Implemented generic callback functionality for alerts.

include/libtorrent/alert.hpp

@@ -4,6 +4,7 @@ Copyright (c) 2003, Daniel Wallin
 Copyright (c) 2004-2005, 2008-2009, 2013-2020, 2022, Arvid Norberg
 Copyright (c) 2004, Magnus Jonsson
 Copyright (c) 2016, Alden Torres
+Copyright (c) 2023, Joris Carrier
 All rights reserved.
 
 Redistribution and use in source and binary forms, with or without
@@ -37,6 +38,7 @@ POSSIBILITY OF SUCH DAMAGE.
 #define TORRENT_ALERT_HPP_INCLUDED
 
 #include <string>
+#include <functional>
 
 // OVERVIEW
 //
@@ -186,6 +188,24 @@ namespace alert_category {
 
 #include "libtorrent/aux_/disable_deprecation_warnings_push.hpp"
 
+	template<typename T>
+	struct TORRENT_EXPORT callback_t {
+		using type = std::function<void(const T*)>;
+
+		callback_t(type func) : m_callback(std::move(func)) {}
+
+		bool has_callback() const {
+			return static_cast<bool>(m_callback);
+		}
+
+		void callback() {
+			m_callback(static_cast<const T*>(this));
+		}
+
+	private:
+		type m_callback;
+	};
+
 	// The ``alert`` class is the base class that specific messages are derived from.
 	// alert types are not copyable, and cannot be constructed by the client. The
 	// pointers returned by libtorrent are short lived (the details are described

include/libtorrent/aux_/alert_manager.hpp

@@ -3,6 +3,7 @@
 Copyright (c) 2003-2013, Daniel Wallin
 Copyright (c) 2013, 2015-2020, Arvid Norberg
 Copyright (c) 2016, 2018, 2020, Alden Torres
+Copyright (c) 2023, Joris Carrier
 All rights reserved.
 
 Redistribution and use in source and binary forms, with or without
@@ -88,6 +89,10 @@ namespace aux {
 			T& alert = queue.emplace_back<T>(
 				m_allocations[m_generation], std::forward<Args>(args)...);
 
+			if constexpr (std::is_base_of<callback_t<T>, T>::value) {
+				if (alert.has_callback()) alert.callback();
+			}
+
 			maybe_notify(&alert);
 		}
 		catch (std::bad_alloc const&)