std::experimental::optional<T>::value

 

C++

Compiler support

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Standard library

Standard library headers

Named requirements

Feature test macros (C++20)

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Concepts library (C++20)

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Technical specifications

Symbols index

External libraries

 

Experimental

Technical Specification

Filesystem library (filesystem TS)

Library fundamentals (library fundamentals TS)

Library fundamentals 2 (library fundamentals TS v2)

Library fundamentals 3 (library fundamentals TS v3)

Extensions for parallelism (parallelism TS)

Extensions for parallelism 2 (parallelism TS v2)

Extensions for concurrency (concurrency TS)

Extensions for concurrency 2 (concurrency TS v2)

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std::execution

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2D Graphics

 

Library fundamentals

experimental::optional

experimental::any

experimental::basic_string_view

experimental::sample

experimental::shared_ptr

experimental::weak_ptr

experimental::apply

experimental::invocation_typeexperimental::raw_invocation_type

experimental::search

experimental::default_searcherexperimental::make_default_searcher

experimental::boyer_moore_searcherexperimental::make_boyer_moore_searcher

experimental::boyer_moore_horspool_searcherexperimental::make_boyer_moore_horspool_searcher

Type-erased and polymorphic allocators

Variable templates for type traits

 

std::experimental::optional

Member functions

optional::optional

optional::~optional

optional::operator=

Observers

optional::operator->optional::operator*

optional::operator bool

optional::value

optional::value_or

Modifiers

optional::emplace

optional::swap

Non-member functions

operator==operator!=operator<operator<=operator>operator>=

make_optional

swap

Helper classes

hash

nullopt_t

in_place_t

bad_optional_access

Helper objects

nullopt

in_place

 

constexpr T& value() &; constexpr const T & value() const &;	(1)	(library fundamentals TS)
constexpr T&& value() &&; constexpr const T&& value() const &&;	(2)	(library fundamentals TS)

Returns the contained value.

1) Equivalent to return bool(*this) ? *val : throw bad_optional_access();.

2) Equivalent to return bool(*this) ? std::move(*val) : throw bad_optional_access();.

Parameters

(none)

Return value

A reference to the contained value.

Exceptions

std::experimental::bad_optional_access if *this does not contain a value.

Notes

The dereference operator operator*() does not check if this optional contains a value, which may be more efficient than value().

Example

Run this code

#include <experimental/optional>
#include <iostream>
 
int main()
{
    std::experimental::optional<int> opt = {};
 
    try
    {
        int n = opt.value();
    }
    catch (const std::logic_error& e)
    {
        std::cout << e.what() << '\n';
    }
}

Possible output:

optional<T>::value: not engaged

See also

value_or	returns the contained value if available, another value otherwise (public member function)
operator->operator*	accesses the contained value (public member function)
bad_optional_access (library fundamentals TS)	exception indicating checked access to an optional that doesn't contain a value (class)

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