зеркало из https://github.com/mozilla/gecko-dev.git
108 строки
3.4 KiB
C++
108 строки
3.4 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
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* file, You can obtain one at http://mozilla.org/MPL/2.0/. */
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#ifndef mozilla_dom_indexeddb_initializedonce_h__
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#define mozilla_dom_indexeddb_initializedonce_h__
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#include "mozilla/Assertions.h"
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#include "mozilla/Maybe.h"
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#include <type_traits>
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// This file is not exported and is only meant to be included in IndexedDB
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// source files.
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// TODO: However, it is meant to be moved to mfbt, and therefore already placed
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// in the mozilla namespace.
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namespace mozilla {
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enum struct LazyInit { Allow, Forbid };
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namespace ValueCheckPolicies {
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template <typename T>
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struct AllowAnyValue {
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constexpr static bool Check(const T& /*aValue*/) { return true; }
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};
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template <typename T>
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struct ConvertsToTrue {
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constexpr static bool Check(const T& aValue) {
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return static_cast<bool>(aValue);
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}
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};
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} // namespace ValueCheckPolicies
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// A kind of mozilla::Maybe that can only be initialized and cleared once. It
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// cannot be re-initialized. This is a more stateful than a const Maybe<T> in
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// that it can be cleared, but much less stateful than a non-const Maybe<T>
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// which could be reinitialized multiple times. Can only be used with const T
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// to ensure that the contents cannot be modified either.
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// TODO: Make constructors constexpr when Maybe's constructors are constexpr
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// (Bug 1601336).
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template <typename T, LazyInit LazyInit = LazyInit::Forbid,
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template <typename> class ValueCheckPolicy =
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ValueCheckPolicies::AllowAnyValue>
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class InitializedOnce final {
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static_assert(std::is_const_v<T>);
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public:
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template <typename Dummy = void>
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explicit InitializedOnce(
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std::enable_if_t<LazyInit == LazyInit::Allow, Dummy>* = nullptr) {}
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template <typename... Args>
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explicit InitializedOnce(Args&&... aArgs)
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: mMaybe{Some(T{std::forward<Args>(aArgs)...})} {
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MOZ_ASSERT(ValueCheckPolicy<T>::Check(*mMaybe));
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}
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InitializedOnce(const InitializedOnce&) = delete;
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InitializedOnce(InitializedOnce&&) = default;
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InitializedOnce& operator=(const InitializedOnce&) = delete;
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InitializedOnce& operator=(InitializedOnce&&) = delete;
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template <typename... Args, typename Dummy = void>
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std::enable_if_t<LazyInit == LazyInit::Allow, Dummy> init(Args&&... aArgs) {
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MOZ_ASSERT(mMaybe.isNothing());
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MOZ_ASSERT(!mWasReset);
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mMaybe.emplace(T{std::forward<Args>(aArgs)...});
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MOZ_ASSERT(ValueCheckPolicy<T>::Check(*mMaybe));
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}
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explicit operator bool() const { return isSome(); }
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bool isSome() const { return mMaybe.isSome(); }
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bool isNothing() const { return mMaybe.isNothing(); }
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T& operator*() { return *mMaybe; }
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T* operator->() { return mMaybe.operator->(); }
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std::add_const_t<T>& operator*() const { return *mMaybe; }
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std::add_const_t<T>* operator->() const { return mMaybe.operator->(); }
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void reset() {
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MOZ_ASSERT(mMaybe.isSome());
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mMaybe.reset();
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#ifdef DEBUG
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mWasReset = true;
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#endif
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}
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private:
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Maybe<T> mMaybe;
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#ifdef DEBUG
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bool mWasReset = false;
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#endif
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};
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template <typename T, LazyInit LazyInit = LazyInit::Forbid>
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using InitializedOnceMustBeTrue =
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InitializedOnce<T, LazyInit, ValueCheckPolicies::ConvertsToTrue>;
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} // namespace mozilla
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#endif
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