NAME¶

std::experimental::optional - std::experimental::optional

Synopsis¶

Defined in header <experimental/optional>
template< class T > (library fundamentals TS)
class optional;

The class template std::experimental::optional manages an optional contained value,
i.e. a value that may or may not be present.

A common use case for optional is the return value of a function that may fail. As
opposed to other approaches, such as std::pair<T,bool>, optional handles expensive
to construct objects well and is more readable, as the intent is expressed
explicitly.

Any instance of optional<T> at any given point in time either contains a value or
does not contain a value.

If an optional<T> contains a value, the value is guaranteed to be allocated as part
of the optional object footprint, i.e. no dynamic memory allocation ever takes
place. Thus, an optional object models an object, not a pointer, even though the
operator*() and operator->() are defined.

When an object of type optional<T> is contextually converted to bool, the conversion
returns true if the object contains a value and false if it does not contain a
value.

The optional object contains a value in the following conditions:

* The object is initialized with a value of type T
* The object is assigned from another optional that contains a value.

The object does not contain a value in the following conditions:

* The object is default-initialized.
* The object is initialized with a value of std::experimental::nullopt_t or an
optional object that does not contain a value.
* The object is assigned from a value of std::experimental::nullopt_t or from an
optional that does not contain a value

Template parameters¶

T - the type of the value to manage initialization state for. The type must meet the
requirements of Destructible

Member types¶

Member type Definition
value_type T

Member functions¶

constructor constructs the optional object
(public member function)
destructor destroys the contained value, if there is one
(public member function)
operator= assigns contents
(public member function)

Observers¶

operator-> accesses the contained value
operator* (public member function)
operator bool checks whether the object contains a value
(public member function)
value returns the contained value
(public member function)
value_or returns the contained value if available, another value otherwise
(public member function)

Modifiers¶

swap exchanges the contents
(public member function)
emplace constructs the contained value in-place
(public member function)

Member objects¶

Member name Definition
val (private) pointer to the contained value (which points at a data member of the
same object), the name is for exposition only

Non-member functions¶

operator==
operator!=
operator< compares optional objects
operator<= (function template)
operator>
operator>=
make_optional creates an optional object
(function template)
std::swap(std::experimental::optional) specializes the std::swap algorithm
(function)

Helper classes¶

std::hash<std::experimental::optional> specializes the std::hash algorithm
(class template specialization)
nullopt_t indicator of optional type with uninitialized
(library fundamentals TS) state
(class)
in_place_t disambiguation tag type for in-place
(library fundamentals TS) construction of optional types
(class)
bad_optional_access exception indicating checked access to an
(library fundamentals TS) optional that doesn't contain a value
(class)

Helper objects

nullopt an object of type nullopt_t
(library fundamentals TS) (function)
in_place an object of type std::experimental::in_place_t
(library fundamentals TS) (function)