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/* -*- Mode: C++; tab-width: 2; 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/. */
/* A class for optional values and in-place lazy construction. */
#ifndef mozilla_Maybe_h
#define mozilla_Maybe_h
#include <new> // for placement new
#include <ostream>
#include <type_traits>
#include <utility>
#include "mozilla/Alignment.h"
#include "mozilla/Assertions.h"
#include "mozilla/Attributes.h"
#include "mozilla/MemoryChecking.h"
#include "mozilla/OperatorNewExtensions.h"
#include "mozilla/Poison.h"
class nsCycleCollectionTraversalCallback;
template <typename T>
inline void CycleCollectionNoteChild(
nsCycleCollectionTraversalCallback& aCallback, T* aChild, const char* aName,
uint32_t aFlags);
namespace mozilla {
struct Nothing {};
inline constexpr bool operator==(const Nothing&, const Nothing&) {
return true;
}
template <class T>
class Maybe;
namespace detail {
// You would think that poisoning Maybe instances could just be a call
// to mozWritePoison. Unfortunately, using a simple call to
// mozWritePoison generates poor code on MSVC for small structures. The
// generated code contains (always not-taken) branches and does a bunch
// of setup for `rep stos{l,q}`, even though we know at compile time
// exactly how many words we're poisoning. Instead, we're going to
// force MSVC to generate the code we want via recursive templates.
// Write the given poisonValue into p at offset*sizeof(uintptr_t).
template <size_t offset>
inline void WritePoisonAtOffset(void* p, const uintptr_t poisonValue) {
memcpy(static_cast<char*>(p) + offset * sizeof(poisonValue), &poisonValue,
sizeof(poisonValue));
}
template <size_t Offset, size_t NOffsets>
struct InlinePoisoner {
static void poison(void* p, const uintptr_t poisonValue) {
WritePoisonAtOffset<Offset>(p, poisonValue);
InlinePoisoner<Offset + 1, NOffsets>::poison(p, poisonValue);
}
};
template <size_t N>
struct InlinePoisoner<N, N> {
static void poison(void*, const uintptr_t) {
// All done!
}
};
// We can't generate inline code for large structures, though, because we'll
// blow out recursive template instantiation limits, and the code would be
// bloated to boot. So provide a fallback to the out-of-line poisoner.
template <size_t ObjectSize>
struct OutOfLinePoisoner {
static MOZ_NEVER_INLINE void poison(void* p, const uintptr_t) {
mozWritePoison(p, ObjectSize);
}
};
template <typename T>
inline void PoisonObject(T* p) {
const uintptr_t POISON = mozPoisonValue();
std::conditional_t<(sizeof(T) <= 8 * sizeof(POISON)),
InlinePoisoner<0, sizeof(T) / sizeof(POISON)>,
OutOfLinePoisoner<sizeof(T)>>::poison(p, POISON);
}
template <typename T>
struct MaybePoisoner {
static const size_t N = sizeof(T);
static void poison(void* aPtr) {
#ifdef MOZ_DIAGNOSTIC_ASSERT_ENABLED
if (N >= sizeof(uintptr_t)) {
PoisonObject(static_cast<std::remove_cv_t<T>*>(aPtr));
}
#endif
MOZ_MAKE_MEM_UNDEFINED(aPtr, N);
}
};
template <typename T>
constexpr bool IsTriviallyDestructibleAndCopyable =
std::is_trivially_destructible_v<T> &&
(std::is_trivially_copy_constructible_v<T> ||
!std::is_copy_constructible_v<T>);
template <typename T,
bool TriviallyDestructibleAndCopyable =
IsTriviallyDestructibleAndCopyable<T>,
bool Copyable = std::is_copy_constructible_v<T>,
bool Movable = std::is_move_constructible_v<T>>
class Maybe_CopyMove_Enabler;
#define MOZ_MAYBE_COPY_OPS() \
Maybe_CopyMove_Enabler(const Maybe_CopyMove_Enabler& aOther) { \
if (downcast(aOther).isSome()) { \
downcast(*this).emplace(*downcast(aOther)); \
} \
} \
\
Maybe_CopyMove_Enabler& operator=(const Maybe_CopyMove_Enabler& aOther) { \
return downcast(*this).template operator=<T>(downcast(aOther)); \
}
#define MOZ_MAYBE_MOVE_OPS() \
