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std::optional

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Revision as of 07:00, 4 February 2014 by Kreios4004 (Talk | contribs)

Defined in header <optional>
template< class T >
class optional;
(since C++14)

The class template std::optional manages an optional contained value, i.e. a value that semantically 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.

The value is guaranteed to be allocated within the optional object itself, 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.

The value inside an optional object may be in either an initialized or uninitialized state. An 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 optional object is engaged on the following conditions:

  • The object is initialized with a value of type T
  • The object is assigned an engaged optional.

The object is disengaged on the following conditions:

  • The object is default-initialized.
  • The object is initialized with a value of std::nullopt_t or a disengaged optional object.
  • The object is assigned a value of std::nullopt_t or a disengaged optional.

Contents

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

constructs the optional object
(public member function of Template:cpp/utility/optional/title) [edit]
destroys the contained value, if there is one
(public member function of Template:cpp/utility/optional/title) [edit]
assigns contents
(public member function of Template:cpp/utility/optional/title) [edit]
Observers
accesses the contained value
(public member function of Template:cpp/utility/optional/title) [edit]
checks whether the object contains a value
(public member function of Template:cpp/utility/optional/title) [edit]
returns the contained value
(public member function of Template:cpp/utility/optional/title) [edit]
returns the contained value if available, another value otherwise
(public member function of Template:cpp/utility/optional/title) [edit]
Modifiers
exchanges the contents
(public member function of Template:cpp/utility/optional/title) [edit]
constructs the contained value in-place
(public member function of Template:cpp/utility/optional/title) [edit]

Non-member functions

compares optional objects
(function template) [edit]
creates an optional object
(function template) [edit]
specializes the std::swap algorithm
(function) [edit]

Helper classes

specializes the std::hash algorithm
(class template specialization) [edit]