tag/Handler: convert to class with virtual methods

src/TagArchive.cxx

@@ -25,7 +25,7 @@
 
 bool
 tag_archive_scan(ArchiveFile &archive, const char *path_utf8,
-		 const TagHandler &handler, void *handler_ctx) noexcept
+		 TagHandler &handler) noexcept
 try {
 	Mutex mutex;
 
@@ -33,7 +33,7 @@ try {
 	if (!is)
 		return false;
 
-	return tag_stream_scan(*is, handler, handler_ctx);
+	return tag_stream_scan(*is, handler);
 } catch (const std::exception &e) {
 	return false;
 }

src/TagArchive.hxx

@@ -23,7 +23,7 @@
 #include "check.h"
 
 class ArchiveFile;
-struct TagHandler;
+class TagHandler;
 class TagBuilder;
 
 /**
@@ -36,7 +36,7 @@ class TagBuilder;
  */
 bool
 tag_archive_scan(ArchiveFile &archive, const char *path_utf8,
-		 const TagHandler &handler, void *handler_ctx) noexcept;
+		 TagHandler &handler) noexcept;
 
 /**
  * Scan the tags of a song file inside an archive.  Invokes matching

src/TagFile.cxx

@@ -36,21 +36,20 @@ class TagFileScan {
 	const Path path_fs;
 	const char *const suffix;
 
-	const TagHandler &handler;
-	void *handler_ctx;
+	TagHandler &handler;
 
 	Mutex mutex;
 	InputStreamPtr is;
 
 public:
 	TagFileScan(Path _path_fs, const char *_suffix,
-		    const TagHandler &_handler, void *_handler_ctx) noexcept
+		    TagHandler &_handler) noexcept
 		:path_fs(_path_fs), suffix(_suffix),
-		 handler(_handler), handler_ctx(_handler_ctx) ,
+		 handler(_handler),
 		 is(nullptr) {}
 
 	bool ScanFile(const DecoderPlugin &plugin) noexcept {
-		return plugin.ScanFile(path_fs, handler, handler_ctx);
+		return plugin.ScanFile(path_fs, handler);
 	}
 
 	bool ScanStream(const DecoderPlugin &plugin) noexcept {
@@ -72,7 +71,7 @@ public:
 		}
 
 		/* now try the stream_tag() method */
-		return plugin.ScanStream(*is, handler, handler_ctx);
+		return plugin.ScanStream(*is, handler);
 	}
 
 	bool Scan(const DecoderPlugin &plugin) noexcept {
@@ -82,8 +81,7 @@ public:
 };
 
 bool
-tag_file_scan(Path path_fs,
-	      const TagHandler &handler, void *handler_ctx) noexcept
+tag_file_scan(Path path_fs, TagHandler &handler) noexcept
 {
 	assert(!path_fs.IsNull());
 
@@ -95,7 +93,7 @@ tag_file_scan(Path path_fs,
 
 	const auto suffix_utf8 = Path::FromFS(suffix).ToUTF8();
 
-	TagFileScan tfs(path_fs, suffix_utf8.c_str(), handler, handler_ctx);
+	TagFileScan tfs(path_fs, suffix_utf8.c_str(), handler);
 	return decoder_plugins_try([&](const DecoderPlugin &plugin){
 			return tfs.Scan(plugin);
 		});
@@ -104,11 +102,13 @@ tag_file_scan(Path path_fs,
 bool
 tag_file_scan(Path path, TagBuilder &builder) noexcept
 {
-	if (!tag_file_scan(path, full_tag_handler, &builder))
+	FullTagHandler h(builder);
+
+	if (!tag_file_scan(path, h))
 		return false;
 
 	if (builder.empty())
-		ScanGenericTags(path, full_tag_handler, &builder);
+		ScanGenericTags(path, h);
 
 	return true;
 }

src/TagFile.hxx

@@ -23,7 +23,7 @@
 #include "check.h"
 
 class Path;
-struct TagHandler;
+class TagHandler;
 class TagBuilder;
 
 /**
@@ -34,8 +34,7 @@ class TagBuilder;
  * found)
  */
 bool
-tag_file_scan(Path path,
-	      const TagHandler &handler, void *handler_ctx) noexcept;
+tag_file_scan(Path path, TagHandler &handler) noexcept;
 
 /**
  * Scan the tags of a song file.  Invokes matching decoder plugins,

src/TagStream.cxx

@@ -46,7 +46,7 @@ CheckDecoderPlugin(const DecoderPlugin &plugin,
 }
 
 bool
-tag_stream_scan(InputStream &is, const TagHandler &handler, void *ctx)
+tag_stream_scan(InputStream &is, TagHandler &handler) noexcept
 {
 	assert(is.IsReady());
 
@@ -62,44 +62,46 @@ tag_stream_scan(InputStream &is, const TagHandler &handler, void *ctx)
 		mime = (mime_base = GetMimeTypeBase(mime)).c_str();
 
 	return decoder_plugins_try([suffix, mime, &is,
-				    &handler, ctx](const DecoderPlugin &plugin){
+				    &handler](const DecoderPlugin &plugin){
 			try {
 				is.LockRewind();
 			} catch (...) {
 			}
 
 			return CheckDecoderPlugin(plugin, suffix, mime) &&
-				plugin.ScanStream(is, handler, ctx);
+				plugin.ScanStream(is, handler);
 		});
 }
 
 bool
-tag_stream_scan(const char *uri, const TagHandler &handler, void *ctx)
+tag_stream_scan(const char *uri, TagHandler &handler) noexcept
 try {
 	Mutex mutex;
 
 	auto is = InputStream::OpenReady(uri, mutex);
-	return tag_stream_scan(*is, handler, ctx);
+	return tag_stream_scan(*is, handler);
 } catch (const std::exception &e) {
 	return false;
 }
 
 bool
-tag_stream_scan(InputStream &is, TagBuilder &builder)
+tag_stream_scan(InputStream &is, TagBuilder &builder) noexcept
 {
 	assert(is.IsReady());
 
-	if (!tag_stream_scan(is, full_tag_handler, &builder))
+	FullTagHandler h(builder);
+
+	if (!tag_stream_scan(is, h))
 		return false;
 
 	if (builder.empty())
-		ScanGenericTags(is, full_tag_handler, &builder);
+		ScanGenericTags(is, h);
 
 	return true;
 }
 
 bool
-tag_stream_scan(const char *uri, TagBuilder &builder)
+tag_stream_scan(const char *uri, TagBuilder &builder) noexcept
 try {
 	Mutex mutex;
 

src/TagStream.hxx

@@ -23,7 +23,7 @@
 #include "check.h"
 
 class InputStream;
-struct TagHandler;
+class TagHandler;
 class TagBuilder;
 
 /**
@@ -34,10 +34,10 @@ class TagBuilder;
  * found)
  */
 bool
-tag_stream_scan(InputStream &is, const TagHandler &handler, void *ctx);
+tag_stream_scan(InputStream &is, TagHandler &handler) noexcept;
 
 bool
-tag_stream_scan(const char *uri, const TagHandler &handler, void *ctx);
+tag_stream_scan(const char *uri, TagHandler &handler) noexcept;
 
 /**
  * Scan the tags of an #InputStream.  Invokes matching decoder
@@ -48,9 +48,9 @@ tag_stream_scan(const char *uri, const TagHandler &handler, void *ctx);
  * found)
  */
 bool
-tag_stream_scan(InputStream &is, TagBuilder &builder);
+tag_stream_scan(InputStream &is, TagBuilder &builder) noexcept;
 
 bool
-tag_stream_scan(const char *uri, TagBuilder &builder);
+tag_stream_scan(const char *uri, TagBuilder &builder) noexcept;
 
 #endif

src/command/FileCommands.cxx

@@ -137,25 +137,24 @@ IsValidValue(const char *p) noexcept
 	return true;
 }
 
-static void
-print_pair(const char *key, const char *value, void *ctx)
-{
-	auto &r = *(Response *)ctx;
+class PrintCommentHandler final : public NullTagHandler {
+	Response &response;
 
-	if (IsValidName(key) && IsValidValue(value))
-		r.Format("%s: %s\n", key, value);
-}
+public:
+	explicit PrintCommentHandler(Response &_response) noexcept
+		:NullTagHandler(WANT_PAIR), response(_response) {}
 
-static constexpr TagHandler print_comment_handler = {
-	nullptr,
-	nullptr,
-	print_pair,
+	void OnPair(const char *key, const char *value) noexcept override {
+		if (IsValidName(key) && IsValidValue(value))
+			response.Format("%s: %s\n", key, value);
+	}
 };
 
 static CommandResult
 read_stream_comments(Response &r, const char *uri)
 {
-	if (!tag_stream_scan(uri, print_comment_handler, &r)) {
+	PrintCommentHandler h(r);
+	if (!tag_stream_scan(uri, h)) {
 		r.Error(ACK_ERROR_NO_EXIST, "Failed to load file");
 		return CommandResult::ERROR;
 	}
@@ -167,12 +166,13 @@ read_stream_comments(Response &r, const char *uri)
 static CommandResult
 read_file_comments(Response &r, const Path path_fs)
 {
-	if (!tag_file_scan(path_fs, print_comment_handler, &r)) {
+	PrintCommentHandler h(r);
+	if (!tag_file_scan(path_fs, h)) {
 		r.Error(ACK_ERROR_NO_EXIST, "Failed to load file");
 		return CommandResult::ERROR;
 	}
 
-	ScanGenericTags(path_fs, print_comment_handler, &r);
+	ScanGenericTags(path_fs, h);
 
 	return CommandResult::OK;
 

src/command/OtherCommands.cxx

@@ -93,15 +93,6 @@ handle_kill(gcc_unused Client &client, gcc_unused Request request,
 	return CommandResult::KILL;
 }
 