constexpr Maybe_CopyMove_Enabler(Maybe_CopyMove_Enabler&& aOther) { \
if (downcast(aOther).isSome()) { \
downcast(*this).emplace(std::move(*downcast(aOther))); \
downcast(aOther).reset(); \
} \
} \
\
constexpr Maybe_CopyMove_Enabler& operator=( \
Maybe_CopyMove_Enabler&& aOther) { \
downcast(*this).template operator=<T>(std::move(downcast(aOther))); \
\
return *this; \
}
#define MOZ_MAYBE_DOWNCAST() \
static constexpr Maybe<T>& downcast(Maybe_CopyMove_Enabler& aObj) { \
return static_cast<Maybe<T>&>(aObj); \
} \
static constexpr const Maybe<T>& downcast( \
const Maybe_CopyMove_Enabler& aObj) { \
return static_cast<const Maybe<T>&>(aObj); \
}
template <typename T>
class Maybe_CopyMove_Enabler<T, true, true, true> {
public:
Maybe_CopyMove_Enabler() = default;
Maybe_CopyMove_Enabler(const Maybe_CopyMove_Enabler&) = default;
Maybe_CopyMove_Enabler& operator=(const Maybe_CopyMove_Enabler&) = default;
constexpr Maybe_CopyMove_Enabler(Maybe_CopyMove_Enabler&& aOther) {
downcast(aOther).reset();
}
constexpr Maybe_CopyMove_Enabler& operator=(Maybe_CopyMove_Enabler&& aOther) {
downcast(aOther).reset();
return *this;
}
private:
MOZ_MAYBE_DOWNCAST()
};
template <typename T>
class Maybe_CopyMove_Enabler<T, true, false, true> {
public:
Maybe_CopyMove_Enabler() = default;
Maybe_CopyMove_Enabler(const Maybe_CopyMove_Enabler&) = delete;
Maybe_CopyMove_Enabler& operator=(const Maybe_CopyMove_Enabler&) = delete;
constexpr Maybe_CopyMove_Enabler(Maybe_CopyMove_Enabler&& aOther) {
downcast(aOther).reset();
}
constexpr Maybe_CopyMove_Enabler& operator=(Maybe_CopyMove_Enabler&& aOther) {
downcast(aOther).reset();
return *this;
}
private:
MOZ_MAYBE_DOWNCAST()
};
template <typename T>
class Maybe_CopyMove_Enabler<T, false, true, true> {
public:
Maybe_CopyMove_Enabler() = default;
MOZ_MAYBE_COPY_OPS()
MOZ_MAYBE_MOVE_OPS()
private:
MOZ_MAYBE_DOWNCAST()
};
template <typename T>
class Maybe_CopyMove_Enabler<T, false, false, true> {
public:
Maybe_CopyMove_Enabler() = default;
MOZ_MAYBE_MOVE_OPS()
private:
MOZ_MAYBE_DOWNCAST()
};
template <typename T>
class Maybe_CopyMove_Enabler<T, false, true, false> {
public:
Maybe_CopyMove_Enabler() = default;
MOZ_MAYBE_COPY_OPS()
private:
MOZ_MAYBE_DOWNCAST()
};
template <typename T, bool TriviallyDestructibleAndCopyable>
class Maybe_CopyMove_Enabler<T, TriviallyDestructibleAndCopyable, false,
false> {
public:
Maybe_CopyMove_Enabler() = default;
Maybe_CopyMove_Enabler(const Maybe_CopyMove_Enabler&) = delete;
Maybe_CopyMove_Enabler& operator=(const Maybe_CopyMove_Enabler&) = delete;
Maybe_CopyMove_Enabler(Maybe_CopyMove_Enabler&&) = delete;
Maybe_CopyMove_Enabler& operator=(Maybe_CopyMove_Enabler&&) = delete;
};
#undef MOZ_MAYBE_COPY_OPS
#undef MOZ_MAYBE_MOVE_OPS
#undef MOZ_MAYBE_DOWNCAST
template <typename T, bool TriviallyDestructibleAndCopyable =
IsTriviallyDestructibleAndCopyable<T>>
struct MaybeStorage;
template <typename T>
struct MaybeStorage<T, false> {
using NonConstT = std::remove_const_t<T>;
union Union {
Union() {}
constexpr explicit Union(const T& aVal) : val{aVal} {}
template <typename U,
typename = std::enable_if_t<std::is_move_constructible_v<U>>>
constexpr explicit Union(U&& aVal) : val{std::forward<U>(aVal)} {}
~Union() {}
NonConstT val;
char dummy;
} mStorage;
char mIsSome = false; // not bool -- guarantees minimal space consumption
MaybeStorage() = default;
explicit MaybeStorage(const T& aVal) : mStorage{aVal}, mIsSome{true} {}
explicit MaybeStorage(T&& aVal) : mStorage{std::move(aVal)}, mIsSome{true} {}
template <typename... Args>
explicit MaybeStorage(std::in_place_t, Args&&... aArgs) : mIsSome{true} {
::new (KnownNotNull, &mStorage.val) T(std::forward<Args>(aArgs)...);
}
// Copy and move operations are no-ops, since copying is moving is implemented
// by Maybe_CopyMove_Enabler.
