std::optional<T>::and_then

if (*this)
 return std::invoke (std::forward <F>(f), value());
else
 return std::remove_cvref_t <std::invoke_result_t <F, T&>>{};

if (*this)
 return std::invoke (std::forward <F>(f), value());
else
 return std::remove_cvref_t <std::invoke_result_t <F, const T&>>{};

if (*this)
 return std::invoke (std::forward <F>(f), std::move(value()));
else
 return std::remove_cvref_t <std::invoke_result_t <F, T>>{};

if (*this)
 return std::invoke (std::forward <F>(f), std::move(value());
else
 return std::remove_cvref_t <std::invoke_result_t <F, const T>>{};

#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::nullopt, "-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 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';
}

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