std::expected<T,E>::value - cppreference.com
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(1) | (since C++23) |
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(2) | (since C++23) |
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(3) | (since C++23) |
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(4) | (since C++23) |
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(5) | (since C++23) |
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(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
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
Defect reports
The following behavior-changing defect reports were applied retroactively to previously published C++ standards.
| DR | Applied to | Behavior as published | Correct behavior |
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| LWG 3940 | C++23 | E was not required to be copy/move-constructible for overloads (5,6)
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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] |