MaybeStorage(const MaybeStorage&) {}
MaybeStorage& operator=(const MaybeStorage&) { return *this; }
MaybeStorage(MaybeStorage&&) {}
MaybeStorage& operator=(MaybeStorage&&) { return *this; }
~MaybeStorage() {
if (mIsSome) {
mStorage.val.T::~T();
}
}
};
template <typename T>
struct MaybeStorage<T, true> {
using NonConstT = std::remove_const_t<T>;
union Union {
constexpr Union() : dummy() {}
constexpr explicit Union(const T& aVal) : val{aVal} {}
constexpr explicit Union(T&& aVal) : val{std::move(aVal)} {}
template <typename... Args>
constexpr explicit Union(std::in_place_t, Args&&... aArgs)
: val{std::forward<Args>(aArgs)...} {}
NonConstT val;
char dummy;
} mStorage;
char mIsSome = false; // not bool -- guarantees minimal space consumption
constexpr MaybeStorage() = default;
constexpr explicit MaybeStorage(const T& aVal)
: mStorage{aVal}, mIsSome{true} {}
constexpr explicit MaybeStorage(T&& aVal)
: mStorage{std::move(aVal)}, mIsSome{true} {}
template <typename... Args>
constexpr explicit MaybeStorage(std::in_place_t, Args&&... aArgs)
: mStorage{std::in_place, std::forward<Args>(aArgs)...}, mIsSome{true} {}
};
} // namespace detail
template <typename T, typename U = typename std::remove_cv<
typename std::remove_reference<T>::type>::type>
constexpr Maybe<U> Some(T&& aValue);
/*
* Maybe is a container class which contains either zero or one elements. It
* serves two roles. It can represent values which are *semantically* optional,
* augmenting a type with an explicit 'Nothing' value. In this role, it provides
* methods that make it easy to work with values that may be missing, along with
* equality and comparison operators so that Maybe values can be stored in
* containers. Maybe values can be constructed conveniently in expressions using
* type inference, as follows:
*
* void doSomething(Maybe<Foo> aFoo) {
* if (aFoo) // Make sure that aFoo contains a value...
* aFoo->takeAction(); // and then use |aFoo->| to access it.
* } // |*aFoo| also works!
*
* doSomething(Nothing()); // Passes a Maybe<Foo> containing no value.
* doSomething(Some(Foo(100))); // Passes a Maybe<Foo> containing |Foo(100)|.
*
* You'll note that it's important to check whether a Maybe contains a value
* before using it, using conversion to bool, |isSome()|, or |isNothing()|. You
* can avoid these checks, and sometimes write more readable code, using
* |valueOr()|, |ptrOr()|, and |refOr()|, which allow you to retrieve the value
* in the Maybe and provide a default for the 'Nothing' case. You can also use
* |apply()| to call a function only if the Maybe holds a value, and |map()| to
* transform the value in the Maybe, returning another Maybe with a possibly
* different type.
