зеркало из https://github.com/mozilla/gecko-dev.git
366 строки
12 KiB
C++
366 строки
12 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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/* A class for optional values and in-place lazy construction. */
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#ifndef mozilla_Maybe_h
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#define mozilla_Maybe_h
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#include "mozilla/Alignment.h"
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#include "mozilla/Assertions.h"
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#include "mozilla/Move.h"
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#include "mozilla/TypeTraits.h"
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#include <new> // for placement new
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namespace mozilla {
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struct Nothing { };
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/*
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* Maybe is a container class which contains either zero or one elements. It
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* serves two roles. It can represent values which are *semantically* optional,
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* augmenting a type with an explicit 'Nothing' value. In this role, it provides
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* methods that make it easy to work with values that may be missing, along with
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* equality and comparison operators so that Maybe values can be stored in
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* containers. Maybe values can be constructed conveniently in expressions using
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* type inference, as follows:
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*
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* void doSomething(Maybe<Foo> aFoo) {
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* if (aFoo) // Make sure that aFoo contains a value...
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* aFoo->takeAction(); // and then use |aFoo->| to access it.
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* } // |*aFoo| also works!
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*
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* doSomething(Nothing()); // Passes a Maybe<Foo> containing no value.
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* doSomething(Some(Foo(100))); // Passes a Maybe<Foo> containing |Foo(100)|.
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*
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* You'll note that it's important to check whether a Maybe contains a value
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* before using it, using conversion to bool, |isSome()|, or |isNothing()|. You
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* can avoid these checks, and sometimes write more readable code, using
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* |valueOr()|, |ptrOr()|, and |refOr()|, which allow you to retrieve the value
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* in the Maybe and provide a default for the 'Nothing' case. You can also use
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* |apply()| to call a function only if the Maybe holds a value, and |map()| to
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* transform the value in the Maybe, returning another Maybe with a possibly
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* different type.
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*
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* Maybe's other role is to support lazily constructing objects without using
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* dynamic storage. A Maybe directly contains storage for a value, but it's
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* empty by default. |emplace()|, as mentioned above, can be used to construct a
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* value in Maybe's storage. The value a Maybe contains can be destroyed by
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* calling |reset()|; this will happen automatically if a Maybe is destroyed
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* while holding a value.
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*
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* It's a common idiom in C++ to use a pointer as a 'Maybe' type, with a null
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* value meaning 'Nothing' and any other value meaning 'Some'. You can convert
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* from such a pointer to a Maybe value using 'ToMaybe()'.
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*
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* Maybe is inspired by similar types in the standard library of many other
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* languages (e.g. Haskell's Maybe and Rust's Option). In the C++ world it's
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* very similar to std::optional, which was proposed for C++14 and originated in
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* Boost. The most important differences between Maybe and std::optional are:
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*
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* - std::optional<T> may be compared with T. We deliberately forbid that.
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* - std::optional allows in-place construction without a separate call to
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* |emplace()| by using a dummy |in_place_t| value to tag the appropriate
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* constructor.
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* - std::optional has |valueOr()|, equivalent to Maybe's |valueOr()|, but
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* lacks corresponding methods for |refOr()| and |ptrOr()|.
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* - std::optional lacks |map()| and |apply()|, making it less suitable for
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* functional-style code.
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* - std::optional lacks many convenience functions that Maybe has. Most
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* unfortunately, it lacks equivalents of the type-inferred constructor
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* functions |Some()| and |Nothing()|.
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*
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* N.B. GCC has missed optimizations with Maybe in the past and may generate
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* extra branches/loads/stores. Use with caution on hot paths; it's not known
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* whether or not this is still a problem.
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*/
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template<class T>
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class Maybe
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{
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typedef void (Maybe::* ConvertibleToBool)(float*****, double*****);
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void nonNull(float*****, double*****) {}
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bool mIsSome;
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AlignedStorage2<T> mStorage;
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public:
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typedef T ValueType;
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Maybe() : mIsSome(false) { }
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~Maybe() { reset(); }
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explicit Maybe(Nothing) : mIsSome(false) { }
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Maybe(const Maybe& aOther)
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: mIsSome(false)
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{
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if (aOther.mIsSome) {
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emplace(*aOther);
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}
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}
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Maybe(Maybe&& aOther)
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: mIsSome(aOther.mIsSome)
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{
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if (aOther.mIsSome) {
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::new (mStorage.addr()) T(Move(*aOther));
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aOther.reset();
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}
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}
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Maybe& operator=(const Maybe& aOther)
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{
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if (&aOther != this) {
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if (aOther.mIsSome) {
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if (mIsSome) {
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// XXX(seth): The correct code for this branch, below, can't be used
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// due to a bug in Visual Studio 2010. See bug 1052940.
