From 43a97878ce14b72f0981164f87f2e35e14151312 Mon Sep 17 00:00:00 2001 From: Daniel Baumann Date: Sun, 7 Apr 2024 11:22:09 +0200 Subject: Adding upstream version 110.0.1. Signed-off-by: Daniel Baumann --- mfbt/InitializedOnce.h | 247 +++++++++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 247 insertions(+) create mode 100644 mfbt/InitializedOnce.h (limited to 'mfbt/InitializedOnce.h') diff --git a/mfbt/InitializedOnce.h b/mfbt/InitializedOnce.h new file mode 100644 index 0000000000..aac152df35 --- /dev/null +++ b/mfbt/InitializedOnce.h @@ -0,0 +1,247 @@ +/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */ +/* vim: set ts=8 sts=2 et sw=2 tw=80: */ +/* This Source Code Form is subject to the terms of the Mozilla Public + * License, v. 2.0. If a copy of the MPL was not distributed with this + * file, You can obtain one at http://mozilla.org/MPL/2.0/. */ + +// Class template for objects that can only be initialized once. + +#ifndef mozilla_mfbt_initializedonce_h__ +#define mozilla_mfbt_initializedonce_h__ + +#include "mozilla/Assertions.h" +#include "mozilla/Maybe.h" + +#include + +namespace mozilla { + +namespace detail { + +enum struct InitWhen { InConstructorOnly, LazyAllowed }; +enum struct DestroyWhen { EarlyAllowed, InDestructorOnly }; + +namespace ValueCheckPolicies { +template +struct AllowAnyValue { + constexpr static bool Check(const T& /*aValue*/) { return true; } +}; + +template +struct ConvertsToTrue { + constexpr static bool Check(const T& aValue) { + return static_cast(aValue); + } +}; +} // namespace ValueCheckPolicies + +// A kind of mozilla::Maybe that can only be initialized and cleared once. It +// cannot be re-initialized. This is a more stateful than a const Maybe in +// that it can be cleared, but much less stateful than a non-const Maybe +// which could be reinitialized multiple times. Can only be used with const T +// to ensure that the contents cannot be modified either. +// TODO: Make constructors constexpr when Maybe's constructors are constexpr +// (Bug 1601336). +template class ValueCheckPolicy = + ValueCheckPolicies::AllowAnyValue> +class InitializedOnce final { + static_assert(std::is_const_v); + using MaybeType = Maybe>; + + public: + using ValueType = T; + + template + explicit constexpr InitializedOnce( + std::enable_if_t* = + nullptr) {} + + // note: aArg0 is named separately here to disallow calling this with no + // arguments. The default constructor should only be available conditionally + // and is declared above. + template + explicit constexpr InitializedOnce(Arg0&& aArg0, Args&&... aArgs) + : mMaybe{Some(std::remove_const_t{std::forward(aArg0), + std::forward(aArgs)...})} { + MOZ_ASSERT(ValueCheckPolicy::Check(*mMaybe)); + } + + InitializedOnce(const InitializedOnce&) = delete; + InitializedOnce(InitializedOnce&& aOther) : mMaybe{std::move(aOther.mMaybe)} { + static_assert(DestroyWhenVal == DestroyWhen::EarlyAllowed); +#ifdef DEBUG + aOther.mWasReset = true; +#endif + } + InitializedOnce& operator=(const InitializedOnce&) = delete; + InitializedOnce& operator=(InitializedOnce&& aOther) { + static_assert(InitWhenVal == InitWhen::LazyAllowed && + DestroyWhenVal == DestroyWhen::EarlyAllowed); + MOZ_ASSERT(!mWasReset); + MOZ_ASSERT(!mMaybe); + mMaybe.~MaybeType(); + new (&mMaybe) MaybeType{std::move(aOther.mMaybe)}; +#ifdef DEBUG + aOther.mWasReset = true; +#endif + return *this; + } + + template + constexpr std::enable_if_t init( + Args&&... aArgs) { + MOZ_ASSERT(mMaybe.isNothing()); + MOZ_ASSERT(!mWasReset); + mMaybe.emplace(std::remove_const_t{std::forward(aArgs)...