An allocator-aware optional type

Abstract

Library types that can potentially hold allocator-aware (AA) objects should, themselves, be allocator-aware. A PMR container, for example, depends on AA types following a known AA protocol so that it (the container) can uniformly manage its memory. Even types that don't manage their own memory, such as tuple, follow the AA rules when they hold one more more AA elements. (A special case is pair, which is not actually AA but effectively follows the rules through special handling in uses-allocator construction.)

The current definition of std::optional does not follow the rules for an AA type, even when holding an AA value. This limitation makes std::optional unsuitable for storage in an AA container when memory allocation customization is needed.

In this paper, we propose a new, allocator-aware std::pmr::optional usable as a container element type and in any other context where allocator propagation is expected. This new type would not replace the current std::optional, but would resolve to std::optional when the value type is not AA. By automatically handling the distinction between AA and non-AA value types, std::pmr::optional can easily be used in generic code where it is not known in advance if it would be storing an AA type or not.

This is a complete proposal with formal wording. It has not yet been discussed in LEWGI or LEWG.

change history :
- replaced alloc_optional with additional overloads of make_optional that take an allocator_arg_t : it was observed that alloc_optional doesn't actually do any allocation, and that the name is not appropriate
- fixed the order of arguments in the Returns clause of what used to be alloc_optional
- adjusted for LWG 2833
- added discussion on deduction guides
- removed mentions of using a type alias since the type alias approach has proven to be problematic
- modified the specification of std::pmr::optional to cover both AA and non AA types
- added CTAD specification
- modified the specification of std::pmr::optional to be expressed in terms of std::pmr::polymorphic_allocator<>, as opposed to std::pmr::memory_resource

Motivation and Proposal Summary

C++ has been moving towards a consistent PMR allocator model whereby allocator-aware (AA) types adhere to a few rules:

1. Once constructed, an object's allocator never changes.
2. The object's constructors all follow one of two argument protocols so that they work with uses-allocator construction and thus can be inserted as container elements using the container's allocator.
3. An object that can contain an AA element of user-specified type should itself be AA and use its allocator to initialize its AA subobjects.

The current std::optional does not follow the above rules. When disengaged, it forgets its allocator, violating the first rule; its constructors don't follow the AA protocols, so containers cannot pass their allocators to optional elements; and it does not hold on an allocator by which it can initialize it's contained object. As a result, std::optional is not a good fit for situations where allocators are used. A std::pmr::vector<optional<std::pmr::string>>, for example, cannot ensure that all of strings within it use the same allocator, violating a key invariant of PMR containers. If one of the elements of the vector became disengaged and re-engaged, for example, the allocator for that one element could change.

The std::pmr::optional class template proposed here properly adheres to the rules of an AA type when instantiated with an AA value type. It holds a copy of the allocator used to construct it, even when disengaged, and uses that allocator to construct its element when re-engaged. Containers of std::pmr::optional objects can correctly manage their memory.

The basic design of an AA optional is straight-forward: Add an allocator to all of its constructors and use that allocator to construct the value object each type the optional is engaged. However, when holding a non-AA type, there is no reason to pay the overhead of storing an allocator, nor to support the AA constructor interface. For this reason, std::pmr::optional has a different specialisation for AA and non-AAtype.

Design decisions:

Rather than support the entire set of allocator types allowed for C++20 containers allocators, std::pmr::optional works only with std::pmr::polymorphic_allocator. The design keeps the allocator from infecting the optional's type, in the sense that it's not a template parameter. The modern PMR-style allocator model simplifies specification, implementation, and use of std::pmr::optional. Allocator-aware optionals are interoperable even if they use different concrete allocators, because the concrete allocator doesn't change the type, nor does it change the size of the type.

An early draft of this proposal suggested using a type alias where a non-AA std::pmr::optional aliases std::optional, and AA std::pmr::optional aliases a new unnamed type. However, this causes usability issues. Type deduction in cases like :

template <typename T>
void foo(std::pmr::optional<T>);

did not work. The above was equivalent to

template <typename T>
void foo(std::conditional<std::uses_allocator <T, std::pmr::polymorphic_allocator<>>::value ,
alloc_optional<T>,
std::optional<T>>:type);

and using the nested type of the std::condigit logtional as the function template parameter made for an undeduced context.

Feedback items for LEWG:

Should std::pmr::optional be a more allocator generic type ?

In the reflector discussion, Ville Voutilainen and Tomasz Kamińskimasz made the following request : ": The C++ standard library should provide building blocks that allow multiple semantics to be expressed, as opposed to deciding on a narrow subset, and making an allocator-aware optional obey the static properties, namely the propagation traits, of an allocator is a fundamental piece of such genericity. See P2080 https://wg21.link/P2080"

This approach would consist of a basic_optional type :

template <class T, class Alloc>
basic_optional<T, Alloc>

Furthermore, pmr::optional<T> would be basic_optional<T, std::pmr::polymorphic_allocator<>>

Such basic_optional would be "allocator aware" even for types that use an allocator other than std::pmr::polymorphic_allocator. Users of allocators with different propagation traits to std::pmr::polymorphic_allocator would not be required to write their own allocator aware optional implementations as basic_optional would respect the propagation traits of the allocator used, and could be used with allocator aware containers like vector "out of the box". These users are not theoretical. In his feedback, Ville Voutilainen writes :

"I have resource-constrained (read "embedded" if you like) scenarios where I need to specify a memory arena that a particular software component uses. All allocator-aware things in that component need to use the same arena, and it's much preferred if the same arena is used by default, instead of having to specify it explicity as an exception to the default. For that to happen, SOCCC needs to return the same allocator, not create a fresh new one, and POCMA needs to be true. I can't solve this with a per-thread default resource, there are multiple separate components running on the same thread, using separate memory arenas. Our pmr::memory_resource works fine for such scenarios. Our std::polymorphic_allocator does not. Our containers work fine, because they obey the propagation traits. New facilities don't work in such scenarios if they don't support propagation traits that are different from the ones of std::polymorphic_allocator."

One could argue that it makes no sense for such a propagating allocator to be a scoped allocator and vice versa. Scoped allocators exist for the purpose of ensuring consistency between a container and its contained elements' allocator. A propagating allocator is used in exactly those situations where such consistency is not required and might be undesirable. Consider the vector case with a propagating, non-scoped allocator:

template <class T>
class PropAlloc
{
public:
...
using propagate_on_container_copy_assignment = std::true_type;
using propagate_on_container_move_assignment = std::true_type;
using propagate_on_container_swap = std::true_type;
};

using MyString = std::string<char, std::char_traits<char>, PropAlloc<char>>;
using MyVector = std::vector<MyString, PropAlloc<MyString>>;

PropAlloc allocA(1);
PropAlloc allocB(2);

MyVector v(allocA);
v.push_back("hello");
v[0] = MyString("goodbye", allocB);
assert(allocA == v.get_allocator());
assert(allocB == v[0].get_allocator());

Note that, by design, the vector and its elements do not share an allocator. There is nothing wrong with that, but it is not the scoped allocator model. Now, let's do the same with optional:

using MyOptional = std::basic_optional<MyString, PropAlloc<char>>;
MyOptional opt(std::allocator_arg_t, allocA, MyString("hello"));
opt = MyString("goodbye", allocB);
assert(allocA == opt.get_allocator());
assert(allocB == opt.value().get_allocator());

The behavior mimics the vector case, so what's the problem? The problem is "what's the purpose of storing an allocator in optional in the first place?" For vector, the allocator is used by the vector itself to allocate memory. For optional, however, the optional object does not allocate memory, so has no use for an allocator. It stores it for use by the value object, but if the value object's allocator isn't consistent with the optional, then it seems to have no real purpose. Instead, the allocator for the value can be passed directly into the emplace functions, using the existing mechanisms. However, one can conceive a scoped allocator that is not polymorphic and might even use fancy pointer types. Having `basic_optional` does give flexibility to support that.

Such approach adds some complexity to the design and reasoning about what happens. Consider:

basic_optional<T, AllocA > alice;
basic_optional<U, AllocB > bob = alice; // #1
alice = bob // #2

The questions that need to be answered are
1) what requirements are needed from AllocA and AllocB for #1 and #2 to be valid ?
2) what is the resulting allocator ?

One possible approach is to disallow #1 and #2 unless AllocA and AllocB are different specialisations of the same allocator. The user can still achieve construction/assignment by retrieving the underlying value type object :

basic_optional<T, AllocA > alice;
basic_optional<U, AllocB > bob = alice.value();
alice = bob.value()

It's worth noting that this approach increases the number of constructors and assignment operators.

With the approach described above, the resulting allocator is determined by the allocator_traits<AllocB> which are the same as allocator_traits<AllocA>. To ensure this is the case, and to keep the value_type allocator in sync with the basic_optiona allocator, every operation which would modify the allocator according to the allocator_traits<AllocB> needs to be implemented in terms of uses-allocator construction.

Now consider :

alice = basic_optional<T, AllocA >{}

When allocator_traits<AllocA >::propagate_on_container_move_assignment::value == true, allocator_traits<AllocA >::propagate_on_container_copy_assignment::value == false, and T does not support move semantics, we get into a situation where an assignment through optional object propagates the allocator, while the same assignment to a naked value_type objects doesn't. If the user designed the type (i.e. deliberately disabled move semantics) with the intention that the allocator never changes after the object has been constructed, this could cause a problem. We are not aware of such users, but the case is presented for the purposes of documenting that it has been considered.