-static void
-print_tag(TagType type, const char *value, void *ctx)
-{
-	auto &r = *(Response *)ctx;
-
-	if (r.GetClient().tag_mask.Test(type))
-		tag_print(r, type, value);
-}
-
 CommandResult
 handle_listfiles(Client &client, Request args, Response &r)
 {
@@ -149,16 +140,24 @@ handle_listfiles(Client &client, Request args, Response &r)
 	gcc_unreachable();
 }
 
-static constexpr TagHandler print_tag_handler = {
-	nullptr,
-	print_tag,
-	nullptr,
+class PrintTagHandler final : public NullTagHandler {
+	Response &response;
+
+public:
+	explicit PrintTagHandler(Response &_response) noexcept
+		:NullTagHandler(WANT_TAG), response(_response) {}
+
+	void OnTag(TagType type, const char *value) noexcept override {
+		if (response.GetClient().tag_mask.Test(type))
+			tag_print(response, type, value);
+	}
 };
 
 static CommandResult
 handle_lsinfo_absolute(Response &r, const char *uri)
 {
-	if (!tag_stream_scan(uri, print_tag_handler, &r)) {
+	PrintTagHandler h(r);
+	if (!tag_stream_scan(uri, h)) {
 		r.Error(ACK_ERROR_NO_EXIST, "No such file");
 		return CommandResult::ERROR;
 	}

src/decoder/DecoderPlugin.hxx

@@ -26,7 +26,7 @@
 
 struct ConfigBlock;
 class InputStream;
-struct TagHandler;
+class TagHandler;
 class Path;
 class DecoderClient;
 class DetachedSong;
@@ -71,18 +71,14 @@ struct DecoderPlugin {
 	 *
 	 * @return false if the operation has failed
 	 */
-	bool (*scan_file)(Path path_fs,
-			  const TagHandler &handler,
-			  void *handler_ctx) noexcept;
+	bool (*scan_file)(Path path_fs, TagHandler &handler) noexcept;
 
 	/**
 	 * Scan metadata of a file.
 	 *
 	 * @return false if the operation has failed
 	 */
-	bool (*scan_stream)(InputStream &is,
-			    const TagHandler &handler,
-			    void *handler_ctx) noexcept;
+	bool (*scan_stream)(InputStream &is, TagHandler &handler) noexcept;
 
 	/**
 	 * @brief Return a "virtual" filename for subtracks in
@@ -139,20 +135,18 @@ struct DecoderPlugin {
 	 * Read the tag of a file.
 	 */
 	template<typename P>
-	bool ScanFile(P path_fs,
-		      const TagHandler &handler, void *handler_ctx) const noexcept {
+	bool ScanFile(P path_fs, TagHandler &handler) const noexcept {
 		return scan_file != nullptr
-			? scan_file(path_fs, handler, handler_ctx)
+			? scan_file(path_fs, handler)
 			: false;
 	}
 
 	/**
 	 * Read the tag of a stream.
 	 */
-	bool ScanStream(InputStream &is,
-			const TagHandler &handler, void *handler_ctx) const noexcept {
+	bool ScanStream(InputStream &is, TagHandler &handler) const noexcept {
 		return scan_stream != nullptr
-			? scan_stream(is, handler, handler_ctx)
+			? scan_stream(is, handler)
 			: false;
 	}
 

src/decoder/plugins/AdPlugDecoderPlugin.cxx

@@ -83,16 +83,14 @@ adplug_file_decode(DecoderClient &client, Path path_fs)
 
 static void
 adplug_scan_tag(TagType type, const std::string &value,
-		const TagHandler &handler, void *handler_ctx) noexcept
+		TagHandler &handler) noexcept
 {
 	if (!value.empty())
-		tag_handler_invoke_tag(handler, handler_ctx,
-				       type, value.c_str());
+		handler.OnTag(type, value.c_str());
 }
 
 static bool
-adplug_scan_file(Path path_fs,
-		 const TagHandler &handler, void *handler_ctx) noexcept
+adplug_scan_file(Path path_fs, TagHandler &handler) noexcept
 {
 	CEmuopl opl(sample_rate, true, true);
 	opl.init();
@@ -101,16 +99,15 @@ adplug_scan_file(Path path_fs,
 	if (player == nullptr)
 		return false;
 
-	tag_handler_invoke_duration(handler, handler_ctx,
-				    SongTime::FromMS(player->songlength()));
+	handler.OnDuration(SongTime::FromMS(player->songlength()));
 
-	if (handler.tag != nullptr) {
+	if (handler.WantTag()) {
 		adplug_scan_tag(TAG_TITLE, player->gettitle(),
-				handler, handler_ctx);
+				handler);
 		adplug_scan_tag(TAG_ARTIST, player->getauthor(),
-				handler, handler_ctx);
+				handler);
 		adplug_scan_tag(TAG_COMMENT, player->getdesc(),
-				handler, handler_ctx);
+				handler);
 	}
 
 	delete player;

src/decoder/plugins/AudiofileDecoderPlugin.cxx

@@ -256,14 +256,13 @@ audiofile_get_duration(InputStream &is) noexcept
 }
 
 static bool
-audiofile_scan_stream(InputStream &is,
-		      const TagHandler &handler, void *handler_ctx) noexcept
+audiofile_scan_stream(InputStream &is, TagHandler &handler) noexcept
 {
 	const auto duration = audiofile_get_duration(is);
 	if (duration.IsNegative())
 		return false;
 
-	tag_handler_invoke_duration(handler, handler_ctx, SongTime(duration));
+	handler.OnDuration(SongTime(duration));
 	return true;
 }
 

src/decoder/plugins/DsdLib.cxx

@@ -115,9 +115,8 @@ dsdlib_valid_freq(uint32_t samplefreq) noexcept
 
 #ifdef ENABLE_ID3TAG
 void
-dsdlib_tag_id3(InputStream &is,
-	       const TagHandler &handler,
-	       void *handler_ctx, offset_type tagoffset)
+dsdlib_tag_id3(InputStream &is, TagHandler &handler,
+	       offset_type tagoffset)
 {
 	if (tagoffset == 0 || !is.KnownSize())
 		return;
@@ -150,7 +149,7 @@ dsdlib_tag_id3(InputStream &is,
 	if (id3_tag == nullptr)
 		return;
 
-	scan_id3_tag(id3_tag, handler, handler_ctx);
+	scan_id3_tag(id3_tag, handler);
 
 	id3_tag_delete(id3_tag);
 	return;

src/decoder/plugins/DsdLib.hxx

@@ -26,7 +26,7 @@
 
 #include <stdint.h>
 
-struct TagHandler;
+class TagHandler;
 class DecoderClient;
 class InputStream;
 
@@ -79,8 +79,7 @@ dsdlib_valid_freq(uint32_t samplefreq) noexcept;
  * DSF and DSDIFF files are imported
  */
 void
-dsdlib_tag_id3(InputStream &is,
-	       const TagHandler &handler,
-	       void *handler_ctx, offset_type tagoffset);
+dsdlib_tag_id3(InputStream &is, TagHandler &handler,
+	       offset_type tagoffset);
 
 #endif

src/decoder/plugins/DsdiffDecoderPlugin.cxx

@@ -186,8 +186,8 @@ dsdiff_read_prop(DecoderClient *client, InputStream &is,
 
 static void
 dsdiff_handle_native_tag(InputStream &is,
-			 const TagHandler &handler,
-			 void *handler_ctx, offset_type tagoffset,
+			 TagHandler &handler,
+			 offset_type tagoffset,
 			 TagType type)
 {
 	if (!dsdlib_skip_to(nullptr, is, tagoffset))
@@ -212,7 +212,7 @@ dsdiff_handle_native_tag(InputStream &is,
 		return;
 
 	string[length] = '\0';
-	tag_handler_invoke_tag(handler, handler_ctx, type, label);
+	handler.OnTag(type, label);
 	return;
 }
 
@@ -228,8 +228,7 @@ static bool
 dsdiff_read_metadata_extra(DecoderClient *client, InputStream &is,
 			   DsdiffMetaData *metadata,
 			   DsdiffChunkHeader *chunk_header,
-			   const TagHandler &handler,
-			   void *handler_ctx)
+			   TagHandler &handler)
 {
 
 	/* skip from DSD data to next chunk header */
@@ -286,17 +285,17 @@ dsdiff_read_metadata_extra(DecoderClient *client, InputStream &is,
 	if (id3_offset != 0) {
 		/* a ID3 tag has preference over the other tags, do not process
 		   other tags if we have one */
-		dsdlib_tag_id3(is, handler, handler_ctx, id3_offset);
+		dsdlib_tag_id3(is, handler, id3_offset);
 		return true;
 	}
 #endif
 
 	if (artist_offset != 0)
-		dsdiff_handle_native_tag(is, handler, handler_ctx,
+		dsdiff_handle_native_tag(is, handler,
 					 artist_offset, TAG_ARTIST);
 
 	if (title_offset != 0)
-		dsdiff_handle_native_tag(is, handler, handler_ctx,
+		dsdiff_handle_native_tag(is, handler,
 					 title_offset, TAG_TITLE);
 	return true;
 }
@@ -449,9 +448,7 @@ dsdiff_stream_decode(DecoderClient &client, InputStream &is)
 }
 
 static bool
-dsdiff_scan_stream(InputStream &is,
-		   gcc_unused const TagHandler &handler,
-		   gcc_unused void *handler_ctx) noexcept
+dsdiff_scan_stream(InputStream &is, TagHandler &handler) noexcept
 {
 	DsdiffMetaData metadata;
 	DsdiffChunkHeader chunk_header;
@@ -469,11 +466,11 @@ dsdiff_scan_stream(InputStream &is,
 	uint64_t n_frames = metadata.chunk_size / metadata.channels;
 	auto songtime = SongTime::FromScale<uint64_t>(n_frames,
 						      sample_rate);
-	tag_handler_invoke_duration(handler, handler_ctx, songtime);
+	handler.OnDuration(songtime);
 
