std::expected<T,E>::or_else - cppreference.com (original) (raw)

Primary template
template< class F > constexpr auto or_else( F&& f ) &; (1) (since C++23)
template< class F > constexpr auto or_else( F&& f ) const&; (2) (since C++23)
template< class F > constexpr auto or_else( F&& f ) &&; (3) (since C++23)
template< class F > constexpr auto or_else( F&& f ) const&&; (4) (since C++23)
void partial specialization
template< class F > constexpr auto or_else( F&& f ) &; (5) (since C++23)
template< class F > constexpr auto or_else( F&& f ) const&; (6) (since C++23)
template< class F > constexpr auto or_else( F&& f ) &&; (7) (since C++23)
template< class F > constexpr auto or_else( F&& f ) const&&; (8) (since C++23)

If *this contains an unexpected value, invokes f with the unexpected value of *this as the argument and returns its result. Otherwise, returns a std::expected object that represents an expected value.

1-4) The expected value is initialized with the expected value val of *this.

Given type G as:

If G is not a specialization of std::expected, or std::is_same_v<G::value_type, T> is false, the program is ill-formed.

1,2) These overloads participate in overload resolution only if std::is_constructible_v<T, decltype((_val_))> is true.

3,4) These overloads participate in overload resolution only if std::is_constructible_v<T, decltype(std::move(_val_))> is true.

[edit] Parameters

f - a suitable function or Callable object that returns a std::expected

[edit] Return value

Overload Value of has_value()
true false
(1,2) G(std::in_place, val) std::invoke(std::forward<F>(f), error())
(3,4) G(std::in_place, std::move(val)) std::invoke(std::forward<F>(f), std::move(error()))
(5,6) G() std::invoke(std::forward<F>(f), error())
(7,8) std::invoke(std::forward<F>(f), std::move(error()))

[edit] Notes

Feature-test macro Value Std Feature
__cpp_lib_expected 202211L (C++23) Monadic functions for std::expected

[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 3938 C++23 the expected value was obtained by value()[1] changed to **this
LWG 3973 C++23 the expected value was obtained by **this[2] changed to val
  1. ↑ value() requires E to be copy constructible (see LWG issue 3843), where operator* does not.
  2. ↑ **this can trigger argument-dependent lookup.

[edit] See also

| | returns the expected itself if it contains an expected value; otherwise, returns an expected containing the transformed unexpected value (public member function) [edit] | | ---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |