local members within a function,
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// which will be enforced by the clang static analysis.
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template <typename T>
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class MOZ_STATIC_LOCAL_CLASS MOZ_INHERIT_TYPE_ANNOTATIONS_FROM_TEMPLATE_ARGS
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NeverDestroyed {
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public:
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static_assert(
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!std::is_trivially_destructible_v<T>,
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"NeverDestroyed is unnecessary for trivially destructable types");
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// Allow constructing the inner type.
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// This isn't constexpr, as it requires invoking placement-new. See the
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// comment on `mStorage`.
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template <typename... U>
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explicit NeverDestroyed(U&&... aArgs) {
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new (mStorage) T(std::forward<U>(aArgs)...);
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}
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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<T*>(mStorage); }
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T* get() { return reinterpret_cast<T*>(mStorage); }
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// Block copy & move constructor, as the type is not safe to copy.
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NeverDestroyed(const NeverDestroyed&) = delete;
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NeverDestroyed& operator=(const NeverDestroyed&) = delete;
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private:
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// Correctly aligned storage for the type. We unfortunately can't use a union
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// for alignment & constexpr initialization as that would require an explicit
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// destructor declaration, making `NeverDestroyed` non-trivially destructable.
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alignas(T) char mStorage[sizeof(T)];
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};
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}; // namespace mozilla
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#endif // mozilla_NeverDestroyed_h
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