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Showing content from https://en.cppreference.com/w/cpp/language/../preprocessor/../experimental/optional.html below:

std::experimental::optional - cppreference.com

std::experimental::optional Merged into ISO C++ The functionality described on this page was merged into the mainline ISO C++ standard as of 3/2016, see std::optional (since C++17)

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.

[edit] Template parameters T - the type of the value to manage initialization state for. The type must meet the requirements of Destructible. [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 [edit] Helper classes [edit] Helper objects

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