Even better example,

    import std;

    int main(int argc, char** argv)
    {
        using namespace std::literals;

        auto foo = "foo:"s;
        foo += argv[0];

        std::map<std::string, int> m{
          {foo, 123}
        , {"count: "s + std::to_string(argc), 456}
        };

        std::println("{}", m);
    }