The issues discussed above are avoided in the current design because allocator traits of std::pmr::polymorphic_allocator<> do not allow for allocator to change after the object has been created. Similarly, because only one allocator family is supported, there is no need to consider cases where the rhs allocator is of an unrelated type. However, the application of the current design is limited to a subset of users of allocator aware standard containers. Users who need allocators with propagation traits that are different to that of std::pmr::polymorphic_allocator<> would still need to write their own alllocator aware optional. If we can express the semantics of the current design in terms of the propagation traits of std::polymorphic_allocator, then we can easily extend the design to cover other propagation traits. The remaining work is deciding what to do with allocators that are unrelated and perhaps even non-convertible. For such questions, there's an approach that's forward-compatible and allows relaxations/extensions in the future.

Should std::pmr::optional be placed in the <optional> header?

Should the deduction guide for mixed optional unpack or not?

Consider

std::optional x1 = value_type();
std::pmr::optional copy1 = x1; // #1

std::optional x2 = value_type_aa();
std::pmr::optional copy2 = x2; // #2

std::pmr::optional x3 = value_type();
std::optional copy3 = x3; // #3

std::pmr::optional x4 = value_type_aa();
std::optional copy4 = x4; // #4

What should the types of x1, x2, x3, and x4 in the above example be ?

The current proposal favours unpacking, and the types are std::pmr::optional<value_type> , std::pmr::optional<value_type_aa>, std::optional<value_type> and std::optional<value_type_aa>. This seems to fit with the idea that std::pmr::optional and std::optional are types that are close enough that they deserve to be treated equally wherever they can. However, it does mean that they are somehow blessed by the standard to work together in a way a user defined type which inherits from std::optional wouldn't be.

Which allocator should be used in value_or ?

It is not all that obvious which allocator should be used for the object returned by value_or. Should the decision be left to the type or should it be mandated by the optional ? That is, should the object be constructed by plain copy/move construction or with uses-allocator construction?

Should assignments use is_­constructible trait ?

It's not construct from U we need, it's construct from U using the allocator. Do we need a new trait ?

Do we need an “allocator_aware” concept?

We might consider an “allocator aware” (AA) concept that requires a get_allocator() method that returns something convertible to std::pmr::polymorphic_allocator<>. If we used this concept instead of the std::uses_allocator trait, the allocator would require zero extra space in most (but not all) cases. (The exception would be a type that stores its allocator outside of its footprint and whose footprint is smaller than polymorphic_allocator<>.)

Proposed wording

Add new paragraphs after 20.6.1/p1 [optional.general]

[2] pmr::optional is a class template such that for a type T, where uses_allocator<T, pmr::polymorphic_allocator<>> is true, an optional object of type pmr::optional<T> will construct the contained object, if any, with an allocator of type pmr::polymorphic_allocator<>.

Modify 20.6.2 Header synopsisl [optional.syn]

namespace std {
// [optional.optional], class template optional
template<class T>
class optional;
namespace pmr {
// [pmroptional.optional], class template optional
template<class T>
class optional;
}
...

Modify 20.6.3 Class template optional [optional.optional]

// [optional.ctor], constructors
...
template<class U>
explicit(see below) optional(const pmr::optional<U>&);
template<class U>
explicit(see below) optional(pmr::optional<U>&&);
...
// [optional.assign], assignment
...
template<class U> optional& operator=(const pmr::optional&);
template<class U> optional& operator=(pmr::optional&&) noexcept(see below);
...
template<class T>
optional(T) -> optional<T>;
template<class T>
optional(const pmr::optional<T>&) -> optional<T>;
template<class T>
optional(pmr::optional<T>&&) -> optional<T>;

Modify 20.6.3.1 Constructors [optional.ctor]

template<class U> explicit(see below) optional(const optional<U>& v); ...
Mandates:
- is_­constructible_­v<T, optional<U>&> is false,
- is_­constructible_­v<T, pmr::optional<U>&> is false,
- is_­constructible_­v<T, optional<U>&&> is false,
- is_­constructible_­v<T, pmr::optional<U>&&> is false,
- is_­constructible_­v<T, const optional<U>&> is false,
- is_­constructible_­v<T, const pmr::optional<U>&> is false,
- is_­constructible_­v<T, const optional<U>&&> is false,
- is_­constructible_­v<T, const pmr::optional<U>&&> is false,
- is_­convertible_­v<optional<U>&, T> is false,
- is_­convertible_­v<pmr::optional<U>&, T> is false,
- is_­convertible_­v<optional<U>&&, T> is false,
- is_­convertible_­v<pmr::optional<U>&&, T> is false,
- is_­convertible_­v<const optional<U>&, T> is false, and
- is_­convertible_­v<const pmr::optional<U>&, T> is false,
- is_­convertible_­v<const optional<U>&&, T> is false,and
- is_­convertible_­v<const pmr::optional<U>&&, T> is false ...

template<class U> explicit(see below) optional(optional<U>&& v); ...
Mandates:
- is_­constructible_­v<T, optional<U>&> is false,
- is_­constructible_­v<T, pmr::optional<U>&> is false,
- is_­constructible_­v<T, optional<U>&&> is false,
- is_­constructible_­v<T, pmr::optional<U>&&> is false,
- is_­constructible_­v<T, const optional<U>&> is false,
- is_­constructible_­v<T, const pmr::optional<U>&> is false,
- is_­constructible_­v<T, const optional<U>&&> is false,
- is_­constructible_­v<T, const pmr::optional<U>&&> is false,
- is_­convertible_­v<optional<U>&, T> is false,
- is_­convertible_­v<pmr::optional<U>&, T> is false,
- is_­convertible_­v<optional<U>&&, T> is false,
- is_­convertible_­v<pmr::optional<U>&&, T> is false,
- is_­convertible_­v<const optional<U>&, T> is false, and
- is_­convertible_­v<const pmr::optional<U>&, T> is false,
- is_­convertible_­v<const optional<U>&&, T> is false, and
- is_­convertible_­v<const pmr::optional<U>&&, T> is false ...

...

template<class U> explicit(see below) optional(const pmr::optional<U>& rhs);
Effects: If rhs contains a value, initializes the contained value as if direct-non-list-initializing an object of type T with the expression *rhs.
Ensures: bool(rhs) == bool(*this).
Throws: Any exception thrown by the selected constructor of T.
Remarks:
The expression inside explicit is equivalent to:
!is_convertible_v<const U&, T>
Mandates:
- is_­constructible_­v<T, const U&> is true,
- is_same<T,U> is false,
- is_­constructible_­v<T, optional<U>&> is false,
- is_­constructible_­v<T, pmr::optional<U>&> is false,
- is_­constructible_­v<T, optional<U>&&> is false,
- is_­constructible_­v<T, pmr::optional<U>&&> is false,
- is_­constructible_­v<T, const optional<U>&> is false,
- is_­constructible_­v<T, const pmr::optional<U>&> is false,
- is_­constructible_­v<T, const optional<U>&&> is false,
- is_­constructible_­v<T, const pmr::optional<U>&&> is false,
- is_­convertible_­v<optional<U>&, T> is false,
- is_­convertible_­v<pmr::optional<U>&, T> is false,
- is_­convertible_­v<optional<U>&&, T> is false,
- is_­convertible_­v<pmr::optional<U>&&, T> is false,
- is_­convertible_­v<const optional<U>&, T> is false,
- is_­convertible_­v<const pmr::optional<U>&, T> is false,
- is_­convertible_­v<const optional<U>&&, T> is false, and
- is_­convertible_­v<const pmr::optional<U>&&, T> is false


template<class U> explicit(see below) optional(pmr::optional<U>&& rhs);
Effects: If rhs contains a value, initializes the contained value as if direct-non-list-initializing an object of type T with the expression std​::​move(*rhs). std​::​move(*rhs) as the constructor argument. bool(rhs) is unchanged.
Ensures: bool(rhs) == bool(*this).
Throws: Any exception thrown by the selected constructor of T.
Remarks:The expression inside explicit is equivalent to:
!is_convertible_v<U, T>
Mandates:
- is_­constructible_­v<T,U&&> is true,
- is_same<T,U> is false,
- is_­constructible_­v<T, optional<U>&> is false,
- is_­constructible_­v<T, pmr::optional<U>&> is false,
- is_­constructible_­v<T, optional<U>&&> is false,
- is_­constructible_­v<T, pmr::optional<U>&&> is false,
- is_­constructible_­v<T, const optional<U>&> is false,
- is_­constructible_­v<T, const pmr::optional<U>&> is false,
- is_­constructible_­v<T, const optional<U>&&> is false,
- is_­constructible_­v<T, const pmr::optional<U>&&> is false,
- is_­convertible_­v<optional<U>&, T> is false,
- is_­convertible_­v<pmr::optional<U>&, T> is false,
- is_­convertible_­v<optional<U>&&, T> is false,
- is_­convertible_­v<pmr::optional<U>&&, T> is false,
- is_­convertible_­v<const optional<U>&, T> is false,
- is_­convertible_­v<const pmr::optional<U>&, T> is false,
- is_­convertible_­v<const optional<U>&&, T> is false, and
- is_­convertible_­v<const pmr::optional<U>&&, T> is false

Modify 20.6.3.3 Assignment [optional.assign]

