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

 

C++

Compiler support

Freestanding and hosted

Language

Standard library

Standard library headers

Named requirements

Feature test macros (C++20)

Language support library

Concepts library (C++20)

Metaprogramming library (C++11)

Diagnostics library

General utilities library

Strings library

Containers library

Iterators library

Ranges library (C++20)

Algorithms library

Numerics library

Localizations library

Input/output library

Filesystem library (C++17)

Regular expressions library (C++11)

Concurrency support library (C++11)

Technical specifications

Symbols index

External libraries

 

Utilities library

Language support Type support (basic types, RTTI) Library feature-test macros (C++20) Dynamic memory management Program utilities Coroutine support (C++20) Variadic functions is_constant_evaluated (C++20) is_within_lifetime (C++26) initializer_list (C++11) source_location (C++20) Three-way comparison (C++20) three_way_comparablethree_way_comparable_with (C++20)(C++20) strong_ordering (C++20) weak_ordering (C++20) partial_ordering (C++20) common_comparison_category (C++20) compare_three_way_result (C++20) compare_three_way (C++20) strong_order (C++20) weak_order (C++20) partial_order (C++20) compare_strong_order_fallback (C++20) compare_weak_order_fallback (C++20) compare_partial_order_fallback (C++20)    is_eqis_neqis_ltis_lteqis_gtis_gteq (C++20)(C++20)(C++20)(C++20)(C++20)(C++20)

General utilities Date and time Function objects Formatting library (C++20) bitset hash (C++11) Relational operators (deprecated in C++20) rel_ops::operator!=rel_ops::operator> rel_ops::operator<=rel_ops::operator>= Integer comparison functions cmp_equalcmp_lesscmp_less_than (C++20)(C++20)(C++20)    cmp_not_equalcmp_greatercmp_greater_than (C++20)(C++20)(C++20) in_range (C++20) Swap and type operations swap ranges::swap (C++20) exchange (C++14) declval (C++11) to_underlying (C++23) forward (C++11) forward_like (C++23) move (C++11) move_if_noexcept (C++11) as_const (C++17) Common vocabulary types pair tuple (C++11) optional (C++17) any (C++17) variant (C++17) tuple_size (C++11) tuple_element (C++11) apply (C++17) make_from_tuple (C++17) expected (C++23) Elementary string conversions to_chars (C++17) from_chars (C++17) chars_format (C++17)

 

std::expected

Member functions

expected::expected (C++23)

expected::~expected (C++23)

expected::operator= (C++23)

Observers

expected::operator->expected::operator* (C++23)(C++23)

expected::operator boolexpected::has_value (C++23)(C++23)

expected::value (C++23)

expected::error (C++23)

expected::value_or (C++23)

Monadic operations

expected::and_then (C++23)

expected::or_else (C++23)

expected::transform (C++23)

expected::transform_error (C++23)

Modifiers

expected::emplace (C++23)

expected::swap (C++23)

Non-member functions

operator== (C++23)

swap (C++23)

Helper classes

unexpected (C++23)

bad_expected_access (C++23)

unexpect_tunexpect (C++23)(C++23)

 

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:

• for overloads (1-2), std::remove_cvref_t<std::invoke_result_t<F, decltype(error())>>;
• for overloads (3-4), std::remove_cvref_t<std::invoke_result_t<F, decltype(std::move(error()))>>.

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

See also

transform_error (C++23)

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