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

< cpp‎ | utility‎ | expected
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 error value, invokes f and returns its result; otherwise, returns a std::expected object that contains a copy of value(). The contained value (error()) is passed as an argument to f.

Let G be:

The return type is G, which must be a specialization of std::expected, and std::is_same_v<G::value_type, T> must be true.

1-2) Equivalent to 
if (has_value())
{
    if constexpr (std::is_void_v<T>)
        return G();
    else
        return G(std::in_place, value());
}
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(value())> is true.
3-4) Equivalent to 
if (has_value())
{
    if constexpr (std::is_void_v<T>)
        return G();
    else
        return G(std::in_place, std::move(value()));
}
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(value()))> 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 Comment
__cpp_lib_expected 202211L (C++23) Monadic functions for std::expected

Example

This section is incomplete
Reason: no example

See also

(C++23)
 returns the expected itself if it contains an expected value; otherwise, returns an expected containing the transformed unexpected value
(public member function)