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 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 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.
   2) Same as (1), except that U is std::remove_cv_t<std::invoke_result_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-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.
   4) Same as (3), except that U is std::remove_cv_t<std::invoke_result_t<F, const T>>.

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-test macro Value Std Feature
   __cpp_lib_optional 202110L (C++23) Monadic operations in std::optional

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)