 	/* Read additional metadata and created tags if available */
 	dsdiff_read_metadata_extra(nullptr, is, &metadata, &chunk_header,
-				   handler, handler_ctx);
+				   handler);
 
 	return true;
 }

src/decoder/plugins/DsfDecoderPlugin.cxx

@@ -325,9 +325,7 @@ dsf_stream_decode(DecoderClient &client, InputStream &is)
 }
 
 static bool
-dsf_scan_stream(InputStream &is,
-		gcc_unused const TagHandler &handler,
-		gcc_unused void *handler_ctx) noexcept
+dsf_scan_stream(InputStream &is, TagHandler &handler) noexcept
 {
 	/* check DSF metadata */
 	DsfMetaData metadata;
@@ -342,11 +340,11 @@ dsf_scan_stream(InputStream &is,
 	const auto n_blocks = metadata.n_blocks;
 	auto songtime = SongTime::FromScale<uint64_t>(n_blocks * DSF_BLOCK_SIZE,
 						      sample_rate);
-	tag_handler_invoke_duration(handler, handler_ctx, songtime);
+	handler.OnDuration(songtime);
 
 #ifdef ENABLE_ID3TAG
 	/* Add available tags from the ID3 tag */
-	dsdlib_tag_id3(is, handler, handler_ctx, metadata.id3_offset);
+	dsdlib_tag_id3(is, handler, metadata.id3_offset);
 #endif
 	return true;
 }

src/decoder/plugins/FaadDecoderPlugin.cxx

@@ -414,16 +414,14 @@ faad_stream_decode(DecoderClient &client, InputStream &is)
 }
 
 static bool
-faad_scan_stream(InputStream &is,
-		 const TagHandler &handler, void *handler_ctx) noexcept
+faad_scan_stream(InputStream &is, TagHandler &handler) noexcept
 {
 	auto result = faad_get_file_time(is);
 	if (!result.first)
 		return false;
 
 	if (!result.second.IsNegative())
-		tag_handler_invoke_duration(handler, handler_ctx,
-					    SongTime(result.second));
+		handler.OnDuration(SongTime(result.second));
 	return true;
 }
 

src/decoder/plugins/FfmpegDecoderPlugin.cxx

@@ -572,21 +572,20 @@ FfmpegParseMetaData(DecoderClient &client,
 }
 
 static void
-FfmpegScanMetadata(const AVStream &stream,
-		   const TagHandler &handler, void *handler_ctx)
+FfmpegScanMetadata(const AVStream &stream, TagHandler &handler) noexcept
 {
-	FfmpegScanDictionary(stream.metadata, handler, handler_ctx);
+	FfmpegScanDictionary(stream.metadata, handler);
 }
 
 static void
 FfmpegScanMetadata(const AVFormatContext &format_context, int audio_stream,
-		   const TagHandler &handler, void *handler_ctx)
+		   TagHandler &handler) noexcept
 {
 	assert(audio_stream >= 0);
 
-	FfmpegScanDictionary(format_context.metadata, handler, handler_ctx);
+	FfmpegScanDictionary(format_context.metadata, handler);
 	FfmpegScanMetadata(*format_context.streams[audio_stream],
-			   handler, handler_ctx);
+			   handler);
 }
 
 #if LIBAVFORMAT_VERSION_INT >= AV_VERSION_INT(56, 1, 0)
@@ -595,8 +594,8 @@ static void
 FfmpegScanTag(const AVFormatContext &format_context, int audio_stream,
 	      TagBuilder &tag)
 {
-	FfmpegScanMetadata(format_context, audio_stream,
-			   full_tag_handler, &tag);
+	FullTagHandler h(tag);
+	FfmpegScanMetadata(format_context, audio_stream, h);
 }
 
 /**
@@ -828,7 +827,7 @@ ffmpeg_decode(DecoderClient &client, InputStream &input)
 
 static bool
 FfmpegScanStream(AVFormatContext &format_context,
-		 const TagHandler &handler, void *handler_ctx)
+		 TagHandler &handler) noexcept
 {
 	const int find_result =
 		avformat_find_stream_info(&format_context, nullptr);
@@ -841,22 +840,19 @@ FfmpegScanStream(AVFormatContext &format_context,
 
 	const AVStream &stream = *format_context.streams[audio_stream];
 	if (stream.duration != (int64_t)AV_NOPTS_VALUE)
-		tag_handler_invoke_duration(handler, handler_ctx,
-					    FromFfmpegTime(stream.duration,
-							   stream.time_base));
+		handler.OnDuration(FromFfmpegTime(stream.duration,
+						  stream.time_base));
 	else if (format_context.duration != (int64_t)AV_NOPTS_VALUE)
-		tag_handler_invoke_duration(handler, handler_ctx,
-					    FromFfmpegTime(format_context.duration,
-							   AV_TIME_BASE_Q));
+		handler.OnDuration(FromFfmpegTime(format_context.duration,
+						  AV_TIME_BASE_Q));
 
-	FfmpegScanMetadata(format_context, audio_stream, handler, handler_ctx);
+	FfmpegScanMetadata(format_context, audio_stream, handler);
 
 	return true;
 }
 
 static bool
-ffmpeg_scan_stream(InputStream &is,
-		   const TagHandler &handler, void *handler_ctx) noexcept
+ffmpeg_scan_stream(InputStream &is, TagHandler &handler) noexcept
 {
 	AVInputFormat *input_format = ffmpeg_probe(nullptr, is);
 	if (input_format == nullptr)
@@ -877,7 +873,7 @@ ffmpeg_scan_stream(InputStream &is,
 		avformat_close_input(&f);
 	};
 
-	return FfmpegScanStream(*f, handler, handler_ctx);
+	return FfmpegScanStream(*f, handler);
 }
 
 /**

src/decoder/plugins/FfmpegMetaData.cxx

@@ -43,46 +43,42 @@ static constexpr struct tag_table ffmpeg_tags[] = {
 static void
 FfmpegScanTag(TagType type,
 	      AVDictionary *m, const char *name,
-	      const TagHandler &handler, void *handler_ctx)
+	      TagHandler &handler) noexcept
 {
 	AVDictionaryEntry *mt = nullptr;
 
 	while ((mt = av_dict_get(m, name, mt, 0)) != nullptr)
-		tag_handler_invoke_tag(handler, handler_ctx,
-				       type, mt->value);
+		handler.OnTag(type, mt->value);
 }
 
 static void
-FfmpegScanPairs(AVDictionary *dict,
-		const TagHandler &handler, void *handler_ctx)
+FfmpegScanPairs(AVDictionary *dict, TagHandler &handler) noexcept
 {
 	AVDictionaryEntry *i = nullptr;
 
 	while ((i = av_dict_get(dict, "", i, AV_DICT_IGNORE_SUFFIX)) != nullptr)
-		tag_handler_invoke_pair(handler, handler_ctx,
-					i->key, i->value);
+		handler.OnPair(i->key, i->value);
 }
 
 void
-FfmpegScanDictionary(AVDictionary *dict,
-		     const TagHandler &handler, void *handler_ctx)
+FfmpegScanDictionary(AVDictionary *dict, TagHandler &handler) noexcept
 {
-	if (handler.tag != nullptr) {
+	if (handler.WantTag()) {
 		for (unsigned i = 0; i < TAG_NUM_OF_ITEM_TYPES; ++i)
 			FfmpegScanTag(TagType(i), dict, tag_item_names[i],
-				      handler, handler_ctx);
+				      handler);
 
 		for (const struct tag_table *i = ffmpeg_tags;
 		     i->name != nullptr; ++i)
 			FfmpegScanTag(i->type, dict, i->name,
-				      handler, handler_ctx);
+				      handler);
 
 		for (const struct tag_table *i = musicbrainz_txxx_tags;
 		     i->name != nullptr; ++i)
 			FfmpegScanTag(i->type, dict, i->name,
-				      handler, handler_ctx);
+				      handler);
 	}
 
-	if (handler.pair != nullptr)
-		FfmpegScanPairs(dict, handler, handler_ctx);
+	if (handler.WantPair())
+		FfmpegScanPairs(dict, handler);
 }

src/decoder/plugins/FfmpegMetaData.hxx

@@ -21,10 +21,9 @@
 #define MPD_FFMPEG_METADATA_HXX
 
 struct AVDictionary;
-struct TagHandler;
+class TagHandler;
 
 void
-FfmpegScanDictionary(AVDictionary *dict,
-		     const TagHandler &handler, void *handler_ctx);
+FfmpegScanDictionary(AVDictionary *dict, TagHandler &handler) noexcept;
 
 #endif

src/decoder/plugins/FlacDecoderPlugin.cxx

@@ -69,8 +69,7 @@ flac_write_cb(const FLAC__StreamDecoder *dec, const FLAC__Frame *frame,
 }
 
 static bool
-flac_scan_file(Path path_fs,
-	       const TagHandler &handler, void *handler_ctx) noexcept
+flac_scan_file(Path path_fs, TagHandler &handler) noexcept
 {
 	FlacMetadataChain chain;
 	if (!chain.Read(NarrowPath(path_fs))) {
@@ -80,13 +79,12 @@ flac_scan_file(Path path_fs,
 		return false;
 	}
 
-	chain.Scan(handler, handler_ctx);
+	chain.Scan(handler);
 	return true;
 }
 
 static bool
-flac_scan_stream(InputStream &is,
-		 const TagHandler &handler, void *handler_ctx) noexcept
+flac_scan_stream(InputStream &is, TagHandler &handler) noexcept
 {
 	FlacMetadataChain chain;
 	if (!chain.Read(is)) {
@@ -96,7 +94,7 @@ flac_scan_stream(InputStream &is,
 		return false;
 	}
 
