std::experimental::optional (3) - Linux Manuals

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

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

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

Observers

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

Modifiers

              exchanges the contents
swap (public member function)
              constructs the contained value in-place
emplace (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!= compares optional objects
operator< (function template)
operator<=
operator>
operator>=
                                       creates an optional object
make_optional (function template)
                                       specializes the std::swap algorithm
std::swap(std::experimental::optional) (function)

Helper classes

                                       specializes the std::hash algorithm
std::hash<std::experimental::optional> (class template specialization)

nullopt_t indicator of optional type with uninitialized state
                                       (class)
(library fundamentals TS)

in_place_t disambiguation tag type for in-place construction of optional types
                                       (class)
(library fundamentals TS)

bad_optional_access exception indicating checked access to an optional that doesn't contain a value
                                       (class)
(library fundamentals TS)

Helper objects

nullopt an object of type nullopt_t
                          (function)
(library fundamentals TS)

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