std::expected<T,E>::and_then
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std::expected
expected::and_then
Main template
template< class F >
constexpr auto and_then( F&& f ) &;
(1)
(since C++23)
constexpr auto and_then( F&& f ) &;
template< class F >
constexpr auto and_then( F&& f ) const&;
(2)
(since C++23)
constexpr auto and_then( F&& f ) const&;
template< class F >
constexpr auto and_then( F&& f ) &&;
(3)
(since C++23)
constexpr auto and_then( F&& f ) &&;
template< class F >
constexpr auto and_then( F&& f ) const&&;
(4)
(since C++23)
constexpr auto and_then( F&& f ) const&&;
void partial specialization
template< class F >
constexpr auto and_then( F&& f ) &;
(5)
(since C++23)
constexpr auto and_then( F&& f ) &;
template< class F >
constexpr auto and_then( F&& f ) const&;
(6)
(since C++23)
constexpr auto and_then( F&& f ) const&;
template< class F >
constexpr auto and_then( F&& f ) &&;
(7)
(since C++23)
constexpr auto and_then( F&& f ) &&;
template< class F >
constexpr auto and_then( F&& f ) const&&;
(8)
(since C++23)
constexpr auto and_then( F&& f ) const&&;
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.
5-8) f is invoked without any argument.
Given type U as:
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.
[edit] Parameters
f
-
a suitable function or Callable object that returns a std::expected
[edit] 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())) | |
[edit] Notes
| Feature-test macro | Value | Std | Feature |
|---|---|---|---|
__cpp_lib_expected |
202211L |
(C++23) | Monadic functions for std::expected
|
[edit] Example
This section is incomplete
Reason: no example
Reason: no example
[edit] 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
|
- ↑
value()requiresEto be copy constructible (see LWG issue 3843), whereoperator*does not. - ↑ **this can trigger argument-dependent lookup.