*
* Maybe's other role is to support lazily constructing objects without using
* dynamic storage. A Maybe directly contains storage for a value, but it's
* empty by default. |emplace()|, as mentioned above, can be used to construct a
* value in Maybe's storage. The value a Maybe contains can be destroyed by
* calling |reset()|; this will happen automatically if a Maybe is destroyed
* while holding a value.
*
* It's a common idiom in C++ to use a pointer as a 'Maybe' type, with a null
* value meaning 'Nothing' and any other value meaning 'Some'. You can convert
* from such a pointer to a Maybe value using 'ToMaybe()'.
*
* Maybe is inspired by similar types in the standard library of many other
* languages (e.g. Haskell's Maybe and Rust's Option). In the C++ world it's
* very similar to std::optional, which was proposed for C++14 and originated in
* Boost. The most important differences between Maybe and std::optional are:
*
* - std::optional<T> may be compared with T. We deliberately forbid that.
* - std::optional has |valueOr()|, equivalent to Maybe's |valueOr()|, but
* lacks corresponding methods for |refOr()| and |ptrOr()|.
* - std::optional lacks |map()| and |apply()|, making it less suitable for
* functional-style code.
* - std::optional lacks many convenience functions that Maybe has. Most
* unfortunately, it lacks equivalents of the type-inferred constructor
* functions |Some()| and |Nothing()|.
*/
template <class T>
class MOZ_INHERIT_TYPE_ANNOTATIONS_FROM_TEMPLATE_ARGS Maybe
: private detail::MaybeStorage<T>,
public detail::Maybe_CopyMove_Enabler<T> {
template <typename, bool, bool, bool>
friend class detail::Maybe_CopyMove_Enabler;
template <typename U, typename V>
friend constexpr Maybe<V> Some(U&& aValue);
struct SomeGuard {};
template <typename U>
constexpr Maybe(U&& aValue, SomeGuard)
: detail::MaybeStorage<T>{std::forward<U>(aValue)} {}
using detail::MaybeStorage<T>::mIsSome;
using detail::MaybeStorage<T>::mStorage;
void poisonData() { detail::MaybePoisoner<T>::poison(&mStorage.val); }
public:
using ValueType = T;
MOZ_ALLOW_TEMPORARY constexpr Maybe() = default;
MOZ_ALLOW_TEMPORARY MOZ_IMPLICIT constexpr Maybe(Nothing) : Maybe{} {}
template <typename... Args>
constexpr explicit Maybe(std::in_place_t, Args&&... aArgs)
: detail::MaybeStorage<T>{std::in_place, std::forward<Args>(aArgs)...} {}
/**
* Maybe<T> can be copy-constructed from a Maybe<U> if T is constructible from
* a const U&.
*/
template <typename U,
typename = std::enable_if_t<std::is_constructible_v<T, const U&>>>
MOZ_IMPLICIT Maybe(const Maybe<U>& aOther) {
if (aOther.isSome()) {
emplace(*aOther);
}
}
/**
* Maybe<T> can be move-constructed from a Maybe<U> if T is constructible from
* a U&&.
*/
template <typename U,
typename = std::enable_if_t<std::is_constructible_v<T, U&&>>>
MOZ_IMPLICIT Maybe(Maybe<U>&& aOther) {
if (aOther.isSome()) {
emplace(std::move(*aOther));
aOther.reset();
}
}
template <typename U,
typename = std::enable_if_t<std::is_constructible_v<T, const U&>>>
Maybe& operator=(const Maybe<U>& aOther) {
if (aOther.isSome()) {
if (mIsSome) {
ref() = aOther.ref();
} else {
emplace(*aOther);
}
} else {
reset();
}
return *this;
}
template <typename U,
typename = std::enable_if_t<std::is_constructible_v<T, U&&>>>
Maybe& operator=(Maybe<U>&& aOther) {
if (aOther.isSome()) {
if (mIsSome) {
ref() = std::move(aOther.ref());
} else {
emplace(std::move(*aOther));
}
aOther.reset();
} else {
reset();
}
return *this;
}
constexpr Maybe& operator=(Nothing) {
reset();
return *this;
}
/* Methods that check whether this Maybe contains a value */
constexpr explicit operator bool() const { return isSome(); }
constexpr bool isSome() const { return mIsSome; }
constexpr bool isNothing() const { return !mIsSome; }
/* Returns the contents of this Maybe<T> by value. Unsafe unless |isSome()|.
