std::optional::and_then - cppreference.com (original) (raw)

If *this contains a value, invokes f with the contained value as an argument, and returns the result of that invocation; otherwise, returns an empty std::optional.

The return type (see below) must be a specialization of std::optional (unlike transform()). Otherwise, the program is ill-formed.

[edit] Parameters

[edit] Return value

The result of f or an empty std::optional, as described above.

[edit] Notes

Some languages call this operation flatmap.

[edit] Example

#include #include #include #include #include #include #include #include   std::optional to_int(std::string_view sv) { int r{}; auto [ptr, ec]{std::from_chars(sv.data(), sv.data() + sv.size(), r)}; if (ec == std::errc()) return r; else return std::nullopt; }   int main() { using namespace std::literals;   const std::vector<std::optional<std::string>> v { "1234", "15 foo", "bar", "42", "5000000000", " 5", std::nullopt, "-43" };   for (auto&& x : v | std::views::transform( [](auto&& o) { // debug print the content of input optional std::cout << std::left << std::setw(13) << std::quoted(o.value_or("nullopt")) << " -> ";   return o // if optional is nullopt convert it to optional with "" string .or_else([]{ return std::optional{""s}; }) // flatmap from strings to ints (making empty optionals where it fails) .and_then(to_int) // map int to int + 1 .transform([](int n) { return n + 1; }) // convert back to strings .transform([](int n) { return std::to_string(n); }) // replace all empty optionals that were left by // and_then and ignored by transforms with "NaN" .value_or("NaN"s); })) std::cout << x << '\n'; }

Output:

"1234" -> 1235 "15 foo" -> 16 "bar" -> NaN "42" -> 43 "5000000000" -> NaN " 5" -> NaN "nullopt" -> NaN "-43" -> -42

[edit] See also

| | returns the contained value if available, another value otherwise (public member function) [edit] | | --------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | | | returns an optional containing the transformed contained value if it exists, or an empty optional otherwise (public member function) [edit] | | | returns the optional itself if it contains a value, or the result of the given function otherwise (public member function) [edit] |