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552 lines
14 KiB
C++
552 lines
14 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/Attributes.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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#include <type_traits>
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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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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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MOZ_IMPLICIT 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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/**
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* Maybe<T*> can be copy-constructed from a Maybe<U*> if U* and T* are
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* compatible, or from Maybe<decltype(nullptr)>.
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*/
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template<typename U,
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typename =
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typename std::enable_if<std::is_pointer<T>::value &&
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(std::is_same<U, decltype(nullptr)>::value ||
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(std::is_pointer<U>::value &&
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std::is_base_of<typename std::remove_pointer<T>::type,
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typename std::remove_pointer<U>::type>::value))>::type>
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MOZ_IMPLICIT
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Maybe(const Maybe<U>& aOther)
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: mIsSome(false)
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{
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if (aOther.isSome()) {
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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(false)
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{
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if (aOther.mIsSome) {
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emplace(Move(*aOther));
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aOther.reset();
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}
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}
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/**
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* Maybe<T*> can be move-constructed from a Maybe<U*> if U* and T* are
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* compatible, or from Maybe<decltype(nullptr)>.
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*/
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template<typename U,
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typename =
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typename std::enable_if<std::is_pointer<T>::value &&
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(std::is_same<U, decltype(nullptr)>::value ||
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(std::is_pointer<U>::value &&
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std::is_base_of<typename std::remove_pointer<T>::type,
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typename std::remove_pointer<U>::type>::value))>::type>
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MOZ_IMPLICIT
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Maybe(Maybe<U>&& aOther)
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: mIsSome(false)
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{
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if (aOther.isSome()) {
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emplace(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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emplace(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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explicit operator bool() const { return isSome(); }
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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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/*
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* Returns the contents of this Maybe<T> by value. If |isNothing()|, returns
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* the default value provided.
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*/
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template<typename V>
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T valueOr(V&& aDefault) const
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{
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if (isSome()) {
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return ref();
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}
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return Forward<V>(aDefault);
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}
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/*
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* Returns the contents of this Maybe<T> by value. If |isNothing()|, returns
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* the value returned from the function or functor provided.
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*/
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template<typename F>
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T valueOrFrom(F&& aFunc) const
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{
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if (isSome()) {
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return ref();
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}
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return aFunc();
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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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/*
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* Returns the contents of this Maybe<T> by pointer. If |isNothing()|,
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* returns the default value provided.
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*/
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T* ptrOr(T* aDefault)
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{
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if (isSome()) {
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return ptr();
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}
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return aDefault;
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}
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const T* ptrOr(const T* aDefault) const
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{
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if (isSome()) {
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return ptr();
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}
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return aDefault;
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}
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/*
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* Returns the contents of this Maybe<T> by pointer. If |isNothing()|,
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* returns the value returned from the function or functor provided.
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*/
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template<typename F>
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T* ptrOrFrom(F&& aFunc)
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{
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if (isSome()) {
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return ptr();
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}
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return aFunc();
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}
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template<typename F>
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const T* ptrOrFrom(F&& aFunc) const
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{
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if (isSome()) {
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return ptr();
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}
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return aFunc();
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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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/*
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* Returns the contents of this Maybe<T> by ref. If |isNothing()|, returns
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* the default value provided.
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*/
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T& refOr(T& aDefault)
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{
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if (isSome()) {
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return ref();
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}
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return aDefault;
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}
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const T& refOr(const T& aDefault) const
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{
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if (isSome()) {
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return ref();
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}
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return aDefault;
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}
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/*
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* Returns the contents of this Maybe<T> by ref. If |isNothing()|, returns the
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* value returned from the function or functor provided.
