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std::optional::transform(3) C++ Standard Libary std::optional::transform(3) NAME std::optional::transform - std::optional::transform Synopsis template< class F > (1) (since C++23) constexpr auto transform( F&& f ) &; template< class F > (2) (since C++23) constexpr auto transform( F&& f ) const&; template< class F > (3) (since C++23) constexpr auto transform( F&& f ) &&; template< class F > (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 invoca- tion; 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 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::op- tional<U>. The program is ill-formed if the variable definition U x(std::invoke(std::forward<F>(f), **this)); is ill-formed. 2) Same as (1), except that U is std::remove_cv_t<std::invoke_re- sult_t<F, const T&>>. 3) Let 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-initial- ized from std::invoke(std::forward<F>(f), std::move(**this)). Otherwise, re- turns 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. 4) Same as (3), except that U is std::remove_cv_t<std::invoke_re- sult_t<F, const T>>. Parameters f - a suitable function or Callable object whose call signature re- turns a non-reference type Return value An std::optional containing the result of f or an empty std::op- tional, as described above. Notes Because transform directly constructs a U object at the right loca- tion, 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::op- tional 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 See also value_or returns the contained value if available, another value otherwise (public member function) and_then returns the result of the given function on the contained value if it (C++23) exists, or an empty optional otherwise (public member function) or_else returns the optional itself if it contains a value, or the result of the (C++23) given function otherwise (public member function) http://cppreference.com 2024.06.10 std::optional::transform(3)
NAME | Synopsis | Parameters | Return value | Notes | Example | Output: | See also
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