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/*
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ref() = aOther.ref();
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*/
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reset();
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emplace(*aOther);
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} else {
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emplace(*aOther);
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}
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} else {
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reset();
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}
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}
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return *this;
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}
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Maybe& operator=(Maybe&& aOther)
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{
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MOZ_ASSERT(this != &aOther, "Self-moves are prohibited");
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if (aOther.mIsSome) {
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if (mIsSome) {
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ref() = Move(aOther.ref());
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} else {
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mIsSome = true;
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::new (mStorage.addr()) T(Move(*aOther));
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}
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aOther.reset();
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} else {
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reset();
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}
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return *this;
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}
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/* Methods that check whether this Maybe contains a value */
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operator ConvertibleToBool() const { return mIsSome ? &Maybe::nonNull : 0; }
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bool isSome() const { return mIsSome; }
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bool isNothing() const { return !mIsSome; }
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/* Returns the contents of this Maybe<T> by value. Unsafe unless |isSome()|. */
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T value() const
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{
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MOZ_ASSERT(mIsSome);
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return ref();
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}
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/* Returns the contents of this Maybe<T> by pointer. Unsafe unless |isSome()|. */
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T* ptr()
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{
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MOZ_ASSERT(mIsSome);
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return &ref();
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}
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const T* ptr() const
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{
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MOZ_ASSERT(mIsSome);
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return &ref();
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}
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T* operator->()
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{
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MOZ_ASSERT(mIsSome);
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return ptr();
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}
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const T* operator->() const
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{
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MOZ_ASSERT(mIsSome);
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return ptr();
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}
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/* Returns the contents of this Maybe<T> by ref. Unsafe unless |isSome()|. */
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T& ref()
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{
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MOZ_ASSERT(mIsSome);
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return *mStorage.addr();
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}
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const T& ref() const
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{
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MOZ_ASSERT(mIsSome);
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return *mStorage.addr();
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}
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T& operator*()
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{
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MOZ_ASSERT(mIsSome);
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return ref();
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}
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const T& operator*() const
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{
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MOZ_ASSERT(mIsSome);
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return ref();
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}
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/* If |isSome()|, empties this Maybe and destroys its contents. */
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void reset()
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{
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if (isSome()) {
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ref().~T();
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mIsSome = false;
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}
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}
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/*
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* Constructs a T value in-place in this empty Maybe<T>'s storage. The
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* arguments to |emplace()| are the parameters to T's constructor.
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*
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* WARNING: You can't pass a literal nullptr to these methods without
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* hitting GCC 4.4-only (and hence B2G-only) compile errors.
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*/
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void emplace()
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{
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MOZ_ASSERT(!mIsSome);
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::new (mStorage.addr()) T();
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mIsSome = true;
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}
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template<typename T1>
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void emplace(T1&& t1)
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{
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MOZ_ASSERT(!mIsSome);
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::new (mStorage.addr()) T(Forward<T1>(t1));
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mIsSome = true;
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}
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template<typename T1, typename T2>
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void emplace(T1&& t1, T2&& t2)
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{
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MOZ_ASSERT(!mIsSome);
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::new (mStorage.addr()) T(Forward<T1>(t1), Forward<T2>(t2));
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mIsSome = true;
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}
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template<typename T1, typename T2, typename T3>
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void emplace(T1&& t1, T2&& t2, T3&& t3)
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{
