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{{cpp/title|n=experimental::|optional}}
{{cpp/experimental/optional/navbar}}
{{ddcl | header=experimental/optional | since=libfund_ts |
template< class T >
class optional;
}}

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

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

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

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

When an object of type optional<T> is [[cpp/language/implicit_cast|contextually converted to bool]], the conversion returns {{c|true}} if the object ''contains a value'' and {{c|false}} if it ''does not contain a value''.

The {{tt|optional}} object ''contains a value'' in the following conditions:

* The object is initialized with a value of type {{tt|T}}
* The object is assigned from another {{tt|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 {{lc|std::experimental::nullopt_t}} or an  {{tt|optional}} object that ''does not contain a value''.
* The object is assigned from a value of {{lc|std::experimental::nullopt_t}} or from an {{tt|optional}} that ''does not contain a value''

===Template parameters===
{{par begin}}
{{par | T | the type of the value to manage initialization state for. The type must meet the requirements of {{concept|Destructible}} }}
{{par end}}

===Member types===
{{dsc begin}}
{{dsc hitem | Member type | Definition}}
{{dsc | {{tt|value_type}} | {{tt|T}} }}
{{dsc end}}

===Member functions===
{{dsc begin}}
{{dsc inc | cpp/experimental/optional/dsc constructor}}
{{dsc inc | cpp/experimental/optional/dsc destructor}}
{{dsc inc | cpp/experimental/optional/dsc operator{{=}}}}

{{dsc h2 | Observers}}
{{dsc inc | cpp/experimental/optional/dsc operator*}}
{{dsc inc | cpp/experimental/optional/dsc operator bool}}
{{dsc inc | cpp/experimental/optional/dsc value}}
{{dsc inc | cpp/experimental/optional/dsc value_or}}

{{dsc h2 | Modifiers}}
{{dsc inc | cpp/experimental/optional/dsc swap}}
{{dsc inc | cpp/experimental/optional/dsc emplace}}
{{dsc end}}

===Member objects===
{{dsc begin}}
{{dsc hitem | Member name | Definition}}
{{dsc | {{tt|val}} {{mark|private}} | pointer to the contained value (which points at a data member of the same object), the name is for exposition only}}
{{dsc end}}

===Non-member functions===
{{dsc begin}}
{{dsc inc | cpp/experimental/optional/dsc operator_cmp}}
{{dsc inc | cpp/experimental/optional/dsc make_optional}}
{{dsc inc | cpp/experimental/optional/dsc swap2}}
{{dsc end}}

===Helper classes===
{{dsc begin}}
{{dsc inc | cpp/experimental/optional/dsc hash}}
{{dsc inc | cpp/experimental/optional/dsc nullopt_t}}
{{dsc end}}

===Helper objects===
{{dsc begin}}
{{dsc inc | cpp/experimental/optional/dsc nullopt}}
{{dsc end}}

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