...
template<class U> optional& operator=(const optional<U>& rhs); ...
Remarks: ... This operator shall not participate in overload resolution unless
- is_­constructible_­v<T, const U&> is true,
- is_assignable<T&, const U&> is true,
- is_­constructible_­v<T, optional<U>&> is false,
- is_­constructible_­v<T, pmr::optional<U>&> is false,
- is_­constructible_­v<T, optional<U>&&> is false,
- is_­constructible_­v<T, pmr::optional<U>&&> is false,
- is_­constructible_­v<T, const optional<U>&> is false,
- is_­constructible_­v<T, const pmr::optional<U>&> is false,
- is_­constructible_­v<T, const optional<U>&&> is false,
- is_­constructible_­v<T, const pmr::optional<U>&&> is false,
- is_­convertible_­v<optional<U>&, T> is false,
- is_­convertible_­v<pmr::optional<U>&, T> is false,
- is_­convertible_­v<optional<U>&&, T> is false,
- is_­convertible_­v<pmr::optional<U>&&, T> is false,
- is_­convertible_­v<const optional<U>&, T> is false,
- is_­convertible_­v<const pmr::optional<U>&, T> is false,
- is_­convertible_­v<const optional<U>&&, T> is false,
- is_­convertible_­v<const pmr::optional<U>&&, T> is false,
- is_assignable_v<T&, optional<U>&> is false,
- is_assignable_v<T&, pmr::optional<U>&> is false,
- is_assignable_v<T&, optional<U>&&> is false,
- is_assignable_v<T&, pmr::optional<U>&&> is false,
- is_assignable_v<T&, const optional<U>&> is false, and
- is_assignable_v<T&, const pmr::optional<U>&> is false,
- is_assignable_v<T&, const optional<U>&&> is false,and
- is_assignable_v<T&, const pmr::optional<U>&&> is false

template<class U> optional& operator=(optional<U>&& rhs); ...
Remarks: ... This operator shall not participate in overload resolution unless
- is_­constructible_­v<T, const U&> is true,
- is_assignable<T&, const U&> is true,
- is_­constructible_­v<T, optional<U>&> is false,
- is_­constructible_­v<T, pmr::optional<U>&> is false,
- is_­constructible_­v<T, optional<U>&&> is false,
- is_­constructible_­v<T, pmr::optional<U>&&> is false,
- is_­constructible_­v<T, const optional<U>&> is false,
- is_­constructible_­v<T, const pmr::optional<U>&> is false,
- is_­constructible_­v<T, const optional<U>&&> is false,
- is_­constructible_­v<T, const pmr::optional<U>&&> is false,
- is_­convertible_­v<optional<U>&, T> is false,
- is_­convertible_­v<pmr::optional<U>&, T> is false,
- is_­convertible_­v<optional<U>&&, T> is false,
- is_­convertible_­v<pmr::optional<U>&&, T> is false,
- is_­convertible_­v<const optional<U>&, T> is false,
- is_­convertible_­v<const pmr::optional<U>&, T> is false,
- is_­convertible_­v<const optional<U>&&, T> is false,
- is_­convertible_­v<const pmr::optional<U>&&, T> is false,
- is_assignable_v<T&, optional<U>&> is false,
- is_assignable_v<T&, pmr::optional<U>&> is false,
- is_assignable_v<T&, optional<U>&&> is false,
- is_assignable_v<T&, pmr::optional<U>&&> is false,
- is_assignable_v<T&, const optional<U>&> is false, and
- is_assignable_v<T&, const pmr::optional<U>&> is false,
- is_assignable_v<T&, const optional<U>&&> is false,and
- is_assignable_v<T&, const pmr::optional<U>&&> is false

template<class U> optional& operator=(const pmr::optional<U>& rhs);
Effects: See Table

Table — optional operator=(const pmr::optional<U>&) effects

	*this contains a value	*this does not contain a value
rhs contains a value	assigns *rhs to the contained value	initializes the contained value as if direct-non-list-initializing an object of type T with *rhs.
rhs does not contain a value	destroys the contained value by calling val->T::~T()	no effect

Returns: *this.
Ensures: bool(rhs) == bool(*this).
Remarks: If any exception is thrown, the result of the expression bool(*this) remains unchanged. If an exception is thrown during the call to T's constructor, the state of *rhs.val is determined by the exception safety guarantee of T's constructor. If an exception is thrown during the call to T's assignment, the state of *val and *rhs.val is determined by the exception safety guarantee of T's assignment.
Mandates:
- is_same<T,U> is false,
- is_­constructible_­v<T, const U&> is true,
- is_assignable<T&, const U&> is true,
- is_­constructible_­v<T, optional<U>&> is false,
- is_­constructible_­v<T, pmr::optional<U>&> is false,
- is_­constructible_­v<T, optional<U>&&> is false,
- is_­constructible_­v<T, pmr::optional<U>&&> is false,
- is_­constructible_­v<T, const optional<U>&> is false,
- is_­constructible_­v<T, const pmr::optional<U>&> is false,
- is_­constructible_­v<T, const optional<U>&&> is false,
- is_­constructible_­v<T, const pmr::optional<U>&&> is false,
- is_­convertible_­v<const U&, T> is true,
- is_­convertible_­v<optional<U>&, T> is false,
- is_­convertible_­v<pmr::optional<U>&, T> is false,
- is_­convertible_­v<optional<U>&&, T> is false,
- is_­convertible_­v<pmr::optional<U>&&, T> is false,
- is_­convertible_­v<const optional<U>&, T> is false,
- is_­convertible_­v<const pmr::optional<U>&, T> is false,
- is_­convertible_­v<const optional<U>&&, T> is false,
- is_­convertible_­v<const pmr::optional<U>&&, T> is false,
- is_assignable_v<T&, optional<U>&> is false,
- is_assignable_v<T&, pmr::optional<U>&> is false,
- is_assignable_v<T&, optional<U>&&> is false,
- is_assignable_v<T&, pmr::optional<U>&&> is false,
- is_assignable_v<T&, const optional<U>&> is false,
- is_assignable_v<T&, const pmr::optional<U>&> is false,
- is_assignable_v<T&, const optional<U>&&> is false, and
- is_assignable_v<T&, const pmr::optional<U>&&> is false

template<class U> optional& operator=(pmr::optional<U>&& rhs);
Effects: See Table

Table — optional operator=(pmr::optional<U>&&) effects

	*this contains a value	*this does not contain a value
rhs contains a value	assigns *rhs to the contained value	initializes the contained value as if direct-non-list-initializing an object of type T with std​::​move(*rhs)
rhs does not contain a value	destroys the contained value by calling val->T::~T()	no effect

Returns: *this.
Ensures: bool(rhs) == bool(*this).
Remarks: If any exception is thrown, the result of the expression bool(*this) remains unchanged. If an exception is thrown during the call to T's constructor, the state of *rhs.val is determined by the exception safety guarantee of T's constructor. If an exception is thrown during the call to T's assignment, the state of *val and *rhs.val is determined by the exception safety guarantee of T's assignment.
Mandates:
- is_­constructible_­v<T, const U&> is true,
- is_assignable<T&, const U&> is true,
- is_­constructible_­v<T, std::optional<U>&> is false,
- is_­constructible_­v<T, pmr::optional<U>&> is false,
- is_­constructible_­v<T, std::optional<U>&&> is false,
- is_­constructible_­v<T, pmr::optional<U>&&> is false,
- is_­constructible_­v<T, const std::optional<U>&> is false,
- is_­constructible_­v<T, const pmr::optional<U>&> is false,
- is_­constructible_­v<T, const std::optional<U>&&> is false,
- is_­constructible_­v<T, const pmr::optional<U>&&> is false,
- is_­convertible_­v<const U&, T> is true,
- is_­convertible_­v<std::optional<U>&, T> is false,
- is_­convertible_­v<pmr::optional<U>&, T> is false,
- is_­convertible_­v<std::optional<U>&&, T> is false,
- is_­convertible_­v<pmr::optional<U>&&, T> is false,
- is_­convertible_­v<const std::optional<U>&, T> is false,
- is_­convertible_­v<const pmr::optional<U>&, T> is false,
- is_­convertible_­v<const std::optional<U>&&, T> is false,
- is_­convertible_­v<const pmr::optional<U>&&, T> is false,
- is_assignable_v<T&, std::optional<U>&> is false,
- is_assignable_v<T&, pmr::optional<U>&> is false,
- is_assignable_v<T&, std::optional<U>&&> is false,
- is_assignable_v<T&, pmr::optional<U>&&> is false,
- is_assignable_v<T&, const std::optional<U>&> is false,
- is_assignable_v<T&, const pmr::optional<U>&> is false,
- is_assignable_v<T&, const std::optional<U>&&> is false, and
- is_assignable_v<T&, const pmr::optional<U>&&> is false

Insert new sections after 20.6.3 Class template optional [optional.optional]

Header <optional> synopsis [pmroptional.sys]

namespace std::pmr {
// [pmroptional.optional]
template<class T>
class optional;

// [pmroptional.traits], allocator-related traits
template<class T>
struct uses_allocator<T, polymorphic_allocator<>>;

template<class T>
constexpr bool uses-poly-alloc = uses_allocator<T, pmr::polymorphic_allocator<>>::value; // exposition only

// [pmroptional.relops], relational operators
template<class T, class U>
bool operator==(const optional<T>&, const optional<U>&);
template<class T, class U>
bool operator==(const optional<T>&, const std::optional<U>&);
template<class T, class U>
bool operator==(const std::optional<T>&, const optional<U>&);
template<class T, class U>
bool operator!=(const optional<T>&, const optional<U>&);
template<class T, class U>
bool operator!=(const optional<T>&, const std::optional<U>&);
template<class T, class U>
bool operator!=(const std::optional<T>&, const optional<U>&);
template<class T, class U>
bool operator<(const optional<T>&, const optional<U>&);
template<class T, class U>
bool operator<(const optional<T>&, const std::optional<U>&);
template<class T, class U>
bool operator<(const std::optional<T>&, const optional<U>&);
template<class T, class U>
bool operator>(const optional<T>&, const optional<U>&);
template<class T, class U>
bool operator>(const optional<T>&, const std::optional<U>&);
template<class T, class U>
bool operator>(const std::optional<T>&, const optional<U>&);
template<class T, class U>
bool operator<=(const optional<T>&, const optional<U>&);
template<class T, class U>
bool operator<=(const optional<T>&, const std::optional<U>&);
template<class T, class U>
bool operator<=(const std::optional<T>&, const optional<U>&);
template<class T, class U>
bool operator>=(const optional<T>&, const optional<U>&);
template<class T, class U>
bool operator>=(const optional<T>&, const std::optional<U>&);
template<class T, class U>
bool operator>=(const std::optional<T>&, const optional<U>&);
template<class T, three_way_comparable_with<T> U>
compare_three_way_result_t<T,U>
operator<=>(const optional<T>&, const optional<U>&);
template<class T, three_way_comparable_with<T> U>
compare_three_way_result_t<T,U>
operator<=>(const optional<T>&, const std::optional<U>&);
template<class T, three_way_comparable_with<T> U>
compare_three_way_result_t<T,U>
operator<=>(const std::optional<T>&, const optional<U>&);

