std::experimental::optional(3)

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
   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-> accesses the contained value
   operator* (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< compares optional objects
   operator<= (function template)
   operator>
   operator>=
   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 indicator of optional type with uninitialized
   (library fundamentals TS) state
                        (class)
   in_place_t disambiguation tag type for in-place
   (library fundamentals TS) construction of optional types
                        (class)
   bad_optional_access exception indicating checked access to an
   (library fundamentals TS) optional that doesn't contain a value
                        (class)
   Helper objects
   nullopt an object of type nullopt_t
   (library fundamentals TS) (function)
   in_place an object of type std::experimental::in_place_t
   (library fundamentals TS) (function)

Category:
     * Noindexed pages