format.h

template <class T>
static void process(int depth, std::vector <std::pair <int, std::string>>&args, T&& t) {
    std::stringstream stream;
    stream << t;

    std::string arg = stream.str();
    for (auto &i : args) {
        if (i.first > depth)
            throw std::runtime_error("invalid argument index");
        else if (i.first == depth)
            i.second = arg;
    }
}

template <class T, class... ArgvT>
static void process(int depth, std::vector <std::pair <int, std::string>>& args, T&& t, ArgvT&&... argv) {
    std::stringstream stream;
    stream << t;

    std::string arg = stream.str();
    for (auto &i : args)
        if (i.first == depth)
            i.second = arg;

    process(++depth, args, std::forward<ArgvT>(argv)...);
}

static bool is_size_t(const std::string& str) {
    for (auto i : str)
        if (i < '0' || i > '9')
            return false;
    return true;
}

template <class... ArgvT>
std::string format(const std::string& f, ArgvT&&... argv) {
    std::vector <std::pair <int, std::string>> args;

    for (int i = 0; i < f.size(); ++i) {
        if (f[i] == '{') {
            int j = i + 1;
            for (; j < f.size(); ++j)
                if (f[j] == '}')
                    break;

            if (j == f.size())
                throw std::runtime_error("invalid brackets use");

            std::string number = f.substr(i + 1, j - i - 1);
            if (is_size_t(number))
                args.push_back(std::make_pair(std::stoi(number), ""));
        }
    }

    process(0, args, std::forward<ArgvT>(argv)...);

    std::string res = "";
    int ind = 0;
    for (int i = 0; i < f.size(); ++i) {
        if (f[i] == '{') {
            while (f[i] != '}')
                ++i;

            res += args[ind++].second;
        }
        else
            res += f[i];
    }

    return res;
}