// [pmroptional.nullops], comparison with nullopt
template<class T> bool operator==(const optional<T>&, nullopt_t) noexcept;
template<class T>
strong_ordering operator<=>(const optional<T>&, nullopt_t) noexcept;

// [pmroptional.comp.with.t], comparison with T
template<class T, class U> bool operator==(const optional<T>&, const U&);
template<class T, class U> bool operator==(const T&, const optional<U>&);
template<class T, class U> bool operator!=(const optional<T>&, const U&);
template<class T, class U> bool operator!=(const T&, const optional<U>&);
template<class T, class U> bool operator<(const optional<T>&, const U&);
template<class T, class U> bool operator<(const T&, const optional<U>&);
template<class T, class U> bool operator>(const optional<T>&, const U&);
template<class T, class U> bool operator>(const T&, const optional<U>&);
template<class T, class U> bool operator<=(const optional<T>&, const U&);
template<class T, class U> bool operator<=(const T&, const optional<U>&);
template<class T, class U> bool operator>=(const optional<T>&, const U&);
template<class T, class U> bool operator>=(const T&, const optional<U>&);
template<class T, three_way_comparable_with<T> U>
compare_three_way_result_t<T,U>
operator<=>(const optional<T>&, const U&);

// [pmroptional.specalg], specialized algorithms
template<class T>
void swap(optional<T>&, optional<T>&) ;

template<class T>
constexpr optional<see below> make_optional(T&&);
template<class T>
constexpr optional<T> make_optional();
template<class T, class Arg0, class... Args>
constexpr optional<T> make_optional(Arg0 arg0&&, Args&&... args);
template<class T, class U, class... Args>
constexpr optional<T> make_optional(initializer_list<U> il, Args&&... args);

template<class T>
constexpr optional<see below> make_optional(allocator_arg_t, const polymorphic_allocator<>& a, T&&);
template<class T, class... Args>
constexpr optional<T> make_optional(allocator_arg_t, const polymorphic_allocator<>& a, Args&&... args);
template<class T, class U, class... Args>
constexpr optional<T> make_optional(allocator_arg_t, const polymorphic_allocator<>& a, initializer_list<U> il, Args&&... args);

// [pmroptional.hash], hash support
template<class T> struct hash<optional<T>>;
}

Class template optional [optional.pmroptional]

namespace std::pmr {
template<class T>
class optional see below{
public:
using value_type = T;
using allocator_type = polymorphic_allocator<>; // not always defined, see below

// [pmroptional.ctor], constructors
optional() noexcept;
optional(nullopt_t) noexcept;
optional(const optional&);
optional(optional&&) noexcept(see below);
optional(const std::optional&);
optional(std::optional&&);
template<class... Args>
explicit optional(in_place_t, Args&&...);
template<class U, class... Args>
explicit optional(in_place_t, initializer_list<U>, Args&&...);
template<class U = T>
explicit(see below) optional(U&&);
template<class U>
explicit(see below) optional(const optional<U>&);
template<class U>
explicit(see below) optional(optional<U>&&);
template<class U>
explicit(see below) optional(const std::optional<U>&);
template<class U>
explicit(see below) optional(std::optional<U>&&);

// allocator-extended constructors
optional(allocator_arg_t, const polymorphic_allocator<>&) noexcept; // not always provided, see below
optional(allocator_arg_t, const polymorphic_allocator<>&, nullopt_t) noexcept; // not always provided, see below
optional(allocator_arg_t, const polymorphic_allocator<>&, const optional&); // not always provided, see below
optional(allocator_arg_t, const polymorphic_allocator<>&, optional&&) noexcept(see below); // not always provided, see below
optional(allocator_arg_t, const polymorphic_allocator<>&, const std::optional&); // not always provided, see below
optional(allocator_arg_t, const polymorphic_allocator<>&, std::optional&&); // not always provided, see below
template<class... Args>
explicit optional(allocator_arg_t, const polymorphic_allocator<>&, in_place_t, Args&&...); // not always provided, see below
template<class U, class... Args>
explicit optional(allocator_arg_t, const polymorphic_allocator<>&, in_place_t, initializer_list<U>, Args&&...); // not always provided, see below
template<class U = T>
explicit(see below) optional(allocator_arg_t, const polymorphic_allocator<>&,U&&); // not always provided, see below
template<class U>
explicit(see below) optional(allocator_arg_t, const polymorphic_allocator<>&,const optional<U>&); // not always provided, see below
template<class U>
explicit(see below) optional(allocator_arg_t, const polymorphic_allocator<>&,optional<U>&&); // not always provided, see below
template<class U>
explicit(see below) optional(allocator_arg_t, const polymorphic_allocator<>&,const std::optional<U>&); // not always provided, see below
template<class U>
explicit(see below) optional(allocator_arg_t, const polymorphic_allocator<>&,std::optional<U>&&); // not always provided, see below

// [pmroptional.dtor], destructor
~optional();

// [pmroptional.assign], assignment
optional& operator=(nullopt_t) noexcept;
optional& operator=(const optional&);
optional& operator=(optional&&) noexcept(see below);
template<class U = T> optional& operator=(U&&);
template<class U> optional& operator=(const optional<U>&);
template<class U> optional& operator=(optional<U>&&);
template<class U> optional& operator=(const std::optional<U>&);
template<class U> optional& operator=(std::optional<U>&&);
template<class... Args>> T& emplace(Args&&...);
template<class U, class... Args> T& emplace(initializer_list<U>, Args&&...);

// [pmroptional.swap], swap
void swap(optional&) noexcept(see below);

// [pmroptional.observe], observers
const T* operator->() const;
T* operator->();
const T& operator*() const&;
T& operator*() &;
T&& operator*() &&;
const T&& operator*() const&&;
explicit operator bool() const noexcept;
bool has_value() const noexcept;
const T& value() const&;
T& value() &;
T&& value() &&;
const T&& value() const&&;
template<class U> T value_or(U&&) const&;
template<class U> T value_or(U&&) &&;
template<class U> T value_or(allocator_arg_t, const polymorphic_allocator<>&, U&&) const&; // not always provided, see below
template<class U> T value_or(allocator_arg_t, const polymorphic_allocator<>&, U&&) &&; // not always provided, see below
allocator_type get_allocator() const noexcept;

// [pmroptional.mod], modifiers
void reset() noexcept;

private:
T* val; // exposition only
const polymorphic_allocator<> alloc; // exposition only; not always provided, see below
};

template<class T>
optional(T) -> optional<T>;
template<class T>
optional(const std::optional<T>&) -> optional<T>;
template<class T>
optional(std::optional<T>&&) -> optional<T>;

}

Any instance of optional<T> at any given time either contains a value or does not contain a value. When an instance of optional<T> contains a value, it means that an object of type T is allocated within the storage of the optional object. Implementations are not permitted to use additional storage, such as dynamic memory, to allocate its contained value. The contained value shall be allocated in a region of the optional<T> storage suitably aligned for the type T and, if uses-poly-alloc is true, constructed by uses-allocator construction with allocator alloc. When an object of type optional<T> is contextually converted to bool, the conversion returns true if the object contains a value; otherwise the conversion returns false.

Member val is provided for exposition only. When an optional<T> object contains a value, val points to the contained value.

If and only if uses-poly-alloc is false, optional<T> publicly inherits from std::optional<T>

If and only if uses-poly-alloc is true, the following applies :
- allocator_type will be defined,
- exposition only member alloc will be provided and will be used to create the contained value, if any, and
- if T provides a get_allocator() member function whose return type is convertible to polymorphic_allocator<>, the implementation is allowed to assume that the allocator used for constructing the element is the same allocator as returned by T::get_allocator(). [Note: In such an implementation, pmr::optional<pmr::string> will be zero-overhead compared to optional<pmr::string>. -- end note]

T shall be a type other than cv in_­place_­t, cv allocator_arg_­t, or cv nullopt_­t that meets the Cpp17Destructible requirements (Table 30).

