make feature_flags proper type-safe

Author: arvidn

docs/upgrade_to_1.2.rst

@@ -120,7 +120,7 @@ plugins
 =======
 
 libtorrent session plugins no longer have all callbacks called unconditionally.
-The callback has to register which callbacks it's interested in receiving by returning a bitmask from ``std::uint32_t implemented_features()``.
+The callback has to register which callbacks it's interested in receiving by returning a bitmask from ``feature_flags_t implemented_features()``.
 The return value is documented in the plugin class.
 
 RSS functions removed

include/libtorrent/extensions.hpp

@@ -34,6 +34,7 @@ POSSIBILITY OF SUCH DAMAGE.
 #define TORRENT_EXTENSIONS_HPP_INCLUDED
 
 #include "libtorrent/units.hpp"
+#include "libtorrent/flags.hpp"
 
 // OVERVIEW
 //
@@ -182,6 +183,12 @@ namespace libtorrent {
 	struct session_handle;
 	struct peer_connection_handle;
 
+	struct feature_flags_tag;
+
+	// these are flags that can be returned by implemented_features()
+	// indicating which callbacks this plugin is interested in
+	using feature_flags_t = flags::bitfield_flag<std::uint8_t, feature_flags_tag>;
+
 	// this is the base class for a session plugin. One primary feature
 	// is that it is notified of all torrents that are added to the session,
 	// and can add its own torrent_plugins.
@@ -190,34 +197,29 @@ namespace libtorrent {
 		// hidden
 		virtual ~plugin() {}
 
-		// these are flags that can be returned by implemented_features()
-		// indicating which callbacks this plugin is interested in
-		enum feature_flags_t
-		{
-			// include this bit if your plugin needs to alter the order of the
-			// optimistic unchoke of peers. i.e. have the on_optimistic_unchoke()
-			// callback be called.
-			optimistic_unchoke_feature = 1,
-
-			// include this bit if your plugin needs to have on_tick() called
-			tick_feature = 2,
-
-			// include this bit if your plugin needs to have on_dht_request()
-			// called
-			dht_request_feature = 4,
-
-			// include this bit if your plugin needs to have on_alert()
-			// called
-			alert_feature = 8,
-		};
+		// include this bit if your plugin needs to alter the order of the
+		// optimistic unchoke of peers. i.e. have the on_optimistic_unchoke()
+		// callback be called.
+		static constexpr feature_flags_t optimistic_unchoke_feature = 1_bit;
+
+		// include this bit if your plugin needs to have on_tick() called
+		static constexpr feature_flags_t tick_feature = 2_bit;
+
+		// include this bit if your plugin needs to have on_dht_request()
+		// called
+		static constexpr feature_flags_t dht_request_feature = 3_bit;
+
+		// include this bit if your plugin needs to have on_alert()
+		// called
+		static constexpr feature_flags_t alert_feature = 4_bit;
 
 		// This function is expected to return a bitmask indicating which features
 		// this plugin implements. Some callbacks on this object may not be called
 		// unless the corresponding feature flag is returned here. Note that
 		// callbacks may still be called even if the corresponding feature is not
 		// specified in the return value here. See feature_flags_t for possible
 		// flags to return.
-		virtual std::uint32_t implemented_features() { return 0; }
+		virtual feature_flags_t implemented_features() { return {}; }
 
 		// this is called by the session every time a new torrent is added.
 		// The ``torrent*`` points to the internal torrent object created

src/session.cpp

@@ -35,11 +35,21 @@ POSSIBILITY OF SUCH DAMAGE.
 #include "libtorrent/extensions/ut_metadata.hpp"
 #include "libtorrent/extensions/smart_ban.hpp"
 #include "libtorrent/session.hpp"
+#include "libtorrent/extensions.hpp"
 #include "libtorrent/aux_/session_impl.hpp"
 #include "libtorrent/aux_/session_call.hpp"
 
 namespace libtorrent {
 
+#ifndef TORRENT_DISABLE_EXTENSIONS
+	// declared in extensions.hpp
+	// remove this once C++17 is required
+	constexpr feature_flags_t plugin::optimistic_unchoke_feature;
+	constexpr feature_flags_t plugin::tick_feature;
+	constexpr feature_flags_t plugin::dht_request_feature;
+	constexpr feature_flags_t plugin::alert_feature;
+#endif
+
 	settings_pack min_memory_usage()
 	{
 		settings_pack set;

src/session_impl.cpp

@@ -815,7 +815,7 @@ namespace aux {
 //		TORRENT_ASSERT(is_single_thread());
 		TORRENT_ASSERT_VAL(ext, ext);
 
-		std::uint32_t const features = ext->implemented_features();
+		feature_flags_t const features = ext->implemented_features();
 
 		m_ses_extensions[plugins_all_idx].push_back(ext);
 

test/test_direct_dht.cpp

@@ -47,7 +47,7 @@ namespace
 
 struct test_plugin : plugin
 {
-	std::uint32_t implemented_features() override
+	feature_flags_t implemented_features() override
 	{
 		return plugin::dht_request_feature;
 	}