#include <string>
#include <iostream>

template <auto from, auto to>
auto substr(const std::string& input) -> std::string {
    return input.substr(from, to-from);
}

struct X {
    std::string input;
    X(const std::string& input) : input{input} { }
    template <auto from, auto to>
    auto substr() -> std::string {
        return input.substr(from, to-from);
    }
};

main: () -> int = {
    test_string: std::string = "The rain in Spain flows mainly down the drain";
    std::cout << substr<4,8>(test_string) << "\n";

    x: X = test_string;
    std::cout << x.substr<4,8>() << "\n";
}