Constructors [pmroptional.ctor]

optional() noexcept;
optional(nullopt_t) noexcept;
Effects: If uses-poly-alloc is true, alloc is default initialized
Ensures: *this does not contain a value.
Remarks: No contained value is initialized.

optional(const optional&);
Effects: If uses-poly-alloc is true, alloc is default initialized. If rhs contains a value, constructs the contained value with uses-allocator construction ([allocator.uses.construction]) with alloc and the expression *rhs.
Ensures: bool(rhs) == bool(*this).
Throws: Any exception thrown by the selected constructor of T.
Mandates:is_­copy­_constructible_­v<T> is true.

optional(optional&& rhs) noexcept(see below)
Effects: If uses-poly-alloc is true, alloc is initialized with rhs.alloc. If rhs contains a value, constructs the contained value with uses-allocator construction ([allocator.uses.construction]) with alloc and the expression std​::​move(*rhs). bool(rhs) is unchanged.
Ensures: bool(rhs) == bool(*this).
Throws: Any exception thrown by the selected constructor of T.
Remarks: The expression inside noexcept is equivalent to is_­nothrow_­move_­constructible_­v<T>.
Mandates:is_­move_­constructible_­v<T> is true.

optional(const std::optional&);
Effects: If uses-poly-alloc is true, alloc is default initialized. If rhs contains a value, initializes the contained value with the expression *rhs.
Ensures: bool(rhs) == bool(*this).
Throws: Any exception thrown by the selected constructor of T.
Mandates:is_­copy­_constructible_­v<T> is true.

optional(std::optional&& rhs)
Effects: If uses-poly-alloc is true, alloc is initialized with rhs.alloc. If rhs contains a value, constructs the contained value with uses-allocator construction ([allocator.uses.construction]) with alloc and the expression std​::​move(*rhs). bool(rhs) is unchanged.
Ensures: bool(rhs) == bool(*this).
Throws: Any exception thrown by the selected constructor of T.
Mandates:is_­move_­constructible_­v<T> is true.

template<class... Args> explicit optional(in_place_t, Args&&... args);
Effects: If uses-poly-alloc is true, alloc is default initialized. Constructs the contained value with uses-allocator construction ([allocator.uses.construction]) with alloc and std​::​forward<Args>(args).....
Ensures: *this contains a value..
Throws: Any exception thrown by the selected constructor of T.
Mandates:is_­copy_­constructible_­v<T> is true.

template<class... Args> explicit optional(in_place_t, initializer_list<U> il, Args&&... args);
Effects: If uses-poly-alloc is true, alloc is default initialized. Constructs the contained value with uses-allocator construction ([allocator.uses.construction]) with alloc, il, and std​::​forward<Args>(args).....
Ensures: *this contains a value..
Throws: Any exception thrown by the selected constructor of T.
Mandates:is_­constructible_­v<T, initializer_­list<U>&, Args&&...> is true.

template<class U = T> explicit(see below) optional(U&& rhs);
Effects:If uses-poly-alloc is true, alloc is default initialized. Constructs the contained value with uses-allocator construction ([allocator.uses.construction]) with alloc and std​::​forward<U>(rhs).
Ensures: *this contains a value.
Throws: Any exception thrown by the selected constructor of T.
Remarks:The expression inside explicit is equivalent to:
!is_convertible_v<U, T>
Mandates:
- is_­constructible_­v<T, U&&> is true,
- is_­same_­v<remove_­cvref_­t<U>, in_­place_­t> is false, and
- is_­same_­v<remove_­cvref_­t<U>,optional> is false.

template<class U> explicit(see below) optional(const optional<U>& rhs);
Effects:If uses-poly-alloc is true, alloc is default initialized. If rhs contains a value, constructs the contained value with uses-allocator construction ([allocator.uses.construction]) with alloc and *rhs.
Ensures: bool(rhs) == bool(*this).
Throws: Any exception thrown by the selected constructor of T.
Remarks:The expression inside explicit is equivalent to:
!is_convertible_v<const U&, T>
Mandates:
- is_­constructible_­v<T, const U&> is true,
- is_­constructible_­v<T, std::optional<U>&> is false,
- is_­constructible_­v<T, optional<U>&> is false,
- is_­constructible_­v<T, std::optional<U>&&> is false,
- is_­constructible_­v<T, optional<U>&&> is false,
- is_­constructible_­v<T, const std::optional<U>&> is false,
- is_­constructible_­v<T, const optional<U>&> is false,
- is_­constructible_­v<T, const std::optional<U>&&> is false,
- is_­constructible_­v<T, const optional<U>&&> is false,
- is_­convertible_­v<std::optional<U>&, T> is false,
- is_­convertible_­v<optional<U>&, T> is false,
- is_­convertible_­v<std::optional<U>&&, T> is false,
- is_­convertible_­v<optional<U>&&, T> is false,
- is_­convertible_­v<const std::optional<U>&, T> is false,
- is_­convertible_­v<const optional<U>&, T> is false,
- is_­convertible_­v<const std::optional<U>&&, T> is false, and
- is_­convertible_­v<const optional<U>&&, T> is false


template<class U> explicit(see below) optional(optional<U>&& rhs);
Effects:If uses-poly-alloc is true, alloc is default initialized. If rhs contains a value, constructs the contained value with uses-allocator construction ([allocator.uses.construction]) with alloc and std​::​move(*rhs). bool(rhs) is unchanged.
Ensures: bool(rhs) == bool(*this).
Throws: Any exception thrown by the selected constructor of T.
Remarks:The expression inside explicit is equivalent to:
!is_convertible_v<U, T>Mandates:
- is_­constructible_­v<T,U&&> is true,
- is_­constructible_­v<T, std::optional<U>&> is false,
- is_­constructible_­v<T, optional<U>&> is false,
- is_­constructible_­v<T, std::optional<U>&&> is false,
- is_­constructible_­v<T, optional<U>&&> is false,
- is_­constructible_­v<T, const std::optional<U>&> is false,
- is_­constructible_­v<T, const optional<U>&> is false,
- is_­constructible_­v<T, const std::optional<U>&&> is false,
- is_­constructible_­v<T, const optional<U>&&> is false,
- is_­convertible_­v<std::optional<U>&, T> is false,
- is_­convertible_­v<optional<U>&, T> is false,
- is_­convertible_­v<std::optional<U>&&, T> is false,
- is_­convertible_­v<optional<U>&&, T> is false,
- is_­convertible_­v<const std::optional<U>&, T> is false,
- is_­convertible_­v<const optional<U>&, T> is false,
- is_­convertible_­v<const std::optional<U>&&, T> is false, and
- is_­convertible_­v<const optional<U>&&, T> is false


template<class U> explicit(see below) optional(const std::optional<U>& rhs);
Effects:If uses-poly-alloc is true, alloc is default initialized. If rhs contains a value, constructs the contained value with uses-allocator construction ([allocator.uses.construction]) with alloc and *rhs.
Ensures: bool(rhs) == bool(*this).
Throws: Any exception thrown by the selected constructor of T.
Remarks:The expression inside explicit is equivalent to:
!is_convertible_v<const U&, T>
Mandates:
- is_­constructible_­v<T, const U&> is true,
- is_­constructible_­v<T, std::optional<U>&> is false,
- is_­constructible_­v<T, optional<U>&> is false,
- is_­constructible_­v<T, std::optional<U>&&> is false,
- is_­constructible_­v<T, optional<U>&&> is false,
- is_­constructible_­v<T, const std::optional<U>&> is false,
- is_­constructible_­v<T, const optional<U>&> is false,
- is_­constructible_­v<T, const std::optional<U>&&> is false,
- is_­constructible_­v<T, const optional<U>&&> is false,
- is_­convertible_­v<std::optional<U>&, T> is false,
- is_­convertible_­v<optional<U>&, T> is false,
- is_­convertible_­v<std::optional<U>&&, T> is false,
- is_­convertible_­v<optional<U>&&, T> is false,
- is_­convertible_­v<const std::optional<U>&, T> is false,
- is_­convertible_­v<const optional<U>&, T> is false,
- is_­convertible_­v<const std::optional<U>&&, T> is false, and
- is_­convertible_­v<const optional<U>&&, T> is false


template<class U> explicit(see below) optional(std::optional<U>&& rhs);
Effects:If uses-poly-alloc is true, alloc is default initialized. If rhs contains a value, constructs the contained value with uses-allocator construction ([allocator.uses.construction]) with alloc and std​::​move(*rhs). bool(rhs) is unchanged.
Ensures: bool(rhs) == bool(*this).
Throws: Any exception thrown by the selected constructor of T.
Remarks:
The expression inside explicit is equivalent to:
!is_convertible_v<U, T>
Mandates:
- is_­constructible_­v<T,U&&> is true,
- is_­constructible_­v<T, std::optional<U>&> is false,
- is_­constructible_­v<T, optional<U>&> is false,
- is_­constructible_­v<T, std::optional<U>&&> is false,
- is_­constructible_­v<T, optional<U>&&> is false,
- is_­constructible_­v<T, const std::optional<U>&> is false,
- is_­constructible_­v<T, const optional<U>&> is false,
- is_­constructible_­v<T, const std::optional<U>&&> is false,
- is_­constructible_­v<T, const optional<U>&&> is false,
- is_­convertible_­v<std::optional<U>&, T> is false,
- is_­convertible_­v<optional<U>&, T> is false,
- is_­convertible_­v<std::optional<U>&&, T> is false,
- is_­convertible_­v<optional<U>&&, T> is false,
- is_­convertible_­v<const std::optional<U>&, T> is false,
- is_­convertible_­v<const optional<U>&, T> is false,
- is_­convertible_­v<const std::optional<U>&&, T> is false, and
- is_­convertible_­v<const optional<U>&&, T> is false


optional(allocator_arg_t, const polymorphic_allocator<>&) noexcept;
optional(allocator_arg_t, const polymorphic_allocator<>&, nullopt_t) noexcept;
optional(allocator_arg_t, const polymorphic_allocator<>&, const optional&);
optional(allocator_arg_t, const polymorphic_allocator<>&, optional&&) noexcept(see below);
optional(allocator_arg_t, const polymorphic_allocator<>&, const std::optional&);
optional(allocator_arg_t, const polymorphic_allocator<>&, std::optional&&);
template<class... Args>
explicit optional(allocator_arg_t, const polymorphic_allocator<>&, in_place_t, Args&&...);
template<class U, class... Args>
explicit optional(allocator_arg_t, const polymorphic_allocator<>&, in_place_t, initializer_list<U>, Args&&...);
template<class U = T>
explicit(see below) optional(allocator_arg_t, const polymorphic_allocator<>&,U&&);
template<class U>
explicit(see below) optional(allocator_arg_t, const polymorphic_allocator<>&,const optional<U>&);
template<class U>
explicit(see below) optional(allocator_arg_t, const polymorphic_allocator<>&,optional<U>&&);
template<class U>
explicit(see below) optional(allocator_arg_t, const polymorphic_allocator<>&,const std::optional<U>&);
template<class U>
explicit(see below) optional(allocator_arg_t, const polymorphic_allocator<>&,std::optional<U>&&);
Constraints: uses_allocator<T,polymorphic_allocator<>> is true;
Effects: Equivalent to the preceding constructors except that alloc is initialized with a.

