std::optional<T>::transform
template< class F > constexpr auto transform( F&& f ) &; |
(1) | (since C++23) |
template< class F > constexpr auto transform( F&& f ) const&; |
(2) | (since C++23) |
template< class F > constexpr auto transform( F&& f ) &&; |
(3) | (since C++23) |
template< class F > constexpr auto transform( F&& f ) const&&; |
(4) | (since C++23) |
Returns an std::optional that contains the result of invocation of f on the contained value if *this contains a value. Otherwise, returns an empty std::optional of such type.
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.
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 directly-non-list-initialized from
std::invoke(std::forward<F>(f), this->value()). 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->value())); 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 directly-non-list-initialized from
std::invoke(std::forward<F>(f), std::move(this->value())). 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->value()))); is ill-formed.
Parameters
| f | - | a suitable function or Callable object whose call signature returns a non-reference type |
Return value
An std::optional containing the result of f or an empty std::optional, as described above.
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 testing macro: __cpp_lib_monadic_optional.
Example
#include <iostream>
#include <optional>
struct A { friend std::ostream& operator<< (std::ostream& os, A) { return os << 'A'; } };
struct B { friend std::ostream& operator<< (std::ostream& os, B) { return os << 'B'; } };
struct C { friend std::ostream& operator<< (std::ostream& os, C) { return os << 'C'; } };
struct D { friend std::ostream& operator<< (std::ostream& os, D) { return os << 'D'; } };
auto A_to_B(A in) { B out; std::cout << in << " => " << out << '\n'; return out; }
auto B_to_C(B in) { C out; std::cout << in << " => " << out << '\n'; return out; }
auto C_to_D(C in) { D out; std::cout << in << " => " << out << '\n'; return out; }
int main()
{
for (std::optional<A> o_A : { std::optional<A>{ A{} }, std::optional<A>{/*empty*/} })
{
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");
}
}
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
See also
| returns the contained value if available, another value otherwise (public member function) |
|
|
(C++23)
|
returns the result of the given function on the contained value if it exists, or an empty optional otherwise (public member function) |
|
(C++23)
|
returns the optional itself if it contains a value, or the result of the given function otherwise (public member function) |
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