< cpp‎ | utility‎ | expected
Utilities library
Language support
Type support (basic types, RTTI)
Library feature-test macros (C++20)
Dynamic memory management
Program utilities
Coroutine support (C++20)
Variadic functions
Debugging support
Three-way comparison
General utilities
Date and time
Function objects
Formatting library (C++20)
Relational operators (deprecated in C++20)
Integer comparison functions
Swap and type operations
Common vocabulary types
Elementary string conversions

T is not (possibly cv-qualified) void
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)
T is (possibly cv-qualified) void
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 if T is (possibly cv-qualified) void.

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.


[edit] Parameters


[edit] Return value

1-4) The expected value contained in *this.
5,6) (none)

[edit] 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.

[edit] Example

[edit] 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

[edit] See also

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