Use NTTP in C++20 as a replacement for macros like CROW_ROUTE

[Rucadi]

With C++20, There is the possibility to use NTTP (Non-type template parameter), while the use of CROW_ROUTE is elegant, I in general dislike macro usage when it can be substituted by a more in-language solution.

It is common to declare something like:

template<std::size_t N>
struct fixed_string {
    char value[N];

    constexpr fixed_string(const char (&str)[N]) {
        std::copy_n(str, N, value);
    }

    constexpr auto operator<=>(const fixed_string&) const = default; 
};

template<std::size_t N>
fixed_string(const char (&)[N]) -> fixed_string<N>;

To allow using constant strings as template-parameters, then we can define our new "route" function, which I call route2:

template<fixed_string Url, typename Handler>
auto route2(Handler&& h) {
    constexpr auto tag = crow::black_magic::get_parameter_tag(Url.value);
    return router_
        .template new_rule_tagged<tag>(Url.value)
        (std::forward<Handler>(h));
}

which makes it possible to define the "hello world" as:

int main()
{
    crow::SimpleApp app;

    app.route2<"/">([]() {
        return "Hello, world!";
    });

    app.port(18080).run();
}

Without macro usage.

Would this be a desirable feature for Crow?

[gittiver]

Yes, definitely. But we have to change to C++ 20 for that.

[Rucadi]

Could I prepare a PR which is conditioned by __cpp_nontype_template_args switch https://en.cppreference.com/w/cpp/feature_test.html ?

So this does not affect <c++20 code? Or this will be considered once the project decides to move to c++20?

[gittiver]

if it will work it would be a reason to switch to C++ 20. __cpp_nontype_template_args would be helpful for transition, but if it makes things muh more complicated I would say, leave it out.