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MOZ_ASSERT(!mIsSome);
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::new (mStorage.addr()) T(Forward<T1>(t1), Forward<T2>(t2), Forward<T3>(t3));
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mIsSome = true;
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}
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template<typename T1, typename T2, typename T3, typename T4>
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void emplace(T1&& t1, T2&& t2, T3&& t3, T4&& t4)
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{
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MOZ_ASSERT(!mIsSome);
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::new (mStorage.addr()) T(Forward<T1>(t1), Forward<T2>(t2), Forward<T3>(t3),
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Forward<T4>(t4));
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mIsSome = true;
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}
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template<typename T1, typename T2, typename T3, typename T4, typename T5>
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void emplace(T1&& t1, T2&& t2, T3&& t3, T4&& t4, T5&& t5)
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{
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MOZ_ASSERT(!mIsSome);
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::new (mStorage.addr()) T(Forward<T1>(t1), Forward<T2>(t2), Forward<T3>(t3),
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Forward<T4>(t4), Forward<T5>(t5));
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mIsSome = true;
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}
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template<typename T1, typename T2, typename T3, typename T4, typename T5,
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typename T6>
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void emplace(T1&& t1, T2&& t2, T3&& t3, T4&& t4, T5&& t5, T6&& t6)
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{
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MOZ_ASSERT(!mIsSome);
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::new (mStorage.addr()) T(Forward<T1>(t1), Forward<T2>(t2), Forward<T3>(t3),
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Forward<T4>(t4), Forward<T5>(t5), Forward<T6>(t6));
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mIsSome = true;
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}
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template<typename T1, typename T2, typename T3, typename T4, typename T5,
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typename T6, typename T7>
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void emplace(T1&& t1, T2&& t2, T3&& t3, T4&& t4, T5&& t5, T6&& t6,
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T7&& t7)
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{
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MOZ_ASSERT(!mIsSome);
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::new (mStorage.addr()) T(Forward<T1>(t1), Forward<T2>(t2), Forward<T3>(t3),
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Forward<T4>(t4), Forward<T5>(t5), Forward<T6>(t6),
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Forward<T7>(t7));
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mIsSome = true;
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}
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template<typename T1, typename T2, typename T3, typename T4, typename T5,
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typename T6, typename T7, typename T8>
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void emplace(T1&& t1, T2&& t2, T3&& t3, T4&& t4, T5&& t5, T6&& t6,
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T7&& t7, T8&& t8)
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{
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MOZ_ASSERT(!mIsSome);
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::new (mStorage.addr()) T(Forward<T1>(t1), Forward<T2>(t2), Forward<T3>(t3),
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Forward<T4>(t4), Forward<T5>(t5), Forward<T6>(t6),
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Forward<T7>(t7), Forward<T8>(t8));
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mIsSome = true;
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}
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template<typename T1, typename T2, typename T3, typename T4, typename T5,
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typename T6, typename T7, typename T8, typename T9>
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void emplace(T1&& t1, T2&& t2, T3&& t3, T4&& t4, T5&& t5, T6&& t6,
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T7&& t7, T8&& t8, T9&& t9)
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{
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MOZ_ASSERT(!mIsSome);
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::new (mStorage.addr()) T(Forward<T1>(t1), Forward<T2>(t2), Forward<T3>(t3),
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Forward<T4>(t4), Forward<T5>(t5), Forward<T6>(t6),
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Forward<T7>(t7), Forward<T8>(t8), Forward<T9>(t9));
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mIsSome = true;
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}
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template<typename T1, typename T2, typename T3, typename T4, typename T5,
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typename T6, typename T7, typename T8, typename T9, typename T10>
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void emplace(T1&& t1, T2&& t2, T3&& t3, T4&& t4, T5&& t5, T6&& t6,
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T7&& t7, T8&& t8, T9&& t9, T10&& t10)
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{
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MOZ_ASSERT(!mIsSome);
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::new (mStorage.addr()) T(Forward<T1>(t1), Forward<T2>(t2), Forward<T3>(t3),
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Forward<T4>(t4), Forward<T5>(t5), Forward<T6>(t6),
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Forward<T7>(t7), Forward<T8>(t8), Forward<T9>(t9),
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Forward<T1>(t10));
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mIsSome = true;
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}
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};
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/*
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* Some() creates a Maybe<T> value containing the provided T value. If T has a
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* move constructor, it's used to make this as efficient as possible.
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*
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* Some() selects the type of Maybe it returns by removing any const, volatile,
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* or reference qualifiers from the type of the value you pass to it. This gives
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* it more intuitive behavior when used in expressions, but it also means that
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* if you need to construct a Maybe value that holds a const, volatile, or
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* reference value, you need to use emplace() instead.
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*/
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template<typename T>
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Maybe<typename RemoveCV<typename RemoveReference<T>::Type>::Type>
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Some(T&& aValue)
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{
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typedef typename RemoveCV<typename RemoveReference<T>::Type>::Type U;
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Maybe<U> value;
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value.emplace(Forward<T>(aValue));
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return value;
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}
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} // namespace mozilla
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#endif /* mozilla_Maybe_h */
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