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)