C++ class template for notification management

Question

Recently, I found myself frequently writing notification managing facilities. That is, one can register as a receiver by providing a callback, and another can send a notification to all registrants by invoking their corresponding callbacks. I had been bothered by re-writing this similar code again and again. Finally, I decided to write a generic one once and for all. Following is the version for cases where the maximum number of registrants is known and limited. So I don't bother using advanced data structures for managing the registered callbacks. Comments pertaining to any aspect of the code are welcome :)

#include <array>
#include <functional>
#include <mutex>
#include <stdexcept>
#include <utility>

template <typename Sig, int max_reg = 16, typename Func = std::function<Sig>>
class NotifyRegistry;

template <int max_reg, typename Func, typename R, typename... Args>
class NotifyRegistry<R(Args...), max_reg, Func>
{
  static_assert(max_reg > 0, "");
public:
  template <typename F> 
  int Register(F&& f) {
    std::lock_guard<std::mutex> lock(mut_);
    int i = -1;
    while (++i < max_reg && reg_[i]);
    if (i == max_reg) throw std::length_error("");
    reg_[i] = std::forward<F>(f);
    return i;
  }
  void Unregister(int id) {
    if (id < 0 || id >= max_reg) throw std::invalid_argument("");
    std::lock_guard<std::mutex> lock(mut_);
    reg_[id] = nullptr;
  }
  void Notify(Args... args) const {
    std::lock_guard<std::mutex> lock(mut_);
    for (auto& f : reg_) {
      if (f) (void)f(std::forward<Args>(args)...);
    }
  }
private:
  std::array<Func, max_reg> reg_;
  mutable std::mutex mut_;
};

Answer 1

Your class seems generally well done and easy to use. I have only a few suggested improvements.

Add text to throws

It would be useful to have something other than a null string when the object throws an error.

Add text to the static_assert

The same issue exists for the static_assert in that it would be useful to provide a meaningful error message instead of an empty string.

Provide some comments

Because the class is compact and generally well written, not much explanation is required, but a brief explanation, especially noting which exceptions might be thrown, would be useful.