#include <cassert>
#include <iostream>
#include <optional>
#include <string>
#include <variant>

std::optional<int> digit(char c) {
    if (c >= '0' && c <= '9') return c - '0';
    return std::nullopt;
}
struct Render {
    std::string operator()(int n) const { return std::to_string(n); }
    std::string operator()(const std::string& s) const { return s; }
};

int main() {
    auto zero = digit('0');
    assert(zero.has_value() && *zero == 0);
    assert(!digit('x'));
    std::variant<int, std::string> message = 7;
    assert(std::visit(Render{}, message) == "7");
    message = std::string("ready");
    assert(std::get_if<int>(&message) == nullptr);
    assert(std::visit(Render{}, message) == "ready");
    std::cout << *zero << ' ' << std::visit(Render{}, message) << '\n';
}