std::experimental:: optional

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 .

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

(constructor)	constructs the optional object (public member function)
(destructor)	destroys the contained value, if there is one (public member function)
operator=	assigns contents (public member function)
Observers
operator-> operator*	accesses the contained value (public member function)
operator bool	checks whether the object contains a value (public member function)
value	returns the contained value (public member function)
value_or	returns the contained value if available, another value otherwise (public member function)
Modifiers
swap	exchanges the contents (public member function)
emplace	constructs the contained value in-place (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!= operator< operator<= operator> operator>=	compares optional objects (function template)
make_optional	creates an optional object (function template)
std::swap (std::experimental::optional)	specializes the std::swap algorithm (function)

Helper classes

std::hash <std::experimental::optional>	specializes the std::hash algorithm (class template specialization)
nullopt_t (library fundamentals TS)	indicator of optional type with uninitialized state (class)
in_place_t (library fundamentals TS)	disambiguation tag type for in-place construction of optional types (class)
bad_optional_access (library fundamentals TS)	exception indicating checked access to an optional that doesn't contain a value (class)

Helper objects

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