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    std::experimental::optional

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    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)
    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

    		 constructs the optional object
    (public member function) [edit]
    		destroys the contained value, if there is one