-	chain.Scan(handler, handler_ctx);
+	chain.Scan(handler);
 	return true;
 }
 
@@ -315,8 +313,7 @@ oggflac_init(gcc_unused const ConfigBlock &block)
 }
 
 static bool
-oggflac_scan_file(Path path_fs,
-		  const TagHandler &handler, void *handler_ctx) noexcept
+oggflac_scan_file(Path path_fs, TagHandler &handler) noexcept
 {
 	FlacMetadataChain chain;
 	if (!chain.ReadOgg(NarrowPath(path_fs))) {
@@ -326,13 +323,12 @@ oggflac_scan_file(Path path_fs,
 		return false;
 	}
 
-	chain.Scan(handler, handler_ctx);
+	chain.Scan(handler);
 	return true;
 }
 
 static bool
-oggflac_scan_stream(InputStream &is,
-		    const TagHandler &handler, void *handler_ctx) noexcept
+oggflac_scan_stream(InputStream &is, TagHandler &handler) noexcept
 {
 	FlacMetadataChain chain;
 	if (!chain.ReadOgg(is)) {
@@ -342,7 +338,7 @@ oggflac_scan_stream(InputStream &is,
 		return false;
 	}
 
-	chain.Scan(handler, handler_ctx);
+	chain.Scan(handler);
 	return true;
 }
 

src/decoder/plugins/FlacMetadata.cxx

@@ -79,11 +79,11 @@ flac_comment_value(const FLAC__StreamMetadata_VorbisComment_Entry *entry,
 static bool
 flac_copy_comment(const FLAC__StreamMetadata_VorbisComment_Entry *entry,
 		  const char *name, TagType tag_type,
-		  const TagHandler &handler, void *handler_ctx)
+		  TagHandler &handler) noexcept
 {
 	const char *value = flac_comment_value(entry, name);
 	if (value != nullptr) {
-		tag_handler_invoke_tag(handler, handler_ctx, tag_type, value);
+		handler.OnTag(tag_type, value);
 		return true;
 	}
 
@@ -92,36 +92,33 @@ flac_copy_comment(const FLAC__StreamMetadata_VorbisComment_Entry *entry,
 
 static void
 flac_scan_comment(const FLAC__StreamMetadata_VorbisComment_Entry *entry,
-		  const TagHandler &handler, void *handler_ctx)
+		  TagHandler &handler) noexcept
 {
-	if (handler.pair != nullptr) {
+	if (handler.WantPair()) {
 		const char *comment = (const char *)entry->entry;
 		const DivideString split(comment, '=');
 		if (split.IsDefined() && !split.empty())
-			tag_handler_invoke_pair(handler, handler_ctx,
-						split.GetFirst(),
-						split.GetSecond());
+			handler.OnPair(split.GetFirst(), split.GetSecond());
 	}
 
 	for (const struct tag_table *i = xiph_tags; i->name != nullptr; ++i)
-		if (flac_copy_comment(entry, i->name, i->type,
-				      handler, handler_ctx))
+		if (flac_copy_comment(entry, i->name, i->type, handler))
 			return;
 
 	for (unsigned i = 0; i < TAG_NUM_OF_ITEM_TYPES; ++i)
 		if (flac_copy_comment(entry,
 				      tag_item_names[i], (TagType)i,
-				      handler, handler_ctx))
+				      handler))
 			return;
 }
 
 static void
 flac_scan_comments(const FLAC__StreamMetadata_VorbisComment *comment,
-		   const TagHandler &handler, void *handler_ctx)
+		   TagHandler &handler) noexcept
 {
 	for (unsigned i = 0; i < comment->num_comments; ++i)
 		flac_scan_comment(&comment->comments[i],
-				  handler, handler_ctx);
+				  handler);
 }
 
 gcc_pure
@@ -136,18 +133,17 @@ flac_duration(const FLAC__StreamMetadata_StreamInfo *stream_info) noexcept
 
 void
 flac_scan_metadata(const FLAC__StreamMetadata *block,
-		   const TagHandler &handler, void *handler_ctx)
+		   TagHandler &handler) noexcept
 {
 	switch (block->type) {
 	case FLAC__METADATA_TYPE_VORBIS_COMMENT:
 		flac_scan_comments(&block->data.vorbis_comment,
-				   handler, handler_ctx);
+				   handler);
 		break;
 
 	case FLAC__METADATA_TYPE_STREAMINFO:
 		if (block->data.stream_info.sample_rate > 0)
-			tag_handler_invoke_duration(handler, handler_ctx,
-						    flac_duration(&block->data.stream_info));
+			handler.OnDuration(flac_duration(&block->data.stream_info));
 		break;
 
 	default:
@@ -159,12 +155,13 @@ Tag
 flac_vorbis_comments_to_tag(const FLAC__StreamMetadata_VorbisComment *comment)
 {
 	TagBuilder tag_builder;
-	flac_scan_comments(comment, add_tag_handler, &tag_builder);
+	AddTagHandler h(tag_builder);
+	flac_scan_comments(comment, h);
 	return tag_builder.Commit();
 }
 
 void
-FlacMetadataChain::Scan(const TagHandler &handler, void *handler_ctx)
+FlacMetadataChain::Scan(TagHandler &handler) noexcept
 {
 	FLACMetadataIterator iterator(*this);
 
@@ -173,6 +170,6 @@ FlacMetadataChain::Scan(const TagHandler &handler, void *handler_ctx)
 		if (block == nullptr)
 			break;
 
-		flac_scan_metadata(block, handler, handler_ctx);
+		flac_scan_metadata(block, handler);
 	} while (iterator.Next());
 }

src/decoder/plugins/FlacMetadata.hxx

@@ -25,7 +25,7 @@
 
 #include <FLAC/metadata.h>
 
-struct TagHandler;
+class TagHandler;
 class MixRampInfo;
 
 class FlacMetadataChain {
@@ -82,7 +82,7 @@ public:
 		return FLAC__Metadata_ChainStatusString[GetStatus()];
 	}
 
-	void Scan(const TagHandler &handler, void *handler_ctx);
+	void Scan(TagHandler &handler) noexcept;
 };
 
 class FLACMetadataIterator {
@@ -126,6 +126,6 @@ flac_vorbis_comments_to_tag(const FLAC__StreamMetadata_VorbisComment *comment);
 
 void
 flac_scan_metadata(const FLAC__StreamMetadata *block,
-		   const TagHandler &handler, void *handler_ctx);
+		   TagHandler &handler) noexcept;
 
 #endif

src/decoder/plugins/FluidsynthDecoderPlugin.cxx

@@ -193,8 +193,7 @@ fluidsynth_file_decode(DecoderClient &client, Path path_fs)
 
 static bool
 fluidsynth_scan_file(Path path_fs,
-		     gcc_unused const TagHandler &handler,
-		     gcc_unused void *handler_ctx) noexcept
+		     gcc_unused TagHandler &handler) noexcept
 {
 	return fluid_is_midifile(path_fs.c_str());
 }

src/decoder/plugins/GmeDecoderPlugin.cxx

@@ -215,15 +215,13 @@ gme_file_decode(DecoderClient &client, Path path_fs)
 
 static void
 ScanGmeInfo(const gme_info_t &info, unsigned song_num, int track_count,
-	    const TagHandler &handler, void *handler_ctx)
+	    TagHandler &handler) noexcept
 {
 	if (info.play_length > 0)
-		tag_handler_invoke_duration(handler, handler_ctx,
-					    SongTime::FromMS(info.play_length));
+		handler.OnDuration(SongTime::FromMS(info.play_length));
 
 	if (track_count > 1)
-		tag_handler_invoke_tag(handler, handler_ctx, TAG_TRACK,
-				       StringFormat<16>("%u", song_num + 1));
+		handler.OnTag(TAG_TRACK, StringFormat<16>("%u", song_num + 1));
 
 	if (info.song != nullptr) {
 		if (track_count > 1) {
@@ -232,33 +230,26 @@ ScanGmeInfo(const gme_info_t &info, unsigned song_num, int track_count,
 				StringFormat<1024>("%s (%u/%d)",
 						   info.song, song_num + 1,
 						   track_count);
-			tag_handler_invoke_tag(handler, handler_ctx,
-					       TAG_TITLE, tag_title);
+			handler.OnTag(TAG_TITLE, tag_title);
 		} else
-			tag_handler_invoke_tag(handler, handler_ctx,
-					       TAG_TITLE, info.song);
+			handler.OnTag(TAG_TITLE, info.song);
 	}
 
 	if (info.author != nullptr)
-		tag_handler_invoke_tag(handler, handler_ctx,
-				       TAG_ARTIST, info.author);
+		handler.OnTag(TAG_ARTIST, info.author);
 
 	if (info.game != nullptr)
-		tag_handler_invoke_tag(handler, handler_ctx,
-				       TAG_ALBUM, info.game);
+		handler.OnTag(TAG_ALBUM, info.game);
 
 	if (info.comment != nullptr)
-		tag_handler_invoke_tag(handler, handler_ctx,
-				       TAG_COMMENT, info.comment);
+		handler.OnTag(TAG_COMMENT, info.comment);
 
 	if (info.copyright != nullptr)
-		tag_handler_invoke_tag(handler, handler_ctx,
-				       TAG_DATE, info.copyright);
+		handler.OnTag(TAG_DATE, info.copyright);
 }
 
 static bool
-ScanMusicEmu(Music_Emu *emu, unsigned song_num,
-	     const TagHandler &handler, void *handler_ctx)
+ScanMusicEmu(Music_Emu *emu, unsigned song_num, TagHandler &handler) noexcept
 {
 	gme_info_t *ti;
 	const char *gme_err = gme_track_info(emu, &ti, song_num);
@@ -271,14 +262,12 @@ ScanMusicEmu(Music_Emu *emu, unsigned song_num,
 
 	AtScopeExit(ti) { gme_free_info(ti); };
 
