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{{cpp/title|optional}}
{{cpp/utility/optional/navbar}}
{{ddcl | header=optional | notes={{mark since c++14}} |
template< class T >
class optional;
}}

The class template {{tt|std::optional}} manages an ''optional'' contained value. The value may be in either initialized or uninitialized state.

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.

The value is guaranteed to be allocated within the {{tt|optional}} object itself, i.e. no dynamic memory allocation ever takes place. Thus, {{tt|optional}} object models an object, not a pointer, even though the {{lc|operator*()}} and {{lc|operator->()}} are defined.

A {{tt|optional}} object with a value in initialized state is called ''engaged'', whereas if the value is in uninitialized state, the object is called ''disengaged''.

The {{tt|optional}} object is ''engaged'' on the following conditions:

* The object is initialized with a value of type {{tt|T}}

* The object is assigned an ''engaged'' {{tt|optional}}.

The object is ''disengaged'' on the following conditions:

* The object is default-initialized.

* The object is initialized with a value of {{lc|std::nullopt_t}} or a ''disengaged'' {{tt|optional}} object.

* The object is assigned a value of {{lc|std::nullopt_t}} or a ''disengaged'' {{tt|optional}}.

===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/utility/optional/dsc constructor}}
{{dsc inc | cpp/utility/optional/dsc destructor}}
{{dsc inc | cpp/utility/optional/dsc operator{{=}}}}

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

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

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

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

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