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std::experimental::optional

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

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

Contents

[edit] Template parameters

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

[edit] Member types

Member type Definition
value_type T

[edit] Member functions

constructs the optional object
(public member function) [edit]
if engaged, destroys the contained value
(public member function) [edit]
assigns contents
(public member function) [edit]
Observers
accesses the contained value
(public member function) [edit]
checks whether the object is in engaged state
(public member function) [edit]
returns the contained value
(public member function) [edit]
returns the contained value if available, another value otherwise
(public member function) [edit]
Modifiers
exchanges the contents
(public member function) [edit]
constructs the contained value in-place
(public member function) [edit]

[edit] 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

[edit] Non-member functions

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

[edit] Helper classes

specializes the std::hash algorithm
(class template specialization) [edit]
(library fundamentals TS)
indicator of optional type with uninitialized state
(class) [edit]

[edit] Helper objects

(library fundamentals TS)
an object of type nullopt_t
(function) [edit]