std::experimental::optional
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experimental::optional | ||||
| Type-erased and polymorphic allocators | ||||
| Variable templates for type traits |
| Member functions | ||||
| Observers | ||||
| Modifiers | ||||
| Non-member functions | ||||
| Helper classes | ||||
| Helper objects | ||||
 | Merged into ISO C++ The functionality described on this page was merged into the mainline ISO C++ standard as of 3/2016, seestd::optional(since C++17) |
Defined in header <experimental/optional> | ||
template<class T> class optional; | (library fundamentals TS) | |
The class templatestd::experimental::optional manages anoptional contained value, i.e. a value that may or may not be present.
A common use case foroptional is the return value of a function that may fail. As opposed to other approaches, such asstd::pair<T,bool>,optional handles expensive to construct objects well and is more readable, as the intent is expressed explicitly.
Any instance ofoptional<T> at any given point in time eithercontains a value ordoes not contain a value.
If anoptional<T>contains a value, the value is guaranteed to be allocated as part of theoptional object footprint, i.e. no dynamic memory allocation ever takes place. Thus, anoptional object models an object, not a pointer, even though theoperator*() andoperator->() are defined.
When an object of type optional<T> iscontextually converted to bool, the conversion returnstrue if the objectcontains a value andfalse if itdoes not contain a value.
Theoptional objectcontains a value in the following conditions:
- The object is initialized with a value of type
T. - The object is assigned from another
optionalthatcontains a value.
The objectdoes not contain a value in the following conditions:
- The object is default-initialized.
- The object is initialized with a value ofstd::experimental::nullopt_t or an
optionalobject thatdoes not contain a value. - The object is assigned from a value ofstd::experimental::nullopt_t or from an
optionalthatdoes 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 ofDestructible. |
[edit]Member types
| Member type | Definition |
value_type | T |
[edit]Member functions
| constructs the optional object (public member function)[edit] | |
| destroys the contained value, if there is one (public member function)[edit] | |
| assigns contents (public member function)[edit] | |
Observers | |
| accesses the contained value (public member function)[edit] | |
| checks whether the object contains a value (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
comparesoptional objects(function template)[edit] | |
creates anoptional object(function template)[edit] | |
| specializes thestd::swap algorithm (function)[edit] |
[edit]Helper classes
| specializes thestd::hash algorithm (class template specialization)[edit] | |
(library fundamentals TS) | indicator of optional type with uninitialized state (class)[edit] |
(library fundamentals TS) | disambiguation tag type for in-place construction of optional types (class)[edit] |
(library fundamentals TS) | exception indicating checked access to an optional that doesn't contain a value (class)[edit] |
[edit]Helper objects
(library fundamentals TS) | an object of typenullopt_t(function)[edit] |
(library fundamentals TS) | an object of typestd::experimental::in_place_t (function)[edit] |