*/
constexpr T value() const;
/**
* Move the contents of this Maybe<T> out of internal storage and return it
* without calling the destructor. The internal storage is also reset to
* avoid multiple calls. Unsafe unless |isSome()|.
*/
T extract() {
MOZ_DIAGNOSTIC_ASSERT(isSome());
auto v = std::move(mStorage.val);
reset();
return v;
}
/**
* Returns the value (possibly |Nothing()|) by moving it out of this Maybe<T>
* and leaving |Nothing()| in its place.
*/
Maybe<T> take() { return std::exchange(*this, Nothing()); }
/*
* Returns the contents of this Maybe<T> by value. If |isNothing()|, returns
* the default value provided.
*/
template <typename V>
constexpr T valueOr(V&& aDefault) const {
if (isSome()) {
return ref();
}
return std::forward<V>(aDefault);
}
/*
* Returns the contents of this Maybe<T> by value. If |isNothing()|, returns
* the value returned from the function or functor provided.
*/
template <typename F>
constexpr T valueOrFrom(F&& aFunc) const {
if (isSome()) {
return ref();
}
return aFunc();
}
/* Returns the contents of this Maybe<T> by pointer. Unsafe unless |isSome()|.
*/
T* ptr();
constexpr const T* ptr() const;
/*
* Returns the contents of this Maybe<T> by pointer. If |isNothing()|,
* returns the default value provided.
*/
T* ptrOr(T* aDefault) {
if (isSome()) {
return ptr();
}
return aDefault;
}
constexpr const T* ptrOr(const T* aDefault) const {
if (isSome()) {
return ptr();
}
return aDefault;
}
/*
* Returns the contents of this Maybe<T> by pointer. If |isNothing()|,
* returns the value returned from the function or functor provided.
*/
template <typename F>
T* ptrOrFrom(F&& aFunc) {
if (isSome()) {
return ptr();
}
return aFunc();
}
template <typename F>
const T* ptrOrFrom(F&& aFunc) const {
if (isSome()) {
return ptr();
}
return aFunc();
}
constexpr T* operator->();
constexpr const T* operator->() const;
/* Returns the contents of this Maybe<T> by ref. Unsafe unless |isSome()|. */
constexpr T& ref();
constexpr const T& ref() const;
/*
* Returns the contents of this Maybe<T> by ref. If |isNothing()|, returns
* the default value provided.
*/
constexpr T& refOr(T& aDefault) {
if (isSome()) {
return ref();
}
return aDefault;
}
constexpr const T& refOr(const T& aDefault) const {
if (isSome()) {
return ref();
}
return aDefault;
}
/*
* Returns the contents of this Maybe<T> by ref. If |isNothing()|, returns the
* value returned from the function or functor provided.
*/
template <typename F>
constexpr T& refOrFrom(F&& aFunc) {
if (isSome()) {
return ref();
}
return aFunc();
}
template <typename F>
constexpr const T& refOrFrom(F&& aFunc) const {
if (isSome()) {
return ref();
}
return aFunc();
}
constexpr T& operator*();
constexpr const T& operator*() const;
/* If |isSome()|, runs the provided function or functor on the contents of
* this Maybe. */
template <typename Func>
constexpr Maybe& apply(Func&& aFunc) {
if (isSome()) {
std::forward<Func>(aFunc)(ref());
}
return *this;
}
template <typename Func>
constexpr const Maybe& apply(Func&& aFunc) const {
if (isSome()) {
std::forward<Func>(aFunc)(ref());
}
return *this;
}
/*
* If |isSome()|, runs the provided function and returns the result wrapped
* in a Maybe. If |isNothing()|, returns an empty Maybe value with the same
* value type as what the provided function would have returned.
*/
template <typename Func>
constexpr auto map(Func&& aFunc) {
if (isSome()) {
return Some(std::forward<Func>(aFunc)(ref()));
}
return Maybe<decltype(std::forward<Func>(aFunc)(ref()))>{};
}
template <typename Func>
constexpr auto map(Func&& aFunc) const {
if (isSome()) {
return Some(std::forward<Func>(aFunc)(ref()));
}
return Maybe<decltype(std::forward<Func>(aFunc)(ref()))>{};
}
/* If |isSome()|, empties this Maybe and destroys its contents. */
constexpr void reset() {
if (isSome()) {
if constexpr (!std::is_trivially_destructible_v<T>) {
ref().T::~T();
poisonData();
}
mIsSome = false;
}
}
/*
* Constructs a T value in-place in this empty Maybe<T>'s storage. The
* arguments to |emplace()| are the parameters to T's constructor.