-	ScanGmeInfo(*ti, song_num, gme_track_count(emu),
-		    handler, handler_ctx);
+	ScanGmeInfo(*ti, song_num, gme_track_count(emu), handler);
 	return true;
 }
 
 static bool
-gme_scan_file(Path path_fs,
-	      const TagHandler &handler, void *handler_ctx) noexcept
+gme_scan_file(Path path_fs, TagHandler &handler) noexcept
 {
 	const auto container = ParseContainerPath(path_fs);
 
@@ -289,7 +278,7 @@ gme_scan_file(Path path_fs,
 
 	AtScopeExit(emu) { gme_delete(emu); };
 
-	return ScanMusicEmu(emu, container.track, handler, handler_ctx);
+	return ScanMusicEmu(emu, container.track, handler);
 }
 
 static std::forward_list<DetachedSong>
@@ -316,8 +305,8 @@ gme_container_scan(Path path_fs)
 
 	auto tail = list.before_begin();
 	for (unsigned i = 0; i < num_songs; ++i) {
-		ScanMusicEmu(emu, i,
-			     add_tag_handler, &tag_builder);
+		AddTagHandler h(tag_builder);
+		ScanMusicEmu(emu, i, h);
 
 		const auto track_name =
 			StringFormat<64>(SUBTUNE_PREFIX "%03u.%s", i+1,

src/decoder/plugins/MadDecoderPlugin.cxx

@@ -1062,16 +1062,14 @@ mp3_decode(DecoderClient &client, InputStream &input_stream)
 }
 
 static bool
-mad_decoder_scan_stream(InputStream &is,
-			const TagHandler &handler, void *handler_ctx) noexcept
+mad_decoder_scan_stream(InputStream &is, TagHandler &handler) noexcept
 {
 	const auto result = mad_decoder_total_file_time(is);
 	if (!result.first)
 		return false;
 
 	if (!result.second.IsNegative())
-		tag_handler_invoke_duration(handler, handler_ctx,
-					    SongTime(result.second));
+		handler.OnDuration(SongTime(result.second));
 	return true;
 }
 

src/decoder/plugins/MikmodDecoderPlugin.cxx

@@ -185,8 +185,7 @@ mikmod_decoder_file_decode(DecoderClient &client, Path path_fs)
 }
 
 static bool
-mikmod_decoder_scan_file(Path path_fs,
-			 const TagHandler &handler, void *handler_ctx) noexcept
+mikmod_decoder_scan_file(Path path_fs, TagHandler &handler) noexcept
 {
 	/* deconstify the path because libmikmod wants a non-const
 	   string pointer */
@@ -204,8 +203,7 @@ mikmod_decoder_scan_file(Path path_fs,
 
 	char *title = Player_LoadTitle(path2);
 	if (title != nullptr) {
-		tag_handler_invoke_tag(handler, handler_ctx,
-				       TAG_TITLE, title);
+		handler.OnTag(TAG_TITLE, title);
 #if (LIBMIKMOD_VERSION >= 0x030200)
 		MikMod_free(title);
 #else

src/decoder/plugins/ModplugDecoderPlugin.cxx

@@ -175,20 +175,17 @@ mod_decode(DecoderClient &client, InputStream &is)
 }
 
 static bool
-modplug_scan_stream(InputStream &is,
-		    const TagHandler &handler, void *handler_ctx) noexcept
+modplug_scan_stream(InputStream &is, TagHandler &handler) noexcept
 {
 	ModPlugFile *f = LoadModPlugFile(nullptr, is);
 	if (f == nullptr)
 		return false;
 
-	tag_handler_invoke_duration(handler, handler_ctx,
-				    SongTime::FromMS(ModPlug_GetLength(f)));
+	handler.OnDuration(SongTime::FromMS(ModPlug_GetLength(f)));
 
 	const char *title = ModPlug_GetName(f);
 	if (title != nullptr)
-		tag_handler_invoke_tag(handler, handler_ctx,
-				       TAG_TITLE, title);
+		handler.OnTag(TAG_TITLE, title);
 
 	ModPlug_Unload(f);
 

src/decoder/plugins/MpcdecDecoderPlugin.cxx

@@ -257,14 +257,13 @@ mpcdec_get_file_duration(InputStream &is)
 }
 
 static bool
-mpcdec_scan_stream(InputStream &is,
-		   const TagHandler &handler, void *handler_ctx) noexcept
+mpcdec_scan_stream(InputStream &is, TagHandler &handler) noexcept
 {
 	const auto duration = mpcdec_get_file_duration(is);
 	if (duration.IsNegative())
 		return false;
 
-	tag_handler_invoke_duration(handler, handler_ctx, SongTime(duration));
+	handler.OnDuration(SongTime(duration));
 	return true;
 }
 

src/decoder/plugins/Mpg123DecoderPlugin.cxx

@@ -279,8 +279,7 @@ mpd_mpg123_file_decode(DecoderClient &client, Path path_fs)
 }
 
 static bool
-mpd_mpg123_scan_file(Path path_fs,
-		     const TagHandler &handler, void *handler_ctx) noexcept
+mpd_mpg123_scan_file(Path path_fs, TagHandler &handler) noexcept
 {
 	int error;
 	mpg123_handle *const handle = mpg123_new(nullptr, &error);
@@ -316,7 +315,7 @@ mpd_mpg123_scan_file(Path path_fs,
 		SongTime::FromScale<uint64_t>(num_samples,
 					      audio_format.sample_rate);
 
-	tag_handler_invoke_duration(handler, handler_ctx, duration);
+	handler.OnDuration(duration);
 	return true;
 }
 

src/decoder/plugins/OpusDecoderPlugin.cxx

@@ -207,10 +207,9 @@ MPDOpusDecoder::HandleTags(const ogg_packet &packet)
 	rgi.Clear();
 
 	TagBuilder tag_builder;
+	AddTagHandler h(tag_builder);
 
-	if (ScanOpusTags(packet.packet, packet.bytes,
-			 &rgi,
-			 add_tag_handler, &tag_builder) &&
+	if (ScanOpusTags(packet.packet, packet.bytes, &rgi, h) &&
 	    !tag_builder.empty()) {
 		client.SubmitReplayGain(&rgi);
 
@@ -314,7 +313,7 @@ ReadAndParseOpusHead(OggSyncState &sync, OggStreamState &stream,
 
 static bool
 ReadAndVisitOpusTags(OggSyncState &sync, OggStreamState &stream,
-		     const TagHandler &handler, void *handler_ctx)
+		     TagHandler &handler)
 {
 	ogg_packet packet;
 
@@ -322,12 +321,12 @@ ReadAndVisitOpusTags(OggSyncState &sync, OggStreamState &stream,
 		IsOpusTags(packet) &&
 		ScanOpusTags(packet.packet, packet.bytes,
 			     nullptr,
-			     handler, handler_ctx);
+			     handler);
 }
 
 static void
 VisitOpusDuration(InputStream &is, OggSyncState &sync, OggStreamState &stream,
-		  const TagHandler &handler, void *handler_ctx)
+		  TagHandler &handler)
 {
 	ogg_packet packet;
 
@@ -335,13 +334,12 @@ VisitOpusDuration(InputStream &is, OggSyncState &sync, OggStreamState &stream,
 		const auto duration =
 			SongTime::FromScale<uint64_t>(packet.granulepos,
 						      opus_sample_rate);
-		tag_handler_invoke_duration(handler, handler_ctx, duration);
+		handler.OnDuration(duration);
 	}
 }
 
 static bool
-mpd_opus_scan_stream(InputStream &is,
-		     const TagHandler &handler, void *handler_ctx) noexcept
+mpd_opus_scan_stream(InputStream &is, TagHandler &handler) noexcept
 {
 	InputStreamReader reader(is);
 	OggSyncState oy(reader);
@@ -354,10 +352,10 @@ mpd_opus_scan_stream(InputStream &is,
 
 	unsigned channels;
 	if (!ReadAndParseOpusHead(oy, os, channels) ||
-	    !ReadAndVisitOpusTags(oy, os, handler, handler_ctx))
+	    !ReadAndVisitOpusTags(oy, os, handler))
 		return false;
 
-	VisitOpusDuration(is, oy, os, handler, handler_ctx);
+	VisitOpusDuration(is, oy, os, handler);
 	return true;
 }
 

src/decoder/plugins/OpusTags.cxx

@@ -43,7 +43,7 @@ ParseOpusTagName(const char *name) noexcept
 static void
 ScanOneOpusTag(const char *name, const char *value,
 	       ReplayGainInfo *rgi,
-	       const TagHandler &handler, void *ctx)
+	       TagHandler &handler) noexcept
 {
 	if (rgi != nullptr && strcmp(name, "R128_TRACK_GAIN") == 0) {
 		/* R128_TRACK_GAIN is a Q7.8 fixed point number in
@@ -63,25 +63,25 @@ ScanOneOpusTag(const char *name, const char *value,
 			rgi->album.gain = double(l) / 256.;
 	}
 
-	tag_handler_invoke_pair(handler, ctx, name, value);
+	handler.OnPair(name, value);
 
-	if (handler.tag != nullptr) {
+	if (handler.WantTag()) {
 		TagType t = ParseOpusTagName(name);
 		if (t != TAG_NUM_OF_ITEM_TYPES)
-			tag_handler_invoke_tag(handler, ctx, t, value);
+			handler.OnTag(t, value);
 	}
 }
 
 bool
 ScanOpusTags(const void *data, size_t size,
 	     ReplayGainInfo *rgi,
-	     const TagHandler &handler, void *ctx)
+	     TagHandler &handler) noexcept
 {
 	OpusReader r(data, size);
 	if (!r.Expect("OpusTags", 8))
 		return false;
 
-	if (handler.pair == nullptr && handler.tag == nullptr)
+	if (!handler.WantPair() && !handler.WantTag())
 		return true;
 
 	if (!r.SkipString())
@@ -100,7 +100,7 @@ ScanOpusTags(const void *data, size_t size,
 		if (eq != nullptr && eq > p) {
 			*eq = 0;
 
