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std::optional::and_then(3) C++ Standard Libary std::optional::and_then(3) NAME std::optional::and_then - std::optional::and_then Synopsis template< class F > (1) (since C++23) constexpr auto and_then( F&& f ) &; template< class F > (2) (since C++23) constexpr auto and_then( F&& f ) const&; template< class F > (3) (since C++23) constexpr auto and_then( F&& f ) &&; template< class F > (4) (since C++23) constexpr auto and_then( F&& f ) const&&; 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::op- tional (unlike transform()). Otherwise, the program is ill-formed. 1) Equivalent to if (*this) return std::invoke(std::forward<F>(f), **this); else return std::remove_cvref_t<std::invoke_result_t<F, T&>>{}; 2) Equivalent to if (*this) return std::invoke(std::forward<F>(f), **this); else return std::remove_cvref_t<std::invoke_result_t<F, const T&>>{}; 3) Equivalent to if (*this) return std::invoke(std::forward<F>(f), std::move(**this)); else return std::remove_cvref_t<std::invoke_result_t<F, T>>{}; 4) Equivalent to if (*this) return std::invoke(std::forward<F>(f), std::move(**this)); else return std::remove_cvref_t<std::invoke_result_t<F, const T>>{}; Parameters f - a suitable function or Callable object that returns an std::op- tional Return value The result of f or an empty std::optional, as described above. Notes Some languages call this operation flatmap. Feature-test macro Value Std Feature __cpp_lib_optional 202110L (C++23) Monadic operations in std::op- tional Example // Run this code #include <charconv> #include <iomanip> #include <iostream> #include <optional> #include <ranges> #include <string> #include <string_view> #include <vector> std::optional<int> 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::nul- lopt, "-43" }; for (auto&& x : v | std::views::transform( [](auto&& o) { // debug print the content of input optional<string> 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 option- als 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 See also value_or returns the contained value if available, another value otherwise (public member function) transform returns an optional containing the transformed 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::and_then(3)
NAME | Synopsis | Parameters | Return value | Notes | Example | Output: | See also
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