*/
template <typename... Args>
constexpr void emplace(Args&&... aArgs);
template <typename U>
constexpr std::enable_if_t<std::is_same_v<T, U> &&
std::is_copy_constructible_v<U> &&
!std::is_move_constructible_v<U>>
emplace(U&& aArgs) {
emplace(aArgs);
}
friend std::ostream& operator<<(std::ostream& aStream,
const Maybe<T>& aMaybe) {
if (aMaybe) {
aStream << aMaybe.ref();
} else {
aStream << "<Nothing>";
}
return aStream;
}
};
template <typename T>
class Maybe<T&> {
public:
constexpr Maybe() = default;
constexpr MOZ_IMPLICIT Maybe(Nothing) {}
void emplace(T& aRef) { mValue = &aRef; }
/* Methods that check whether this Maybe contains a value */
constexpr explicit operator bool() const { return isSome(); }
constexpr bool isSome() const { return mValue; }
constexpr bool isNothing() const { return !mValue; }
T& ref() const {
MOZ_DIAGNOSTIC_ASSERT(isSome());
return *mValue;
}
T* operator->() const { return &ref(); }
T& operator*() const { return ref(); }
// Deliberately not defining value and ptr accessors, as these may be
// confusing on a reference-typed Maybe.
// XXX Should we define refOr?
void reset() { mValue = nullptr; }
template <typename Func>
Maybe& apply(Func&& aFunc) {
if (isSome()) {
std::forward<Func>(aFunc)(ref());
}
return *this;
}
template <typename Func>
const Maybe& apply(Func&& aFunc) const {
if (isSome()) {
std::forward<Func>(aFunc)(ref());
}
return *this;
}
template <typename Func>
auto map(Func&& aFunc) {
Maybe<decltype(std::forward<Func>(aFunc)(ref()))> val;
if (isSome()) {
val.emplace(std::forward<Func>(aFunc)(ref()));
}
return val;
}
template <typename Func>
auto map(Func&& aFunc) const {
Maybe<decltype(std::forward<Func>(aFunc)(ref()))> val;
if (isSome()) {
val.emplace(std::forward<Func>(aFunc)(ref()));
}
return val;
}
bool refEquals(const Maybe<T&>& aOther) const {
return mValue == aOther.mValue;
}
bool refEquals(const T& aOther) const { return mValue == &aOther; }
private:
T* mValue = nullptr;
};
template <typename T>
constexpr T Maybe<T>::value() const {
MOZ_DIAGNOSTIC_ASSERT(isSome());
return ref();
}
template <typename T>
T* Maybe<T>::ptr() {
MOZ_DIAGNOSTIC_ASSERT(isSome());
return &ref();
}
template <typename T>
constexpr const T* Maybe<T>::ptr() const {
MOZ_DIAGNOSTIC_ASSERT(isSome());
return &ref();
}
template <typename T>
constexpr T* Maybe<T>::operator->() {
MOZ_DIAGNOSTIC_ASSERT(isSome());
return ptr();
}
template <typename T>
constexpr const T* Maybe<T>::operator->() const {
MOZ_DIAGNOSTIC_ASSERT(isSome());
return ptr();
}
template <typename T>
constexpr T& Maybe<T>::ref() {
MOZ_DIAGNOSTIC_ASSERT(isSome());
return mStorage.val;
}
template <typename T>
constexpr const T& Maybe<T>::ref() const {
MOZ_DIAGNOSTIC_ASSERT(isSome());
return mStorage.val;
}
template <typename T>
constexpr T& Maybe<T>::operator*() {
MOZ_DIAGNOSTIC_ASSERT(isSome());
return ref();
}
template <typename T>
constexpr const T& Maybe<T>::operator*() const {
MOZ_DIAGNOSTIC_ASSERT(isSome());
return ref();
}
template <typename T>
template <typename... Args>
constexpr void Maybe<T>::emplace(Args&&... aArgs) {
MOZ_DIAGNOSTIC_ASSERT(!isSome());
::new (KnownNotNull, &mStorage.val) T(std::forward<Args>(aArgs)...);
mIsSome = true;
}
/*
* Some() creates a Maybe<T> value containing the provided T value. If T has a
* move constructor, it's used to make this as efficient as possible.
