Modify observer_set to only accept and pass const ref arguments

nano/lib/observer_set.hpp

@@ -12,21 +12,25 @@ template <typename... T>
 class observer_set final
 {
 public:
-	void add (std::function<void (T...)> const & observer_a)
+	using observer_type = std::function<void (T const &...)>;
+
+public:
+	void add (observer_type observer)
 	{
 		nano::lock_guard<nano::mutex> lock{ mutex };
-		observers.push_back (observer_a);
+		observers.push_back (observer);
 	}
 
-	void notify (T... args) const
+	void notify (T const &... args) const
 	{
+		// Make observers copy to allow parallel notifications
 		nano::unique_lock<nano::mutex> lock{ mutex };
 		auto observers_copy = observers;
 		lock.unlock ();
 
-		for (auto & i : observers_copy)
+		for (auto const & observer : observers_copy)
 		{
-			i (args...);
+			observer (args...);
 		}
 	}
 
@@ -53,7 +57,7 @@ public:
 
 private:
 	mutable nano::mutex mutex{ mutex_identifier (mutexes::observer_set) };
-	std::vector<std::function<void (T...)>> observers;
+	std::vector<observer_type> observers;
 };
 
 }