# std::experimental::optional

From cppreference.com
< cpp‎ | experimental

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)

Retrieved from "https://en.cppreference.com/mwiki/index.php?title=cpp/experimental/optional&oldid=103235"