std::expected<T,E>:: value

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C++
Utilities library
General utilities
Relational operators (deprecated in C++20)
std::expected
Primary template
constexpr T & value ( ) & ;
(1) (since C++23)
constexpr const T & value ( ) const & ;
(2) (since C++23)
constexpr T && value ( ) && ;
(3) (since C++23)
constexpr const T && value ( ) const && ;
(4) (since C++23)
void partial specialization
constexpr void value ( ) const & ;
(5) (since C++23)
constexpr void value ( ) && ;
(6) (since C++23)

If * this contains an expected value, returns a reference to the contained value. Returns nothing for void partial specialization.

Otherwise, throws an exception of type std:: bad_expected_access < std:: decay_t < E >> that contains a copy of error() .

1,2) If std:: is_copy_constructible_v < E > is false , the program is ill-formed.
3,4) If std:: is_copy_constructible_v < E > or std:: is_constructible_v < E, decltype ( std :: move ( error ( ) ) ) > is false , the program is ill-formed.
5) If std:: is_copy_constructible_v < E > is false , the program is ill-formed.
6) If std:: is_move_constructible_v < E > is false , the program is ill-formed.

Return value

1,2) val
3,4) std :: move ( val )

Exceptions

1,2,5) Throws std:: bad_expected_access ( std:: as_const ( error ( ) ) ) if * this contains an unexpected value.
3,4,6) Throws std:: bad_expected_access ( std :: move ( error ( ) ) ) if * this contains an unexpected value.

Example

This section is incomplete
Reason: no example

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR Applied to Behavior as published Correct behavior
LWG 3940 C++23 E was not required to be copy/move-constructible for overloads (5,6) required

See also

returns the expected value if present, another value otherwise
(public member function)
accesses the expected value
(public member function)
returns the unexpected value
(public member function)
(C++23)
exception indicating checked access to an expected that contains an unexpected value
(class template)
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