Destructor [pmroptional.dtor]

~optional();
Effects: If is_­trivially_­destructible_­v<T> != true and *this contains a value, calls val->T::~T()
Remarks: If is_­trivially_­destructible_­v<T> is true, then this destructor is trivial.

Assignment [pmroptional.assign]

optional<T>& operator=(nullopt_t) noexcept;
Effects: If *this contains a value, calls val->T::~T() to destroy the contained value; otherwise no effect.
Remarks: *this
Ensures: *this does not contain a value.

optional<T>& operator=(const optional& rhs);
Effects: See Table

Table — optional operator=(const optional& rhs) effects

	*this contains a value	*this does not contain a value
rhs contains a value	assigns *rhs to the contained value	initializes the contained value with *rhs. If uses-poly-alloc is true, initialization is done by uses-allocator construction with allocator alloc
rhs does not contain a value	destroys the contained value by calling val->T::~T()	no effect

Returns: *this.
Ensures: bool(rhs) == bool(*this).
Remarks: If any exception is thrown, the result of the expression bool(*this) remains unchanged. If an exception is thrown during the call to T's copy constructor, no effect. If an exception is thrown during the call to T's copy assignment, the state of of its contained value is as defined by the exception safety guarantee of is determined by the exception safety guarantee of T's copy assignment. This operator shall be defined as deleted unless is_­copy_­constructible_­v<T> is true and is_­copy_­assignable_­v<T> is true.

optional<T>& operator=(optional&& rhs) noexcept(see below);
Effects: See Table

Table — optional operator=(optional&& rhs) effects

	*this contains a value	*this does not contain a value
rhs contains a value	assigns std​::​move(*rhs) to the contained value	initializes the contained value with std​::​move(*rhs). If uses-poly-alloc is true, initialization is done by uses-allocator construction with allocator alloc
rhs does not contain a value	destroys the contained value by calling val->T::~T()	no effect

Returns: *this.
Ensures: bool(rhs) == bool(*this).
Remarks: The expression inside noexcept is equivalent to:
is_nothrow_move_assignable_v<T> && is_nothrow_move_constructible_v<T>
If any exception is thrown, the result of the expression bool(*this) remains unchanged. If an exception is thrown during the call to T's move constructor, the state of *rhs.val is determined by the exception safety guarantee of T's move constructor. If an exception is thrown during the call to T's move assignment, the state of *val and *rhs.val is determined by the exception safety guarantee of T's move assignment.
Mandates: is_­move_­constructible_­v<T> is true and is_­move_­assignable_­v<T> is true.

template<class U = T> optional& operator=(U&& v);
Effects: If *this contains a value, assigns std​::​forward<U>(v) to the contained value; otherwise initializes the contained value with std​::​forward<U>(v). If uses-poly-alloc is true, initialization is done by uses-allocator construction with allocator alloc
Returns: *this.
Ensures: *this contains a value.
Remarks: If any exception is thrown, the result of the expression bool(*this) remains unchanged. If an exception is thrown during the call to T's constructor, the state of v is determined by the exception safety guarantee of T's constructor. If an exception is thrown during the call to T's assignment, the state of *val and v is determined by the exception safety guarantee of T's assignment.
Mandates:
- is_­same_­v<remove_­cvref_­t<U>, alloc-optional> is false,
- conjunction_­v<is_­scalar<T>, is_­same<T, decay_­t<U>>> is false,
- is_­constructible_­v<T,U> is true, and
- is_­assignable_­v<T&,U> is true

template<class U> optional& operator=(const optional<U>& rhs);
Effects: See Table

Table — optional operator=(const optional<U>&) effects

	*this contains a value	*this does not contain a value
rhs contains a value	assigns *rhs to the contained value	initializes the contained value with *rhs. If uses-poly-alloc is true, initialization is done by uses-allocator construction with allocator alloc
rhs does not contain a value	destroys the contained value by calling val->T::~T()	no effect

Returns: *this.
Ensures: bool(rhs) == bool(*this).
Remarks: If any exception is thrown, the result of the expression bool(*this) remains unchanged. If an exception is thrown during the call to T's constructor, no effect. If an exception is thrown during the call to T's assignment, the state of *val is determined by the exception safety guarantee of T's assignment.
Mandates:
- is_same<T,U> is false,
- is_­constructible_­v<T, const U&> is true,
- is_assignable<T&, const U&> is true,
- is_­constructible_­v<T, std::optional<U>&> is false,
- is_­constructible_­v<T, optional<U>&> is false,
- is_­constructible_­v<T, std::optional<U>&&> is false,
- is_­constructible_­v<T, optional<U>&&> is false,
- is_­constructible_­v<T, const std::optional<U>&> is false,
- is_­constructible_­v<T, const optional<U>&> is false,
- is_­constructible_­v<T, const std::optional<U>&&> is false,
- is_­constructible_­v<T, const optional<U>&&> is false,
- is_­convertible_­v<std::optional<U>&, T> is false,
- is_­convertible_­v<optional<U>&, T> is false,
- is_­convertible_­v<std::optional<U>&&, T> is false,
- is_­convertible_­v<optional<U>&&, T> is false,
- is_­convertible_­v<const std::optional<U>&, T> is false,
- is_­convertible_­v<const optional<U>&, T> is false,
- is_­convertible_­v<const std::optional<U>&&, T> is false,
- is_­convertible_­v<const optional<U>&&, T> is false,
- is_assignable_v<T&, std::optional<U>&> is false,
- is_assignable_v<T&, optional<U>&> is false,
- is_assignable_v<T&, std::optional<U>&&> is false,
- is_assignable_v<T&, optional<U>&&> is false,
- is_assignable_v<T&, const std::optional<U>&> is false,
- is_assignable_v<T&, const optional<U>&> is false,
- is_assignable_v<T&, const std::optional<U>&&> is false, and
- is_assignable_v<T&, const optional<U>&&> is false

template<class U> optional& operator=(const std::optional<U>& rhs);
Effects: The result of the expression bool(rhs) remains unchanged. See Table

Table — optional operator=(const std::optional<U>&) effects

	*this contains a value	*this does not contain a value
rhs contains a value	assigns std​::​move(*rhs) to the contained value	initializes the contained value with std​::​move(*rhs). If uses-poly-alloc is true, initialization is done by uses-allocator construction with allocator alloc.
rhs does not contain a value	destroys the contained value by calling val->T::~T()	no effect

Returns: *this.
Ensures: bool(rhs) == bool(*this).
Remarks: If any exception is thrown, the result of the expression bool(*this) remains unchanged. If an exception is thrown during the call to T's constructor, no effect. If an exception is thrown during the call to T's assignment, the state of *val is determined by the exception safety guarantee of T's assignment.
Mandates:
- is_­constructible_­v<T, U> is true,
- is_assignable<T&, U> is true,
- is_­constructible_­v<T, std::optional<U>&> is false,
- is_­constructible_­v<T, optional<U>&> is false,
- is_­constructible_­v<T, std::optional<U>&&> is false,
- is_­constructible_­v<T, optional<U>&&> is false,
- is_­constructible_­v<T, const std::optional<U>&> is false,
- is_­constructible_­v<T, const optional<U>&> is false,
- is_­constructible_­v<T, const std::optional<U>&&> is false,
- is_­constructible_­v<T, const optional<U>&&> is false,
- is_­convertible_­v<std::optional<U>&, T> is false,
- is_­convertible_­v<optional<U>&, T> is false,
- is_­convertible_­v<std::optional<U>&&, T> is false,
- is_­convertible_­v<optional<U>&&, T> is false,
- is_­convertible_­v<const std::optional<U>&, T> is false,
- is_­convertible_­v<const optional<U>&, T> is false,
- is_­convertible_­v<const std::optional<U>&&, T> is false,
- is_­convertible_­v<const optional<U>&&, T> is false,
- is_assignable_v<T&, std::optional<U>&> is false,
- is_assignable_v<T&, optional<U>&> is false,
- is_assignable_v<T&, std::optional<U>&&> is false,
- is_assignable_v<T&, optional<U>&&> is false,
- is_assignable_v<T&, const std::optional<U>&> is false,
- is_assignable_v<T&, const optional<U>&> is false,
- is_assignable_v<T&, const std::optional<U>&&> is false, and
- is_assignable_v<T&, const optional<U>&&> is false

template<class U> optional& operator=(optional<U>&& rhs);
Effects: See Table

Table — optional operator=(optional<U>&&) effects

	*this contains a value	*this does not contain a value
rhs contains a value	assigns *rhs to the contained value	initializes the contained value with *rhs. If uses-poly-alloc is true, initialization is done by uses-allocator construction with allocator alloc .
rhs does not contain a value	destroys the contained value by calling val->T::~T()	no effect

