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

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< cpp‎ | utility‎ | expected
 
 
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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)

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.

Contents

[edit] Parameters

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

[edit] Return value

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

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

[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]