NAME

std::expected::or_else - std::expected::or_else

Synopsis

template< class F > (1) (since C++23)
constexpr auto or_else( F&& f ) &;
template< class F > (2) (since C++23)
constexpr auto or_else( F&& f ) const&;
template< class F > (3) (since C++23)
constexpr auto or_else( F&& f ) &&;
template< class F > (4) (since C++23)
constexpr auto or_else( F&& f ) const&&;

If *this contains an unexpected value, invokes f with the argument error() and
returns its result; otherwise, returns a std::expected object that contains a copy
of the contained expected value (obtained from operator*).

1,2) Given type G as std::remove_cvref_t<std::invoke_result_t<F,
decltype(error())>>.
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.
The effect is equivalent to

if (has_value())
{
if constexpr (std::is_void_v<T>)
return G();
else
return G(std::in_place, **this);
}
else
return std::invoke(std::forward<F>(f), error());

These overloads participate in overload resolution only if std::is_void_v<T> or
std::is_constructible_v<T, decltype(**this)> is true.
3,4) Given type G as std::remove_cvref_t<std::invoke_result_t<F,
decltype(std::move(error()))>>.
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.
The effect is equivalent to

if (has_value())
{
if constexpr (std::is_void_v<T>)
return G();
else
return G(std::in_place, std::move(**this));
}
else
return std::invoke(std::forward<F>(f), std::move(error()));

These overloads participate in overload resolution only if std::is_void_v<T> or
std::is_constructible_v<T, decltype(std::move(**this))> is true.

Parameters

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

Return value

The result of f, or a std::expected object that contains a copy of the expected
value, as described above.

Notes

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

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 3938 C++23 or_else was ill-formed if T is not (possibly made well-formed
cv-qualified) void and E is not copyable

See also

returns the expected itself if it contains an expected value;
transform_error otherwise, returns an expected containing the transformed unexpected
value
(public member function)

Category:

* Todo no example