/* -*- 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/. */
/* A class for optional values and in-place lazy construction. */
#ifndef mozilla_Maybe_h
#define mozilla_Maybe_h
#include "mozilla/Alignment.h"
#include "mozilla/Assertions.h"
#include "mozilla/Attributes.h"
#include "mozilla/Move.h"
#include "mozilla/TypeTraits.h"
#include <new> // for placement new
namespace mozilla {
struct Nothing { };
/*
* 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 allows in-place construction without a separate call to
* |emplace()| by using a dummy |in_place_t| value to tag the appropriate
* constructor.
* - 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()|.
*
* N.B. GCC has missed optimizations with Maybe in the past and may generate
* extra branches/loads/stores. Use with caution on hot paths; it's not known
* whether or not this is still a problem.
*/
template<class T>
class Maybe
{
bool mIsSome;
AlignedStorage2<T> mStorage;
public:
typedef T ValueType;
Maybe() : mIsSome(false) { }
~Maybe() { reset(); }
MOZ_IMPLICIT Maybe(Nothing) : mIsSome(false) { }
Maybe(const Maybe& aOther)
: mIsSome(false)
{
if (aOther.mIsSome) {
emplace(*aOther);
}
}
Maybe(Maybe&& aOther)
: mIsSome(false)
{
if (aOther.mIsSome) {
emplace(Move(*aOther));
aOther.reset();
}
}
Maybe& operator=(const Maybe& aOther)
{
if (&aOther != this) {
if (aOther.mIsSome) {
if (mIsSome) {
// XXX(seth): The correct code for this branch, below, can't be used
// due to a bug in Visual Studio 2010. See bug 1052940.
/*
ref() = aOther.ref();
*/
reset();
emplace(*aOther);
} else {
emplace(*aOther);
}
} else {
reset();
}
}
return *this;
}
Maybe& operator=(Maybe&& aOther)
{
MOZ_ASSERT(this != &aOther, "Self-moves are prohibited");
if (aOther.mIsSome) {
if (mIsSome) {
ref() = Move(aOther.ref());
} else {
emplace(Move(*aOther));
}
aOther.reset();
} else {
reset();
}
return *this;
}
/* Methods that check whether this Maybe contains a value */
explicit operator bool() const { return isSome(); }
bool isSome() const { return mIsSome; }
bool isNothing() const { return !mIsSome; }
/* Returns the contents of this Maybe<T> by value. Unsafe unless |isSome()|. */
T value() const
{
MOZ_ASSERT(mIsSome);
return ref();
}
/*
* Returns the contents of this Maybe<T> by value. If |isNothing()|, returns
* the default value provided.
*/
template<typename V>
T valueOr(V&& aDefault) const
{
if (isSome()) {
return ref();
}
return 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>
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()
{
MOZ_ASSERT(mIsSome);
return &ref();
}
const T* ptr() const
{
MOZ_ASSERT(mIsSome);
return &ref();
}
/*
* 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;
}
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();
}
T* operator->()
{
MOZ_ASSERT(mIsSome);
return ptr();
}
const T* operator->() const
{
MOZ_ASSERT(mIsSome);
return ptr();
}
/* Returns the contents of this Maybe<T> by ref. Unsafe unless |isSome()|. */
T& ref()
{
MOZ_ASSERT(mIsSome);
return *mStorage.addr();
}
const T& ref() const
{
MOZ_ASSERT(mIsSome);
return *mStorage.addr();
}
/*
* Returns the contents of this Maybe<T> by ref. If |isNothing()|, returns
* the default value provided.
*/
T& refOr(T& aDefault)
{
if (isSome()) {
return ref();
}
return aDefault;
}
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>
T& refOrFrom(F&& aFunc)
{
if (isSome()) {
return ref();
}
return aFunc();
}
template<typename F>
const T& refOrFrom(F&& aFunc) const
{
if (isSome()) {
return ref();
}
return aFunc();
}
T& operator*()
{
MOZ_ASSERT(mIsSome);
return ref();
}
const T& operator*() const
{
MOZ_ASSERT(mIsSome);
return ref();
}
/* If |isSome()|, runs the provided function or functor on the contents of
* this Maybe. */
template<typename F, typename... Args>
void apply(F&& aFunc, Args&&... aArgs)
{
if (isSome()) {
aFunc(ref(), Forward<Args>(aArgs)...);
}
}
template<typename F, typename... Args>
void apply(F&& aFunc, Args&&... aArgs) const
{
if (isSome()) {
aFunc(ref(), Forward<Args>(aArgs)...);
}
}
/*
* If |isSome()|, runs the provided function and returns the result wrapped
* in a Maybe. If |isNothing()|, returns an empty Maybe value.
*/
template<typename R, typename... FArgs, typename... Args>
Maybe<R> map(R (*aFunc)(T&, FArgs...), Args&&... aArgs)
{
if (isSome()) {
Maybe<R> val;
val.emplace(aFunc(ref(), Forward<Args>(aArgs)...));
return val;
}
return Maybe<R>();
}
template<typename R, typename... FArgs, typename... Args>
Maybe<R> map(R (*aFunc)(const T&, FArgs...), Args&&... aArgs) const
{
if (isSome()) {
Maybe<R> val;
val.emplace(aFunc(ref(), Forward<Args>(aArgs)...));
return val;
}
return Maybe<R>();
}
/* If |isSome()|, empties this Maybe and destroys its contents. */
void reset()
{
if (isSome()) {
ref().~T();
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>
void emplace(Args&&... aArgs)
{
MOZ_ASSERT(!mIsSome);
::new (mStorage.addr()) T(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>
Maybe<typename RemoveCV<typename RemoveReference<T>::Type>::Type>
Some(T&& aValue)
{
typedef typename RemoveCV<typename RemoveReference<T>::Type>::Type U;
Maybe<U> value;
value.emplace(Forward<T>(aValue));
return value;
}
template<typename T>
Maybe<typename RemoveCV<typename RemoveReference<T>::Type>::Type>
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> bool
operator==(const Maybe<T>& aLHS, const Maybe<T>& aRHS)
{
if (aLHS.isNothing() != aRHS.isNothing()) {
return false;
}
return aLHS.isNothing() || *aLHS == *aRHS;
}
template<typename T> 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> bool
operator==(const Maybe<T>& aLHS, const Nothing& aRHS)
{
return aLHS.isNothing();
}
template<typename T> bool
operator!=(const Maybe<T>& aLHS, const Nothing& aRHS)
{
return !(aLHS == aRHS);
}
template<typename T> bool
operator==(const Nothing& aLHS, const Maybe<T>& aRHS)
{
return aRHS.isNothing();
}
template<typename T> 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> 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> bool
operator>(const Maybe<T>& aLHS, const Maybe<T>& aRHS)
{
return !(aLHS < aRHS || aLHS == aRHS);
}
template<typename T> bool
operator<=(const Maybe<T>& aLHS, const Maybe<T>& aRHS)
{
return aLHS < aRHS || aLHS == aRHS;
}
template<typename T> bool
operator>=(const Maybe<T>& aLHS, const Maybe<T>& aRHS)
{
return !(aLHS < aRHS);
}
} // namespace mozilla
#endif /* mozilla_Maybe_h */