Returns: *this.
Ensures: bool(rhs) == bool(*this).
Remarks: If any exception is thrown, the result of the expression bool(*this) remains unchanged. If an exception is thrown during the call to T's move constructor, the state of *rhs.val is determined by the exception safety guarantee of T's move constructor. If an exception is thrown during the call to T's move assignment, the state of *val and *rhs.val is determined by the exception safety guarantee of T's move assignment.
Mandates:
- is_same<T,U> is false,
- is_­constructible_­v<T, const U&> is true,
- is_assignable<T&, const U&> is true,
- is_­constructible_­v<T, std::optional<U>&> is false,
- is_­constructible_­v<T, optional<U>&> is false,
- is_­constructible_­v<T, std::optional<U>&&> is false,
- is_­constructible_­v<T, optional<U>&&> is false,
- is_­constructible_­v<T, const std::optional<U>&> is false,
- is_­constructible_­v<T, const optional<U>&> is false,
- is_­constructible_­v<T, const std::optional<U>&&> is false,
- is_­constructible_­v<T, const optional<U>&&> is false,
- is_­convertible_­v<const U&, T> is true,
- is_­convertible_­v<std::optional<U>&, T> is false,
- is_­convertible_­v<optional<U>&, T> is false,
- is_­convertible_­v<std::optional<U>&&, T> is false,
- is_­convertible_­v<optional<U>&&, T> is false,
- is_­convertible_­v<const std::optional<U>&, T> is false,
- is_­convertible_­v<const optional<U>&, T> is false,
- is_­convertible_­v<const std::optional<U>&&, T> is false,
- is_­convertible_­v<const optional<U>&&, T> is false,
- is_assignable_v<T&, std::optional<U>&> is false,
- is_assignable_v<T&, optional<U>&> is false,
- is_assignable_v<T&, std::optional<U>&&> is false,
- is_assignable_v<T&, optional<U>&&> is false,
- is_assignable_v<T&, const std::optional<U>&> is false,
- is_assignable_v<T&, const optional<U>&> is false,
- is_assignable_v<T&, const std::optional<U>&&> is false, and
- is_assignable_v<T&, const optional<U>&&> is false

template<class U> optional& operator=(std::optional<U>&& rhs);
Effects: See Table

Table — optional operator=(std::optional<U>&&) effects

	*this contains a value	*this does not contain a value
rhs contains a value	assigns *rhs to the contained value	initializes the contained value with *rhs. If uses-poly-alloc is true, initialization is done by uses-allocator construction with allocator alloc
rhs does not contain a value	destroys the contained value by calling val->T::~T()	no effect

Returns: *this.
Ensures: bool(rhs) == bool(*this).
Remarks: If any exception is thrown, the result of the expression bool(*this) remains unchanged. If an exception is thrown during the call to T's constructor, the state of *rhs.val is determined by the exception safety guarantee of T's constructor. If an exception is thrown during the call to T's assignment, the state of *val and *rhs.val is determined by the exception safety guarantee of T's assignment.
Mandates:
- is_­constructible_­v<T, const U&> is true,
- is_assignable<T&, const U&> is true,
- is_­constructible_­v<T, std::optional<U>&> is false,
- is_­constructible_­v<T, optional<U>&> is false,
- is_­constructible_­v<T, std::optional<U>&&> is false,
- is_­constructible_­v<T, optional<U>&&> is false,
- is_­constructible_­v<T, const std::optional<U>&> is false,
- is_­constructible_­v<T, const optional<U>&> is false,
- is_­constructible_­v<T, const std::optional<U>&&> is false,
- is_­constructible_­v<T, const optional<U>&&> is false,
- is_­convertible_­v<const U&, T> is true,
- is_­convertible_­v<std::optional<U>&, T> is false,
- is_­convertible_­v<optional<U>&, T> is false,
- is_­convertible_­v<std::optional<U>&&, T> is false,
- is_­convertible_­v<optional<U>&&, T> is false,
- is_­convertible_­v<const std::optional<U>&, T> is false,
- is_­convertible_­v<const optional<U>&, T> is false,
- is_­convertible_­v<const std::optional<U>&&, T> is false,
- is_­convertible_­v<const optional<U>&&, T> is false,
- is_assignable_v<T&, std::optional<U>&> is false,
- is_assignable_v<T&, optional<U>&> is false,
- is_assignable_v<T&, std::optional<U>&&> is false,
- is_assignable_v<T&, optional<U>&&> is false,
- is_assignable_v<T&, const std::optional<U>&> is false,
- is_assignable_v<T&, const optional<U>&> is false,
- is_assignable_v<T&, const std::optional<U>&&> is false, and
- is_assignable_v<T&, const optional<U>&&> is false

template<class... Args>> T& emplace(Args&&...);
Constraints: is_­constructible_­v<T, Args&&...> is true.
Effects: Calls *this = nullopt. Then initializes the contained value with std​::​forward<Args>(args)..... If uses-poly-alloc is true, initialization is done by uses-allocator construction with allocator alloc.
Ensures: *this contains a value.
Returns: A reference to the new contained value.
Throws: Any exception thrown by the selected constructor of T.
Remarks: If an exception is thrown during the call to T's constructor, *this does not contain a value, and the previous *val (if any) has been destroyed.

template<class U, class... Args> T& emplace(initializer_list<U>, Args&&...);
Constraints: is_­constructible_­v<T, initializer_­list<U>&, Args&&...> is true.
Effects: Calls *this = nullopt. Then initializes the contained value with il and std​::​forward<Args>(args)..... If uses-poly-alloc is true, initialization is done by uses-allocator construction with allocator alloc.
Ensures: *this contains a value.
Returns: A reference to the new contained value.
Throws: Any exception thrown by the selected constructor of T.
Remarks: If an exception is thrown during the call to T's constructor, *this does not contain a value, and the previous *val (if any) has been destroyed.

Swap [pmroptional.swap]

void swap(optional& rhs) noexcept(see below);
Constraints: Lvalues of type T shall be swappable and is_­move_­constructible_­v<T> is true.
Expects:If uses-poly-alloc is true, a.get_allocator() == b.get_allocator().
Effects: See Table x
Throws: Any exceptions thrown by the operations in the relevant part of Table x.
Remarks: The expression inside noexcept is equivalent to: is_nothrow_move_constructible_v<T> ∧∧ is_nothrow_swappable_v<T> If any exception is thrown, the results of the expressions bool(*this) and bool(rhs) remain unchanged. If an exception is thrown during the call to function swap, the state of *val and *rhs.val is determined by the exception safety guarantee of swap for lvalues of T. If an exception is thrown during the call to T’s move constructor, the state of tt>*val and *rhs.val is determined by the exception safety guarantee of T’s move constructor.

Table x — swap(optional&) effects

	*this contains a value	*this does not contain a value
rhs contains a value	calls swap(*(*this), *rhs)	initializes the contained value with std​::​move(*rhs) as the constructor argument, followed by rhs.val->T​::​~T(); If uses-poly-alloc is true, initialization is done by uses-allocator construction with allocator alloc. Postcondition is that *this contains a value and rhs does not contain a value
rhs does not contain a value	initializes the contained value with std​::​move(*(*this)) as the constructor argument, followed by val->T​::​~T(); If uses-poly-alloc is true, initialization is done by uses-allocator construction with allocator alloc. Postcondition is that *this does not contain a value and rhs contains a value	no effect

Throws: Any exceptions thrown by the operations in the relevant part of Table x.

Observers [pmroptional.observe]

const T* operator->() const;
T* operator->();
Expects: *this contains a value.
Returns: val.
Throws: Nothing.

const T& operator*() const&;
T& operator*() &;();
Expects: *this contains a value.
Returns: *val.
Throws: Nothing.

T&& operator*() &&;
const T&& operator*() const&&; &;();
Expects: *this contains a value.
Effects: Equivalent to: return std​::​move(*val);

explicit operator bool() const noexcept;
Returns: true if and only if contains a value.

bool has_value() const noexcept;
Returns: true if and only if contains a value.

const T& value() const&;
T& value() &;
Effects: Equivalent to:
return bool(*this) ? *val : throw bad_optional_access();

T&& value() &&;
const T&& value() const&&;
Effects: Equivalent to:
return bool(*this) ? std::move(*val) : throw bad_optional_access();

template<class U> T value_or(U&& v) const&;
Effects: Equivalent to:
return bool(*this) ? **this : static_cast<T>(std::forward<U>(v));
Remarks: If is_­copy_­constructible_­v<T> && is_­convertible_­v<U&&, T> is false, the program is ill-formed.

template<class U> T value_or(U&& v) &&;
Effects: Equivalent to:
return bool(*this) ? std::move(**this) : static_cast<T>(std::forward<U>(v));
Remarks: If is_­move_­constructible_­v<T> && is_­convertible_­v<U&&, T> is false, the program is ill-formed.

template<class U> T value_or(allocator_arg_t, const polymorphic_allocator<>& a, U&& v) const&;tt>
Constraints: uses_allocator<T,polymorphic_allocator<>> is true;
Effects: if *this contains a value, returns an object of type T initialized by uses-allocator construction with allocator a and **this as the constructor argument. Otherwise, returns an object of type T initialized by uses-allocator construction with allocator a and std::forward<U>(v) as the constructor argument.
Remarks: If is_­copy_­constructible_­v<T> && is_­convertible_­v<U&&, T> is false, the program is ill-formed.

template<class U> T value_or(allocator_arg_t, const polymorphic_allocator<>& a, U&& v) &&;
Constraints: uses_allocator<T,polymorphic_allocator<>> is true;
Effects: if *this contains a value, returns an object of type T initialized by uses-allocator construction with allocator a and std::move(**this) as the constructor argument. Otherwise, returns an object of type T initialized by uses-allocator construction with allocator a and std::forward<U>(v) as the constructor argument.
Remarks: If is_­copy_­constructible_­v<T> && is_­convertible_­v<U&&, T> is false, the program is ill-formed.

allocator_type get_allocator() const noexcept;
Constraints: uses_allocator<T,polymorphic_allocator<>> is true;
Returns: alloc.
Throws: nothing.

Modifiers [pmroptional.mod]

void reset() noexcept;
Effects: If *this contains a value, calls val->T​::​~T() to destroy the contained value; otherwise no effect.
Ensures: *this does not contain a value.

