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

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.

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

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.