}); + MOZ_ASSERT(ValueCheckPolicy::Check(*mMaybe)); + } + + constexpr explicit operator bool() const { return isSome(); } + constexpr bool isSome() const { return mMaybe.isSome(); } + constexpr bool isNothing() const { return mMaybe.isNothing(); } + + constexpr T& operator*() const { return *mMaybe; } + constexpr T* operator->() const { return mMaybe.operator->(); } + + constexpr T& ref() const { return mMaybe.ref(); } + + template + std::enable_if_t + destroy() { + MOZ_ASSERT(mMaybe.isSome()); + maybeDestroy(); + } + + template + std::enable_if_t + maybeDestroy() { + mMaybe.reset(); +#ifdef DEBUG + mWasReset = true; +#endif + } + + template + std::enable_if_t + release() { + MOZ_ASSERT(mMaybe.isSome()); + auto res = std::move(mMaybe.ref()); + destroy(); + return res; + } + + private: + MaybeType mMaybe; +#ifdef DEBUG + bool mWasReset = false; +#endif +}; + +template class ValueCheckPolicy> +class LazyInitializer { + public: + explicit LazyInitializer(InitializedOnce& aLazyInitialized) + : mLazyInitialized{aLazyInitialized} {} + + template + LazyInitializer& operator=(U&& aValue) { + mLazyInitialized.init(std::forward(aValue)); + return *this; + } + + LazyInitializer(const LazyInitializer&) = delete; + LazyInitializer& operator=(const LazyInitializer&) = delete; + + private: + InitializedOnce& + mLazyInitialized; +}; + +} // namespace detail + +// The following *InitializedOnce* template aliases allow to declare class +// member variables that can only be initialized once, but maybe destroyed +// earlier explicitly than in the containing classes destructor. +// The intention is to restrict the possible state transitions for member +// variables that can almost be const, but not quite. This may be particularly +// useful for classes with a lot of members. Uses in other contexts, e.g. as +// local variables, are possible, but probably seldom useful. They can only be +// instantiated with a const element type. Any misuses that cannot be detected +// at compile time trigger a MOZ_ASSERT at runtime. Individually spelled out +// assertions for these aspects are not necessary, which may improve the +// readability of the code without impairing safety. +// +// The base variant InitializedOnce requires initialization in the constructor, +// but allows early destruction using destroy(), and allow move construction. It +// is similar to Maybe in some sense, but a Maybe could be +// reinitialized arbitrarily. InitializedOnce expresses the intent not to do +// this, and prohibits reinitialization. +// +// The Lazy* variants allow default construction, and can be initialized lazily +// using init() in that case, but it cannot be reinitialized either. They do not +// allow early destruction. +// +// The Lazy*EarlyDestructible variants allow lazy initialization, early +// destruction, move construction and move assignment. This should be used only +// when really required. +// +// The *NotNull variants only allow initialization with values that convert to +// bool as true. They are named NotNull because the typical use case is with +// (smart) pointer types, but any other type convertible to bool will also work +// analogously. +// +// There is no variant combining detail::DestroyWhen::InConstructorOnly with +// detail::DestroyWhen::InDestructorOnly because this would be equivalent to a +// const member. +// +// For special cases, e.g. requiring custom value check policies, +// detail::InitializedOnce might be instantiated directly, but be mindful when +// doing this. + +template +using InitializedOnce = + detail::InitializedOnce; + +template +using InitializedOnceNotNull = + detail::InitializedOnce; + +template +using LazyInitializedOnce = + detail::InitializedOnce; + +template +using LazyInitializedOnceNotNull = + detail::InitializedOnce; + +template +using LazyInitializedOnceEarlyDestructible = + detail::InitializedOnce; + +template +using LazyInitializedOnceNotNullEarlyDestructible = + detail::InitializedOnce; + +template class ValueCheckPolicy> +auto do_Init(detail::InitializedOnce& aLazyInitialized) { + return detail::LazyInitializer(aLazyInitialized); +} + +} // namespace mozilla + +#endif -- cgit v1.2.3