NAME¶

std::expected::and_then - std::expected::and_then

Synopsis¶

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

If *this contains an expected value, invokes f and returns its result; otherwise,
returns a std::expected object that contains a copy of error().

If T is not (possibly cv-qualified) void, the contained expected value (obtained
from operator*) is passed as an argument to f; otherwise f takes no argument.

1,2) Given type U as
* std::remove_cvref_t<std::invoke_result_t<F>> if T is (possibly cv-qualified)
void, or
* std::remove_cvref_t<std::invoke_result_t<F, decltype(**this)>> otherwise.
If U is not a specialization of std::expected, or std::is_same_v<U::error_type, E>
is false, the program is ill-formed.
The effect is equivalent to

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

These overloads participate in overload resolution only if
std::is_constructible_v<E, decltype(error())> is true.
3,4) Given type U as
* std::remove_cvref_t<std::invoke_result_t<F>> if T is (possibly cv-qualified)
void, or
* std::remove_cvref_t<std::invoke_result_t<F, decltype(std::move(**this))>>
otherwise.
If U is not a specialization of std::expected, or std::is_same_v<U::error_type, E>
is false, the program is ill-formed.
The effect is equivalent to

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

These overloads participate in overload resolution only if
std::is_constructible_v<E, decltype(std::move(error()))> 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 an error 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 and_then was ill-formed if T is not (possibly made well-formed
cv-qualified) void and E is not copyable

See also¶

unexpect in-place construction tag for unexpected value in expected
unexpect_t (tag)
(C++23)
returns an expected containing the transformed expected value if it
transform exists; otherwise, returns the expected itself
(public member function)

Category:¶

* Todo no example