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*/
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template<typename F>
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T& refOrFrom(F&& aFunc)
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{
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if (isSome()) {
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return ref();
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}
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return aFunc();
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}
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template<typename F>
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const T& refOrFrom(F&& aFunc) const
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{
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if (isSome()) {
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return ref();
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}
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return aFunc();
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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()|, runs the provided function or functor on the contents of
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* this Maybe. */
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template<typename Func>
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Maybe& apply(Func aFunc)
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{
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if (isSome()) {
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aFunc(ref());
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}
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return *this;
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}
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template<typename Func>
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const Maybe& apply(Func aFunc) const
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{
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if (isSome()) {
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aFunc(ref());
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}
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return *this;
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}
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/*
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* If |isSome()|, runs the provided function and returns the result wrapped
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* in a Maybe. If |isNothing()|, returns an empty Maybe value.
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*/
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template<typename Func>
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auto map(Func aFunc) -> Maybe<decltype(aFunc(DeclVal<Maybe<T>>().ref()))>
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{
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using ReturnType = decltype(aFunc(ref()));
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if (isSome()) {
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Maybe<ReturnType> val;
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val.emplace(aFunc(ref()));
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return val;
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}
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return Maybe<ReturnType>();
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}
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template<typename Func>
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auto map(Func aFunc) const -> Maybe<decltype(aFunc(DeclVal<Maybe<T>>().ref()))>
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{
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using ReturnType = decltype(aFunc(ref()));
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if (isSome()) {
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Maybe<ReturnType> val;
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val.emplace(aFunc(ref()));
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return val;
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}
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return Maybe<ReturnType>();
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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::~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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template<typename... Args>
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void emplace(Args&&... aArgs)
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{
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MOZ_ASSERT(!mIsSome);
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::new (mStorage.addr()) T(Forward<Args>(aArgs)...);
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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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template<typename T>
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Maybe<typename RemoveCV<typename RemoveReference<T>::Type>::Type>
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ToMaybe(T* aPtr)
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{
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if (aPtr) {
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return Some(*aPtr);
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}
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return Nothing();
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}
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/*
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* Two Maybe<T> values are equal if
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* - both are Nothing, or
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* - both are Some, and the values they contain are equal.
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*/
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template<typename T> bool
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operator==(const Maybe<T>& aLHS, const Maybe<T>& aRHS)
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{
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if (aLHS.isNothing() != aRHS.isNothing()) {
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return false;
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}
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return aLHS.isNothing() || *aLHS == *aRHS;
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}
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template<typename T> bool
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operator!=(const Maybe<T>& aLHS, const Maybe<T>& aRHS)
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{
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return !(aLHS == aRHS);
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}
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/*
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* We support comparison to Nothing to allow reasonable expressions like:
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* if (maybeValue == Nothing()) { ... }
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*/
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template<typename T> bool
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operator==(const Maybe<T>& aLHS, const Nothing& aRHS)
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{
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return aLHS.isNothing();
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}
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template<typename T> bool
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operator!=(const Maybe<T>& aLHS, const Nothing& aRHS)
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{
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return !(aLHS == aRHS);
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}
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template<typename T> bool
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operator==(const Nothing& aLHS, const Maybe<T>& aRHS)
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{
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return aRHS.isNothing();
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}
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template<typename T> bool
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operator!=(const Nothing& aLHS, const Maybe<T>& aRHS)
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{
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return !(aLHS == aRHS);
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}
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/*
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* Maybe<T> values are ordered in the same way T values are ordered, except that
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* Nothing comes before anything else.
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*/
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template<typename T> bool
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operator<(const Maybe<T>& aLHS, const Maybe<T>& aRHS)
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{
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if (aLHS.isNothing()) {
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return aRHS.isSome();
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}
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if (aRHS.isNothing()) {
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return false;
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}
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return *aLHS < *aRHS;
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}
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template<typename T> bool
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operator>(const Maybe<T>& aLHS, const Maybe<T>& aRHS)
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{
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return !(aLHS < aRHS || aLHS == aRHS);
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}
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template<typename T> bool
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operator<=(const Maybe<T>& aLHS, const Maybe<T>& aRHS)
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{
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return aLHS < aRHS || aLHS == aRHS;
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}
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template<typename T> bool
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operator>=(const Maybe<T>& aLHS, const Maybe<T>& aRHS)
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{
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return !(aLHS < aRHS);
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}
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} // namespace mozilla
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#endif /* mozilla_Maybe_h */
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