*
* Some() selects the type of Maybe it returns by removing any const, volatile,
* or reference qualifiers from the type of the value you pass to it. This gives
* it more intuitive behavior when used in expressions, but it also means that
* if you need to construct a Maybe value that holds a const, volatile, or
* reference value, you need to use emplace() instead.
*/
template <typename T, typename U>
constexpr Maybe<U> Some(T&& aValue) {
return {std::forward<T>(aValue), typename Maybe<U>::SomeGuard{}};
}
template <typename T>
constexpr Maybe<T&> SomeRef(T& aValue) {
Maybe<T&> value;
value.emplace(aValue);
return value;
}
template <typename T>
Maybe<std::remove_cv_t<std::remove_reference_t<T>>> ToMaybe(T* aPtr) {
if (aPtr) {
return Some(*aPtr);
}
return Nothing();
}
/*
* Two Maybe<T> values are equal if
* - both are Nothing, or
* - both are Some, and the values they contain are equal.
*/
template <typename T>
constexpr bool operator==(const Maybe<T>& aLHS, const Maybe<T>& aRHS) {
static_assert(!std::is_reference_v<T>,
"operator== is not defined for Maybe<T&>, compare values or "
"addresses explicitly instead");
if (aLHS.isNothing() != aRHS.isNothing()) {
return false;
}
return aLHS.isNothing() || *aLHS == *aRHS;
}
template <typename T>
constexpr bool operator!=(const Maybe<T>& aLHS, const Maybe<T>& aRHS) {
return !(aLHS == aRHS);
}
/*
* We support comparison to Nothing to allow reasonable expressions like:
* if (maybeValue == Nothing()) { ... }
*/
template <typename T>
constexpr bool operator==(const Maybe<T>& aLHS, const Nothing& aRHS) {
return aLHS.isNothing();
}
template <typename T>
constexpr bool operator!=(const Maybe<T>& aLHS, const Nothing& aRHS) {
return !(aLHS == aRHS);
}
template <typename T>
constexpr bool operator==(const Nothing& aLHS, const Maybe<T>& aRHS) {
return aRHS.isNothing();
}
template <typename T>
constexpr bool operator!=(const Nothing& aLHS, const Maybe<T>& aRHS) {
return !(aLHS == aRHS);
}
/*
* Maybe<T> values are ordered in the same way T values are ordered, except that
* Nothing comes before anything else.
*/
template <typename T>
constexpr bool operator<(const Maybe<T>& aLHS, const Maybe<T>& aRHS) {
if (aLHS.isNothing()) {
return aRHS.isSome();
}
if (aRHS.isNothing()) {
return false;
}
return *aLHS < *aRHS;
}
template <typename T>
constexpr bool operator>(const Maybe<T>& aLHS, const Maybe<T>& aRHS) {
return !(aLHS < aRHS || aLHS == aRHS);
}
template <typename T>
constexpr bool operator<=(const Maybe<T>& aLHS, const Maybe<T>& aRHS) {
return aLHS < aRHS || aLHS == aRHS;
}
template <typename T>
constexpr bool operator>=(const Maybe<T>& aLHS, const Maybe<T>& aRHS) {
return !(aLHS < aRHS);
}
template <typename T>
inline void ImplCycleCollectionTraverse(
nsCycleCollectionTraversalCallback& aCallback, mozilla::Maybe<T>& aField,
const char* aName, uint32_t aFlags = 0) {
if (aField) {
ImplCycleCollectionTraverse(aCallback, aField.ref(), aName, aFlags);
}
}
template <typename T>
inline void ImplCycleCollectionUnlink(mozilla::Maybe<T>& aField) {
if (aField) {
ImplCycleCollectionUnlink(aField.ref());
}
}
} // namespace mozilla
#endif /* mozilla_Maybe_h */