Show how to use format xtd::format class with std::optional.
#include <xtd/xtd>
auto main() -> int {
console::out << format("{}", optional<string> {"string value"}) << environment::new_line;
console::out << format("{}", optional<string> {"string value"}.value()) << environment::new_line;
console::out << format("{}", optional<string> {}.value_or("fallback string")) << environment::new_line;
console::out << format("{}", optional<string> {}) << environment::new_line;
console::out << format("{}", nullopt) << environment::new_line;
}