-			ScanOneOpusTag(p, eq + 1, rgi, handler, ctx);
+			ScanOneOpusTag(p, eq + 1, rgi, handler);
 		}
 
 		delete[] p;

src/decoder/plugins/OpusTags.hxx

@@ -25,11 +25,11 @@
 #include <stddef.h>
 
 struct ReplayGainInfo;
-struct TagHandler;
+class TagHandler;
 
 bool
 ScanOpusTags(const void *data, size_t size,
 	     ReplayGainInfo *rgi,
-	     const TagHandler &handler, void *ctx);
+	     TagHandler &handler) noexcept;
 
 #endif

src/decoder/plugins/SidplayDecoderPlugin.cxx

@@ -405,7 +405,7 @@ GetInfoString(const SidTuneInfo &info, unsigned i) noexcept
 
 static void
 ScanSidTuneInfo(const SidTuneInfo &info, unsigned track, unsigned n_tracks,
-		const TagHandler &handler, void *handler_ctx)
+		TagHandler &handler) noexcept
 {
 	/* title */
 	const char *title = GetInfoString(info, 0);
@@ -416,31 +416,26 @@ ScanSidTuneInfo(const SidTuneInfo &info, unsigned track, unsigned n_tracks,
 		const auto tag_title =
 			StringFormat<1024>("%s (%u/%u)",
 					   title, track, n_tracks);
-		tag_handler_invoke_tag(handler, handler_ctx,
-				       TAG_TITLE, tag_title);
+		handler.OnTag(TAG_TITLE, tag_title);
 	} else
-		tag_handler_invoke_tag(handler, handler_ctx, TAG_TITLE, title);
+		handler.OnTag(TAG_TITLE, title);
 
 	/* artist */
 	const char *artist = GetInfoString(info, 1);
 	if (artist != nullptr)
-		tag_handler_invoke_tag(handler, handler_ctx, TAG_ARTIST,
-				       artist);
+		handler.OnTag(TAG_ARTIST, artist);
 
 	/* date */
 	const char *date = GetInfoString(info, 2);
 	if (date != nullptr)
-		tag_handler_invoke_tag(handler, handler_ctx, TAG_DATE,
-				       date);
+		handler.OnTag(TAG_DATE, date);
 
 	/* track */
-	tag_handler_invoke_tag(handler, handler_ctx, TAG_TRACK,
-			       StringFormat<16>("%u", track));
+	handler.OnTag(TAG_TRACK, StringFormat<16>("%u", track));
 }
 
 static bool
-sidplay_scan_file(Path path_fs,
-		  const TagHandler &handler, void *handler_ctx) noexcept
+sidplay_scan_file(Path path_fs, TagHandler &handler) noexcept
 {
 	const auto container = ParseContainerPath(path_fs);
 	const unsigned song_num = container.track;
@@ -463,13 +458,12 @@ sidplay_scan_file(Path path_fs,
 	const unsigned n_tracks = info.songs;
 #endif
 
-	ScanSidTuneInfo(info, song_num, n_tracks, handler, handler_ctx);
+	ScanSidTuneInfo(info, song_num, n_tracks, handler);
 
 	/* time */
 	const auto duration = get_song_length(tune);
 	if (!duration.IsNegative())
-		tag_handler_invoke_duration(handler, handler_ctx,
-					    SongTime(duration));
+		handler.OnDuration(SongTime(duration));
 
 	return true;
 }
@@ -506,8 +500,8 @@ sidplay_container_scan(Path path_fs)
 	for (unsigned i = 1; i <= n_tracks; ++i) {
 		tune.selectSong(i);
 
-		ScanSidTuneInfo(info, i, n_tracks,
-				add_tag_handler, &tag_builder);
+		AddTagHandler h(tag_builder);
+		ScanSidTuneInfo(info, i, n_tracks, h);
 
 		char track_name[32];
 		/* Construct container/tune path names, eg.

src/decoder/plugins/SndfileDecoderPlugin.cxx

@@ -240,11 +240,11 @@ sndfile_stream_decode(DecoderClient &client, InputStream &is)
 
 static void
 sndfile_handle_tag(SNDFILE *sf, int str, TagType tag,
-		   const TagHandler &handler, void *handler_ctx)
+		   TagHandler &handler) noexcept
 {
 	const char *value = sf_get_string(sf, str);
 	if (value != nullptr)
-		tag_handler_invoke_tag(handler, handler_ctx, tag, value);
+		handler.OnTag(tag, value);
 }
 
 static constexpr struct {
@@ -261,8 +261,7 @@ static constexpr struct {
 };
 
 static bool
-sndfile_scan_stream(InputStream &is,
-		    const TagHandler &handler, void *handler_ctx) noexcept
+sndfile_scan_stream(InputStream &is, TagHandler &handler) noexcept
 {
 	SF_INFO info;
 
@@ -280,11 +279,10 @@ sndfile_scan_stream(InputStream &is,
 		return false;
 	}
 
-	tag_handler_invoke_duration(handler, handler_ctx,
-				    sndfile_duration(info));
+	handler.OnDuration(sndfile_duration(info));
 
 	for (auto i : sndfile_tags)
-		sndfile_handle_tag(sf, i.str, i.tag, handler, handler_ctx);
+		sndfile_handle_tag(sf, i.str, i.tag, handler);
 
 	sf_close(sf);
 

src/decoder/plugins/VorbisDecoderPlugin.cxx

@@ -348,7 +348,7 @@ static void
 VisitVorbisDuration(InputStream &is,
 		    OggSyncState &sync, OggStreamState &stream,
 		    unsigned sample_rate,
-		    const TagHandler &handler, void *handler_ctx)
+		    TagHandler &handler) noexcept
 {
 	ogg_packet packet;
 
@@ -358,12 +358,11 @@ VisitVorbisDuration(InputStream &is,
 	const auto duration =
 		SongTime::FromScale<uint64_t>(packet.granulepos,
 					      sample_rate);
-	tag_handler_invoke_duration(handler, handler_ctx, duration);
+	handler.OnDuration(duration);
 }
 
 static bool
-vorbis_scan_stream(InputStream &is,
-		   const TagHandler &handler, void *handler_ctx) noexcept
+vorbis_scan_stream(InputStream &is, TagHandler &handler) noexcept
 {
 	/* initialize libogg */
 
@@ -397,12 +396,11 @@ vorbis_scan_stream(InputStream &is,
 
 	/* visit the Vorbis comments we just read */
 
-	vorbis_comments_scan(vc.user_comments,
-			     handler, handler_ctx);
+	vorbis_comments_scan(vc.user_comments, handler);
 
 	/* check the song duration by locating the e_o_s packet */
 
-	VisitVorbisDuration(is, sync, stream, vi.rate, handler, handler_ctx);
+	VisitVorbisDuration(is, sync, stream, vi.rate, handler);
 
 	return true;
 }

src/decoder/plugins/WavpackDecoderPlugin.cxx

@@ -578,8 +578,7 @@ wavpack_filedecode(DecoderClient &client, Path path_fs)
  * Reads metainfo from the specified file.
  */
 static bool
-wavpack_scan_file(Path path_fs,
-		  const TagHandler &handler, void *handler_ctx) noexcept
+wavpack_scan_file(Path path_fs, TagHandler &handler) noexcept
 {
 	WavpackContext *wpc;
 
@@ -595,14 +594,13 @@ wavpack_scan_file(Path path_fs,
 
 	const auto duration = GetDuration(wpc);
 	if (!duration.IsNegative())
-		tag_handler_invoke_duration(handler, handler_ctx, SongTime(duration));
+		handler.OnDuration(SongTime(duration));
 
 	return true;
 }
 
 static bool
-wavpack_scan_stream(InputStream &is,
-		    const TagHandler &handler, void *handler_ctx) noexcept
+wavpack_scan_stream(InputStream &is, TagHandler &handler) noexcept
 {
 	WavpackInput isp(nullptr, is);
 
@@ -620,7 +618,7 @@ wavpack_scan_stream(InputStream &is,
 
 	const auto duration = GetDuration(wpc);
 	if (!duration.IsNegative())
-		tag_handler_invoke_duration(handler, handler_ctx, SongTime(duration));
+		handler.OnDuration(SongTime(duration));
 
 	return true;
 }

src/decoder/plugins/WildmidiDecoderPlugin.cxx

@@ -126,8 +126,7 @@ wildmidi_file_decode(DecoderClient &client, Path path_fs)
 }
 
 static bool
-wildmidi_scan_file(Path path_fs,
-		   const TagHandler &handler, void *handler_ctx) noexcept
+wildmidi_scan_file(Path path_fs, TagHandler &handler) noexcept
 {
 	midi *wm = WildMidi_Open(path_fs.c_str());
 	if (wm == nullptr)
@@ -142,7 +141,7 @@ wildmidi_scan_file(Path path_fs,
 	const auto duration =
 		SongTime::FromScale<uint64_t>(info->approx_total_samples,
 					      WILDMIDI_SAMPLE_RATE);
-	tag_handler_invoke_duration(handler, handler_ctx, duration);
+	handler.OnDuration(duration);
 
 	WildMidi_Close(wm);
 

src/lib/xiph/VorbisComments.cxx

@@ -53,13 +53,13 @@ vorbis_comments_to_replay_gain(ReplayGainInfo &rgi, char **comments) noexcept
 static bool
 vorbis_copy_comment(const char *comment,
 		    const char *name, TagType tag_type,
-		    const TagHandler &handler, void *handler_ctx)
+		    TagHandler &handler) noexcept
 {
 	const char *value;
 
 	value = vorbis_comment_value(comment, name);
 	if (value != nullptr) {
-		tag_handler_invoke_tag(handler, handler_ctx, tag_type, value);
+		handler.OnTag(tag_type, value);
 		return true;
 	}
 
