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
optionalthat 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
optionalobject that does not contain a value. - The object is assigned from a value of std::experimental::nullopt_t or from an
optionalthat 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 (public member function) | |
| destroys the contained value, if there is one (public member function) | |
| assigns contents (public member function) | |
Observers | |
| accesses the contained value (public member function) | |
| checks whether the object contains a value (public member function) | |
| returns the contained value (public member function) | |
| returns the contained value if available, another value otherwise (public member function) | |
Modifiers | |
| exchanges the contents (public member function) | |
| 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
compares optional objects (function template) | |
creates an optional object (function template) | |
| specializes the std::swap algorithm (function) |
Helper classes
| specializes the std::hash algorithm (class template specialization) | |
| (library fundamentals TS) |
indicator of optional type with uninitialized state (class) |
| (library fundamentals TS) |
disambiguation tag type for in-place construction of optional types (class) |
| (library fundamentals TS) |
exception indicating checked access to an optional that doesn't contain a value (class) |
Helper objects
| (library fundamentals TS) |
an object of type nullopt_t (function) |
| (library fundamentals TS) |
an object of type std::experimental::in_place_t (function) |