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

From cppreference.net
C++
Utilities library
std::expected
Main template
template < class F >
constexpr auto and_then ( F && f ) & ;
(1) (since C++23)
template < class F >
constexpr auto and_then ( F && f ) const & ;
(2) (since C++23)
template < class F >
constexpr auto and_then ( F && f ) && ;
(3) (since C++23)
template < class F >
constexpr auto and_then ( F && f ) const && ;
(4) (since C++23)
void partial specialization
template < class F >
constexpr auto and_then ( F && f ) & ;
(5) (since C++23)
template < class F >
constexpr auto and_then ( F && f ) const & ;
(6) (since C++23)
template < class F >
constexpr auto and_then ( F && f ) && ;
(7) (since C++23)
template < class F >
constexpr auto and_then ( F && f ) const && ;
(8) (since C++23)

If * this represents an expected value, invokes f and returns its result. Otherwise, returns a std::expected object that contains an unexpected value, which is initialized with the unexpected value of * this .

1-4) f is invoked with the expected value val as the argument.
5-8) f is invoked without any argument.

Given type U as:

1,2) std:: remove_cvref_t < std:: invoke_result_t < F, decltype ( ( val ) ) >>
3,4) std:: remove_cvref_t < std:: invoke_result_t < F, decltype ( std :: move ( val ) ) >>
5-8) std:: remove_cvref_t < std:: invoke_result_t < F >>

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.

1,2) These overloads participate in overload resolution only if std:: is_constructible_v < E, decltype ( error ( ) ) > is true .
3,4) These overloads participate in overload resolution only if std:: is_constructible_v < E, decltype ( std :: move ( error ( ) ) ) > is true .
5,6) These overloads participate in overload resolution only if std:: is_constructible_v < E, decltype ( error ( ) ) > is true .
7,8) These overloads participate in overload resolution only if std:: is_constructible_v < E, decltype ( std :: move ( error ( ) ) ) > is true .

Contents

Parameters

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

Return value

Overload Value of has_value()
true false
( 1,2 ) std:: invoke ( std:: forward < F > ( f ) , val ) U ( std:: unexpect , error ( ) )
( 3,4 ) std:: invoke ( std:: forward < F > ( f ) ,std :: move ( val ) ) U ( std:: unexpect , std :: move ( error ( ) ) )
( 5,6 ) std:: invoke ( std:: forward < F > ( f ) ) U ( std:: unexpect , error ( ) )
( 7,8 ) U ( std:: unexpect , std :: move ( error ( ) ) )

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 the expected value was obtained by value ( ) [1] changed to ** this
LWG 3973 C++23 the expected value was obtained by ** this [2] changed to val
  1. ↑ value() requires E to be copy constructible (see LWG issue 3843 ), where operator* does not.
  2. ↑ ** this can trigger argument-dependent lookup .

See also

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