@@ -67,36 +67,31 @@ vorbis_copy_comment(const char *comment,
 }
 
 static void
-vorbis_scan_comment(const char *comment,
-		    const TagHandler &handler, void *handler_ctx)
+vorbis_scan_comment(const char *comment, TagHandler &handler) noexcept
 {
-	if (handler.pair != nullptr) {
+	if (handler.WantPair()) {
 		const DivideString split(comment, '=');
 		if (split.IsDefined() && !split.empty())
-			tag_handler_invoke_pair(handler, handler_ctx,
-						split.GetFirst(),
-						split.GetSecond());
+			handler.OnPair(split.GetFirst(), split.GetSecond());
 	}
 
 	for (const struct tag_table *i = xiph_tags; i->name != nullptr; ++i)
 		if (vorbis_copy_comment(comment, i->name, i->type,
-					handler, handler_ctx))
+					handler))
 			return;
 
 	for (unsigned i = 0; i < TAG_NUM_OF_ITEM_TYPES; ++i)
 		if (vorbis_copy_comment(comment,
 					tag_item_names[i], TagType(i),
-					handler, handler_ctx))
+					handler))
 			return;
 }
 
 void
-vorbis_comments_scan(char **comments,
-		     const TagHandler &handler, void *handler_ctx)
+vorbis_comments_scan(char **comments, TagHandler &handler) noexcept
 {
 	while (*comments)
-		vorbis_scan_comment(*comments++,
-				    handler, handler_ctx);
+		vorbis_scan_comment(*comments++, handler);
 
 }
 
@@ -104,7 +99,8 @@ std::unique_ptr<Tag>
 vorbis_comments_to_tag(char **comments) noexcept
 {
 	TagBuilder tag_builder;
-	vorbis_comments_scan(comments, add_tag_handler, &tag_builder);
+	AddTagHandler h(tag_builder);
+	vorbis_comments_scan(comments, h);
 	return tag_builder.empty()
 		? nullptr
 		: tag_builder.CommitNew();

src/lib/xiph/VorbisComments.hxx

@@ -25,15 +25,14 @@
 #include <memory>
 
 struct ReplayGainInfo;
-struct TagHandler;
+class TagHandler;
 struct Tag;
 
 bool
 vorbis_comments_to_replay_gain(ReplayGainInfo &rgi, char **comments) noexcept;
 
 void
-vorbis_comments_scan(char **comments,
-		     const TagHandler &handler, void *handler_ctx);
+vorbis_comments_scan(char **comments, TagHandler &handler) noexcept;
 
 std::unique_ptr<Tag>
 vorbis_comments_to_tag(char **comments) noexcept;

src/playlist/plugins/EmbeddedCuePlaylistPlugin.cxx

@@ -71,22 +71,23 @@ public:
 	virtual std::unique_ptr<DetachedSong> NextSong() override;
 };
 
-static void
-embcue_tag_pair(const char *name, const char *value, void *ctx)
-{
-	EmbeddedCuePlaylist *playlist = (EmbeddedCuePlaylist *)ctx;
+class ExtractCuesheetTagHandler final : public NullTagHandler {
+public:
+	std::string cuesheet;
 
-	if (playlist->cuesheet.empty() &&
-	    StringEqualsCaseASCII(name, "cuesheet"))
-		playlist->cuesheet = value;
-}
+	ExtractCuesheetTagHandler() noexcept:NullTagHandler(WANT_PAIR) {}
 
-static constexpr TagHandler embcue_tag_handler = {
-	nullptr,
-	nullptr,
-	embcue_tag_pair,
+	void OnPair(const char *key, const char *value) noexcept override;
 };
 
+void
+ExtractCuesheetTagHandler::OnPair(const char *name, const char *value) noexcept
+{
+	if (cuesheet.empty() &&
+	    StringEqualsCaseASCII(name, "cuesheet"))
+		cuesheet = value;
+}
+
 static std::unique_ptr<SongEnumerator>
 embcue_playlist_open_uri(const char *uri,
 			 gcc_unused Mutex &mutex)
@@ -97,18 +98,21 @@ embcue_playlist_open_uri(const char *uri,
 
 	const auto path_fs = AllocatedPath::FromUTF8Throw(uri);
 
-	auto playlist = std::make_unique<EmbeddedCuePlaylist>();
-
-	tag_file_scan(path_fs, embcue_tag_handler, playlist.get());
-	if (playlist->cuesheet.empty())
-		ScanGenericTags(path_fs, embcue_tag_handler, playlist.get());
+	ExtractCuesheetTagHandler extract_cuesheet;
+	tag_file_scan(path_fs, extract_cuesheet);
+	if (extract_cuesheet.cuesheet.empty())
+		ScanGenericTags(path_fs, extract_cuesheet);
 
-	if (playlist->cuesheet.empty())
+	if (extract_cuesheet.cuesheet.empty())
 		/* no "CUESHEET" tag found */
 		return nullptr;
 
+	auto playlist = std::make_unique<EmbeddedCuePlaylist>();
+
 	playlist->filename = PathTraitsUTF8::GetBase(uri);
 
+	playlist->cuesheet = std::move(extract_cuesheet.cuesheet);
+
 	playlist->next = &playlist->cuesheet[0];
 	playlist->parser = new CueParser();
 

src/tag/ApeTag.cxx

@@ -76,7 +76,7 @@ ForEachValue(const char *value, const char *end, C &&callback)
 static bool
 tag_ape_import_item(unsigned long flags,
 		    const char *key, StringView value,
-		    const TagHandler &handler, void *handler_ctx)
+		    TagHandler &handler) noexcept
 {
 	/* we only care about utf-8 text tags */
 	if ((flags & (0x3 << 1)) != 0)
@@ -85,36 +85,31 @@ tag_ape_import_item(unsigned long flags,
 	const auto begin = value.begin();
 	const auto end = value.end();
 
-	if (handler.pair != nullptr)
-		ForEachValue(begin, end, [handler, handler_ctx,
-					  key](const char *_value) {
-				handler.pair(key, _value, handler_ctx);
+	if (handler.WantPair())
+		ForEachValue(begin, end, [&handler, key](const char *_value) {
+				handler.OnPair(key, _value);
 			});
 
 	TagType type = tag_ape_name_parse(key);
 	if (type == TAG_NUM_OF_ITEM_TYPES)
 		return false;
 
-	ForEachValue(begin, end, [handler, handler_ctx,
-				  type](const char *_value) {
-			tag_handler_invoke_tag(handler, handler_ctx,
-					       type, _value);
+	ForEachValue(begin, end, [&handler, type](const char *_value) {
+			handler.OnTag(type, _value);
 		});
 
 	return true;
 }
 
 bool
-tag_ape_scan2(InputStream &is,
-	      const TagHandler &handler, void *handler_ctx)
+tag_ape_scan2(InputStream &is, TagHandler &handler) noexcept
 {
 	bool recognized = false;
 
-	auto callback = [handler, handler_ctx, &recognized]
+	auto callback = [&handler, &recognized]
 		(unsigned long flags, const char *key,
 		 StringView value) {
-		recognized |= tag_ape_import_item(flags, key, value,
-						  handler, handler_ctx);
+		recognized |= tag_ape_import_item(flags, key, value, handler);
 		return true;
 	};
 

src/tag/ApeTag.hxx

@@ -20,10 +20,8 @@
 #ifndef MPD_APE_TAG_HXX
 #define MPD_APE_TAG_HXX
 
-#include "Table.hxx"
-
 class InputStream;
-struct TagHandler;
+class TagHandler;
 
 /**
  * Scan the APE tags of a stream.
@@ -31,7 +29,6 @@ struct TagHandler;
  * @param path_fs the path of the file in filesystem encoding
  */
 bool
-tag_ape_scan2(InputStream &is,
-	      const TagHandler &handler, void *handler_ctx);
+tag_ape_scan2(InputStream &is, TagHandler &handler) noexcept;
 
 #endif

src/tag/Generic.cxx

@@ -30,9 +30,9 @@
 #include <exception>
 
 bool
-ScanGenericTags(InputStream &is, const TagHandler &handler, void *ctx)
+ScanGenericTags(InputStream &is, TagHandler &handler) noexcept
 {
-	if (tag_ape_scan2(is, handler, ctx))
+	if (tag_ape_scan2(is, handler))
 		return true;
 
 #ifdef ENABLE_ID3TAG
@@ -42,19 +42,19 @@ ScanGenericTags(InputStream &is, const TagHandler &handler, void *ctx)
 		return false;
 	}
 
-	return tag_id3_scan(is, handler, ctx);
+	return tag_id3_scan(is, handler);
 #else
 	return false;
 #endif
 }
 
 bool
-ScanGenericTags(Path path, const TagHandler &handler, void *ctx)
+ScanGenericTags(Path path, TagHandler &handler) noexcept
 try {
 	Mutex mutex;
 
 	auto is = OpenLocalInputStream(path, mutex);
-	return ScanGenericTags(*is, handler, ctx);
+	return ScanGenericTags(*is, handler);
 } catch (...) {
 	LogError(std::current_exception());
 	return false;

src/tag/Generic.hxx

@@ -22,7 +22,7 @@
 
 #include "check.h"
 
-struct TagHandler;
+class TagHandler;
 class InputStream;
 class Path;
 
@@ -31,12 +31,12 @@ class Path;
  * stream does not need to be rewound.
  */
 bool
-ScanGenericTags(InputStream &is, const TagHandler &handler, void *ctx);
+ScanGenericTags(InputStream &is, TagHandler &handler) noexcept;
 
 /**
  * Attempts to scan APE or ID3 tags from the specified file.
  */
 bool
-ScanGenericTags(Path path, const TagHandler &handler, void *ctx);
+ScanGenericTags(Path path, TagHandler &handler) noexcept;
 