Allocator-related traits [pmroptional.traits]

template<class T>
struct uses_allocator<T, polymorphic_allocator<>> :
uses_allocator<T, polymorphic_allocator<>> { };

Relational operators [pmroptional.relops]

template<class T, class U> bool operator==(const optional<T>&, const optional<U>&);
template<class T, class U> bool operator==(const optional<T>&, const std::optional<U>&);
template<class T, class U> bool operator==(const std::optional<T>&, const optional<U>&);
Mandates: The expression *x == *y shall be well-formed and its result shall be convertible to bool. [ Note: T need not be Cpp17EqualityComparable. — end note ]
Returns: If bool(x) != bool(y), false; otherwise if bool(x) == false, true; otherwise *x == *y.

template<class T, class U> bool operator!=(const optional<T>&, const optional<U>&);
template<class T, class U>bool operator!=(const optional<T>&, const std::optional<U>&);
template<class T, class U> bool operator!=(const std::optional<T>&, const optional<U>&);
Mandates: The expression *x != *y shall be well-formed and its result shall be convertible to bool.
Returns: If bool(x) != bool(y), true; otherwise if bool(x) == false, false; otherwise *x != *y.

template<class T, class U> bool operator<(const optional<T>&, const optional<U>&);
template<class T, class U> bool operator<(const optional<T>&, const std::optional<U>&);
template<class T, class U> bool operator<(const std::optional<T>&, const optional<U>&);
Mandates: The expression *x < *y shall be well-formed and its result shall be convertible to bool.
Returns: If !y, false; otherwise if !x, true; otherwise *x < *y.

template<class T, class U> bool operator>(const optional<T>&, const optional<U>&);
template<class T, class U> bool operator>(const optional<T>&, const std::optional<U>&);
template<class T, class U> bool operator>(const std::optional<T>&, const optional<U>&);
Mandates: The expression *x > *y shall be well-formed and its result shall be convertible to bool.
Returns: If !x, false; otherwise if !y, true; otherwise *x > *y.

template<class T, class U> bool operator<=(const optional<T>&, const optional<U>&);
template<class T, class U> bool operator<=(const optional<T>&, const std::optional<U>&);
template<class T, class U> bool operator<=(const std::optional<T>&, const optional<U>&);
Mandates: The expression *x <= *y shall be well-formed and its result shall be convertible to bool.
Returns: If !x, true; otherwise if !y, false; otherwise *x <= *y.

template<class T, class U> bool operator&t;=(const optional<T>&, const optional<U>&);
template<class T, class U> bool operator>=(const optional<T>&, const std::optional<U>&);
template<class T, class U> bool operator>=(const std::optional<T>&, const optional<U>&);
Mandates: The expression *x >= *y shall be well-formed and its result shall be convertible to bool.
Returns: If !y, true; otherwise if !x, false; otherwise *x >= *y.

template<class T, three_way_comparable_with<T> U>
compare_three_way_result_t<T,U>
operator<=>(const optional<T>&, const optional<U>&);
template<class T, three_way_comparable_with<T> U>
compare_three_way_result_t<T,U>
operator<=>(const optional<T>&, const std::optional<U>&);
template<class T, three_way_comparable_with<T> U>
compare_three_way_result_t<T,U>
operator<=>(const std::optional<T>&, const optional<U>&);
Returns: If x && y, *x <=> *y; otherwise bool(x) <=> bool(y).

Comparison with nullopt [pmroptional.nullops]

template<class T> bool operator==(const optional<T>&, nullopt_t) noexcept;
Returns: !x.

template<class T> strong_ordering operator<=>(const optional<T>&, nullopt_t) noexcept;
Returns: bool(x) <=> false.

Comparison with T [pmroptional.comp.with.t]

template<class T, class U> bool operator==(const optional<T>& x, const U& v);
Mandates: The expression *x == v shall be well-formed and its result shall be convertible to bool. [ Note: T need not be Cpp17EqualityComparable. — end note ]
Effects: Equivalent to: return bool(x) ? *x == v : false;

template<class T, class U> bool operator==(const T& t, const optional<U>& v);
Mandates: The expression x == *v shall be well-formed and its result shall be convertible to bool.
Effects: Equivalent to: return bool(v) ? x == *v : false;

template<class T, class U> bool operator!=(const optional<T>& u, const U& v);
Mandates: The expression *x != v shall be well-formed and its result shall be convertible to bool.
Effects: Equivalent to: return bool(x) ? *x != v : true;

template<class T, class U> bool operator!=(const T& t, const optional<U>& v);
Mandates: The expression x != *v shall be well-formed and its result shall be convertible to bool.
Effects: Equivalent to: return bool(v) ? x != *v : true;

template<class T, class U> bool operator<(const optional<T>&, const U&);
Mandates: The expression *x < v shall be well-formed and its result shall be convertible to bool.
Effects: Equivalent to: return bool(x) ? *x < v : true;

template<class T, class U> bool operator<(const T& t, const optional<U>& v);
Mandates: The expression x < *v shall be well-formed and its result shall be convertible to bool.
Effects: Equivalent to: return bool(v) ? x < *v : false;

template<class T, class U> bool operator>(const optional<T>&, const U&);
Mandates: The expression *x > v shall be well-formed and its result shall be convertible to bool.
Effects: Equivalent to: return bool(x) ? *x > v : false;

template<class T, class U> bool operator>(const T& t, const optional<U>& v);
Mandates: The expression x > *v shall be well-formed and its result shall be convertible to bool.
Effects: Equivalent to: return bool(v) ? x > *v : true;

template<class T, class U> bool operator<=(const optional<T>&, const U&);
Mandates: The expression *x <= v shall be well-formed and its result shall be convertible to bool.
Effects: Equivalent to: return bool(x) ? *x <= v : true;

template<class T, class U> bool operator<=(const T& t, const optional<U>& v);
Mandates: The expression x <= *v shall be well-formed and its result shall be convertible to bool.
Effects: Equivalent to: return bool(v) ? x <= *v : false;

template<class T, class U> bool operator>=(const optional<T>&, const U&);
Mandates: The expression *x >= v shall be well-formed and its result shall be convertible to bool.
Effects: Equivalent to: return bool(x) ? *x >= v : false;

template<class T, class U> bool operator>=(const T& t, const optional<U>& v);
Mandates: The expression x >= *v shall be well-formed and its result shall be convertible to bool.
Effects: Equivalent to: return bool(v) ? x >= *v : true;

template<class T, three_way_comparable_with<T> U>
compare_three_way_result_t<T,U>
operator<=>(const optional<T>& x, const U&v);
Effects: Equivalent to: return bool(x) ? *x >=< v : strong_­ordering​::​less ;

Specialized algorithms [pmroptional.specalg]

template<class T> void swap(optional<T>& x, optional<T>& y)
Effects: if x.get_allocator() == y.get_allocator(), calls x.swap(y). Otherwise equvivalent to
std::pmr::optional futureX(allocator_arg_t,x.get_allocator(),y);
std::pmr::optional futureY(allocator_arg_t,y.get_allocator(),x);
futureX.swap(x);
futureY.swap(y);

template<class T> constexpr optional<decay_­t<T>> make_optional(T&&);
Returns: optional<decay_­t<T>>(std​::​forward<T>(v)).
Remarks: Specialization of this function template shall be a constexpr function if uses-poly-alloc::value is false.

template<class T> constexpr optional<T> make_optional();
Effects: Equivalent to: return optional<T>(in_­place).
Remarks: Specialization of this function template shall be a constexpr function if uses-poly-alloc::value is false.

template<class T, class Arg0, class... Args> constexpr optional<T> make_optional(Arg0&& arg0, Args&&... args);
Effects: Equivalent to: return optional<T>(in_­place, std​::​forward<Arg0>(arg0), std​::​forward<Args>(args)...).
Constraints: !is_same< typename remove_cvref_t<ARG0>, allocator_arg_t>
Remarks: Specialization of this function template shall be a constexpr function if uses-poly-alloc::value is false.

template<class T, class... Args> constexpr optional<T> make_optional(Args&&... args);
Effects: Equivalent to: return optional<T>(in_­place, std​::​forward<Args>(args)...).
Remarks: Specialization of this function template shall be a constexpr function if uses-poly-alloc::value is false.

template<class T, class U, class... Args> optional<T>
constexpr make_optional(initializer_list<U> il, Args&&... args);
Effects: Equivalent to: return optional<T>(in_­place, il, std​::​forward<Args>(args)...).
Remarks: Specialization of this function template shall be a constexpr function if uses-poly-alloc::value is false.

template<class T, optional<decay_­t<T>>
make_optional(allocator_arg_t, const polymorphic_allocator<>& a, T&&);
Mandates: uses_allocator<T,polymorphic_allocator<>> is true;
Returns: optional<decay_­t<T>>(allocator_arg, a, in_place, std​::​forward<T>(v)).

template<class T, class... Args> optional<T>
make_optional(allocator_arg_t, const polymorphic_allocator<>& a, Args&&... args);
Mandates: uses_allocator<T,polymorphic_allocator<>> is true;
Effects: Equivalent to: return optional<T>(allocator_arg, a, in_place, std​::​forward<Args>(args)...).

template<class T, class U, class... Args> optional<T>
make_optional(allocator_arg_t, const polymorphic_allocator<>& a, initializer_list<U> il, Args&&... args);
Mandates: uses_allocator<T,polymorphic_allocator<>> is true;
Effects: Equivalent to: return optional<T>(allocator_arg, a, in_place, il, std​::​forward<Args>(args)...).

Hash support [pmroptional.hash]

template<class T> struct hash<optional<T>>;
The specialization hash<optional<T>> is enabled ([unord.hash]) if and only if hash<remove_­const_­t<T>> is enabled. When enabled, for an object o of type optional<T>, if bool(o) == true, then hash<optional<T>>()(o) shall evaluate to the same value as hash<remove_­const_­t<T>>()(*o); otherwise it evaluates to an unspecified value. The member functions are not guaranteed to be noexcept.