std::optional<T>::transform
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std::optional
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optional::transform
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template< class F >
constexpr auto transform( F&& f ) &;
(1)
(since C++23)
constexpr auto transform( F&& f ) &;
template< class F >
constexpr auto transform( F&& f ) const&;
(2)
(since C++23)
constexpr auto transform( F&& f ) const&;
template< class F >
constexpr auto transform( F&& f ) &&;
(3)
(since C++23)
constexpr auto transform( F&& f ) &&;
template< class F >
constexpr auto transform( F&& f ) const&&;
(4)
(since C++23)
constexpr auto transform( F&& f ) const&&;
If *this contains a value, invokes f with the contained value as an argument, and returns an std::optional that contains the result of that invocation; otherwise, returns an empty std::optional.
The type of contained value in the result (denoted by U below) must be a non-array object type, and must not be std::in_place_t or std::nullopt_t ). Otherwise, the program is ill-formed.
1) Let
The program is ill-formed if the variable definition U x(std::invoke (std::forward <F>(f), **this)); is ill-formed.
U be std::remove_cv_t <std::invoke_result_t <F, T&>>. If *this contains a value, returns a std::optional <U> whose contained value is direct-initialized from std::invoke (std::forward <F>(f), **this) (unlike and_then(), which must return an std::optional directly). Otherwise, returns an empty std::optional <U>.The program is ill-formed if the variable definition U x(std::invoke (std::forward <F>(f), **this)); is ill-formed.
3) Let
The program is ill-formed if the variable definition U x(std::invoke (std::forward <F>(f), std::move(**this))); is ill-formed.
U be std::remove_cv_t <std::invoke_result_t <F, T>>. If *this contains a value, returns a std::optional <U> whose contained value is direct-initialized from std::invoke (std::forward <F>(f), std::move(**this)). Otherwise, returns an empty std::optional <U>.The program is ill-formed if the variable definition U x(std::invoke (std::forward <F>(f), std::move(**this))); is ill-formed.
Contents
[edit] Parameters
f
-
a suitable function or Callable object whose call signature returns a non-reference type
[edit] Return value
An std::optional containing the result of f or an empty std::optional , as described above.
[edit] Notes
Because transform directly constructs a U object at the right location, rather than passing it to a constructor, std::is_move_constructible_v <U> can be false.
As the callable f can't return a reference type, it cannot be a pointer to data member.
Some languages call this operation map.
| Feature-test macro | Value | Std | Feature |
|---|---|---|---|
__cpp_lib_optional |
202110L |
(C++23) | Monadic operations in std::optional |
[edit] Example
Run this code
#include <iostream> #include <optional> struct A { /* ... */ }; struct B { /* ... */ }; struct C { /* ... */ }; struct D { /* ... */ }; auto A_to_B(A) -> B { /* ... */ std::cout << "A => B \n"; return {}; } auto B_to_C(B) -> C { /* ... */ std::cout << "B => C \n"; return {}; } auto C_to_D(C) -> D { /* ... */ std::cout << "C => D \n"; return {}; } void try_transform_A_to_D(std::optional <A> o_A) { std::cout << (o_A ? "o_A has a value\n" : "o_A is empty\n"); std::optional <D> o_D = o_A.transform(A_to_B) .transform(B_to_C) .transform(C_to_D); std::cout << (o_D ? "o_D has a value\n\n" : "o_D is empty\n\n"); }; int main() { try_transform_A_to_D( A{} ); try_transform_A_to_D( {} ); }
Output:
o_A has a value A => B B => C C => D o_D has a value o_A is empty o_D is empty
[edit] See also
(C++23)
optional otherwise (public member function) [edit]