 #endif

src/tag/Handler.cxx

@@ -25,19 +25,15 @@
 
 #include <stdlib.h>
 
-static void
-add_tag_duration(SongTime duration, void *ctx)
+void
+AddTagHandler::OnDuration(SongTime duration) noexcept
 {
-	TagBuilder &tag = *(TagBuilder *)ctx;
-
 	tag.SetDuration(duration);
 }
 
-static void
-add_tag_tag(TagType type, const char *value, void *ctx)
+void
+AddTagHandler::OnTag(TagType type, const char *value) noexcept
 {
-	TagBuilder &tag = *(TagBuilder *)ctx;
-
 	if (type == TAG_TRACK || type == TAG_DISC) {
 		/* filter out this extra data and leading zeroes */
 		char *end;
@@ -48,24 +44,10 @@ add_tag_tag(TagType type, const char *value, void *ctx)
 		tag.AddItem(type, value);
 }
 
-const TagHandler add_tag_handler = {
-	add_tag_duration,
-	add_tag_tag,
-	nullptr,
-};
-
-static void
-full_tag_pair(const char *name, gcc_unused const char *value, void *ctx)
+void
+FullTagHandler::OnPair(const char *name, gcc_unused const char *value) noexcept
 {
-	TagBuilder &tag = *(TagBuilder *)ctx;
-
 	if (StringEqualsCaseASCII(name, "cuesheet"))
 		tag.SetHasPlaylist(true);
 }
 
-const TagHandler full_tag_handler = {
-	add_tag_duration,
-	add_tag_tag,
-	full_tag_pair,
-};
-

src/tag/Handler.hxx

@@ -23,19 +23,45 @@
 #include "check.h"
 #include "Type.h"
 #include "Chrono.hxx"
+#include "Compiler.h"
 
-#include <assert.h>
+class TagBuilder;
 
 /**
- * A callback table for receiving metadata of a song.
+ * An interface for receiving metadata of a song.
  */
-struct TagHandler {
+class TagHandler {
+	const unsigned want_mask;
+
+public:
+	static constexpr unsigned WANT_DURATION = 0x1;
+	static constexpr unsigned WANT_TAG = 0x2;
+	static constexpr unsigned WANT_PAIR = 0x4;
+
+	explicit TagHandler(unsigned _want_mask) noexcept
+		:want_mask(_want_mask) {}
+
+	TagHandler(const TagHandler &) = delete;
+	TagHandler &operator=(const TagHandler &) = delete;
+
+	bool WantDuration() const noexcept {
+		return want_mask & WANT_DURATION;
+	}
+
+	bool WantTag() const noexcept {
+		return want_mask & WANT_TAG;
+	}
+
+	bool WantPair() const noexcept {
+		return want_mask & WANT_PAIR;
+	}
+
 	/**
 	 * Declare the duration of a song.  Do not call
 	 * this when the duration could not be determined, because
 	 * there is no magic value for "unknown duration".
 	 */
-	void (*duration)(SongTime duration, void *ctx);
+	virtual void OnDuration(SongTime duration) noexcept = 0;
 
 	/**
 	 * A tag has been read.
@@ -43,56 +69,57 @@ struct TagHandler {
 	 * @param the value of the tag; the pointer will become
 	 * invalid after returning
 	 */
-	void (*tag)(TagType type, const char *value, void *ctx);
+	virtual void OnTag(TagType type, const char *value) noexcept = 0;
 
 	/**
 	 * A name-value pair has been read.  It is the codec specific
 	 * representation of tags.
 	 */
-	void (*pair)(const char *key, const char *value, void *ctx);
+	virtual void OnPair(const char *key, const char *value) noexcept = 0;
 };
 
-static inline void
-tag_handler_invoke_duration(const TagHandler &handler, void *ctx,
-			    SongTime duration) noexcept
-{
-	if (handler.duration != nullptr)
-		handler.duration(duration, ctx);
-}
-
-static inline void
-tag_handler_invoke_tag(const TagHandler &handler, void *ctx,
-		       TagType type, const char *value) noexcept
-{
-	assert((unsigned)type < TAG_NUM_OF_ITEM_TYPES);
-	assert(value != nullptr);
-
-	if (handler.tag != nullptr)
-		handler.tag(type, value, ctx);
-}
-
-static inline void
-tag_handler_invoke_pair(const TagHandler &handler, void *ctx,
-			const char *name, const char *value) noexcept
-{
-	assert(name != nullptr);
-	assert(value != nullptr);
-
-	if (handler.pair != nullptr)
-		handler.pair(name, value, ctx);
-}
+class NullTagHandler : public TagHandler {
+public:
+	explicit NullTagHandler(unsigned _want_mask) noexcept
+		:TagHandler(_want_mask) {}
+
+	void OnDuration(gcc_unused SongTime duration) noexcept override {}
+	void OnTag(gcc_unused TagType type,
+		   gcc_unused const char *value) noexcept override {}
+	void OnPair(gcc_unused const char *key,
+		    gcc_unused const char *value) noexcept override {}
+};
 
 /**
- * This #TagHandler implementation adds tag values to a #TagBuilder object
- * (casted from the context pointer).
+ * This #TagHandler implementation adds tag values to a #TagBuilder
+ * object.
  */
-extern const TagHandler add_tag_handler;
+class AddTagHandler : public NullTagHandler {
+protected:
+	TagBuilder &tag;
+
+	AddTagHandler(unsigned _want_mask, TagBuilder &_builder) noexcept
+		:NullTagHandler(_want_mask), tag(_builder) {}
+
+public:
+	explicit AddTagHandler(TagBuilder &_builder) noexcept
+		:AddTagHandler(WANT_DURATION|WANT_TAG, _builder) {}
+
+	void OnDuration(SongTime duration) noexcept override;
+	void OnTag(TagType type, const char *value) noexcept override;
+};
 
 /**
  * This #TagHandler implementation adds tag values to a #TagBuilder object
  * (casted from the context pointer), and supports the has_playlist
  * attribute.
  */
-extern const TagHandler full_tag_handler;
+class FullTagHandler : public AddTagHandler {
+public:
+	explicit FullTagHandler(TagBuilder &_builder) noexcept
+		:AddTagHandler(WANT_DURATION|WANT_TAG|WANT_PAIR, _builder) {}
+
+	void OnPair(const char *key, const char *value) noexcept override;
+};
 
 #endif

src/tag/Id3Scan.cxx

@@ -100,7 +100,7 @@ import_id3_string(const id3_ucs4_t *ucs4)
 static void
 tag_id3_import_text_frame(const struct id3_frame *frame,
 			  TagType type,
-			  const TagHandler &handler, void *handler_ctx)
+			  TagHandler &handler) noexcept
 {
 	if (frame->nfields != 2)
 		return;
@@ -133,8 +133,7 @@ tag_id3_import_text_frame(const struct id3_frame *frame,
 
 		AtScopeExit(utf8) { free(utf8); };
 
-		tag_handler_invoke_tag(handler, handler_ctx,
-				       type, (const char *)utf8);
+		handler.OnTag(type, (const char *)utf8);
 	}
 }
 
@@ -144,13 +143,13 @@ tag_id3_import_text_frame(const struct id3_frame *frame,
  */
 static void
 tag_id3_import_text(struct id3_tag *tag, const char *id, TagType type,
-		    const TagHandler &handler, void *handler_ctx)
+		    TagHandler &handler) noexcept
 {
 	const struct id3_frame *frame;
 	for (unsigned i = 0;
 	     (frame = id3_tag_findframe(tag, id, i)) != nullptr; ++i)
 		tag_id3_import_text_frame(frame, type,
-					  handler, handler_ctx);
+					  handler);
 }
 
 /**
@@ -164,8 +163,7 @@ tag_id3_import_text(struct id3_tag *tag, const char *id, TagType type,
  */
 static void
 tag_id3_import_comment_frame(const struct id3_frame *frame, TagType type,
-			     const TagHandler &handler,
-			     void *handler_ctx)
+			     TagHandler &handler) noexcept
 {
 	if (frame->nfields != 4)
 		return;
@@ -185,7 +183,7 @@ tag_id3_import_comment_frame(const struct id3_frame *frame, TagType type,
 
 	AtScopeExit(utf8) { free(utf8); };
 
-	tag_handler_invoke_tag(handler, handler_ctx, type, (const char *)utf8);
+	handler.OnTag(type, (const char *)utf8);
 }
 
 /**
@@ -194,13 +192,13 @@ tag_id3_import_comment_frame(const struct id3_frame *frame, TagType type,
  */
 static void
 tag_id3_import_comment(struct id3_tag *tag, const char *id, TagType type,
-		       const TagHandler &handler, void *handler_ctx)
+		       TagHandler &handler) noexcept
 {
 	const struct id3_frame *frame;
 	for (unsigned i = 0;
 	     (frame = id3_tag_findframe(tag, id, i)) != nullptr; ++i)
 		tag_id3_import_comment_frame(frame, type,
-					     handler, handler_ctx);
+					     handler);
 }
 
 /**
@@ -220,8 +218,7 @@ tag_id3_parse_txxx_name(const char *name) noexcept
  */
 static void
 tag_id3_import_musicbrainz(struct id3_tag *id3_tag,
-			   const TagHandler &handler,
-			   void *handler_ctx)
+			   TagHandler &handler) noexcept
 {
 	for (unsigned i = 0;; ++i) {
 		const id3_frame *frame = id3_tag_findframe(id3_tag, "TXXX", i);
@@ -240,15 +237,12 @@ tag_id3_import_musicbrainz(struct id3_tag *id3_tag,
 
 		AtScopeExit(value) { free(value); };
 
-		tag_handler_invoke_pair(handler, handler_ctx,
-					(const char *)name,
-					(const char *)value);
+		handler.OnPair((const char *)name, (const char *)value);
 
 		TagType type = tag_id3_parse_txxx_name((const char*)name);
 
 		if (type != TAG_NUM_OF_ITEM_TYPES)
-			tag_handler_invoke_tag(handler, handler_ctx,
-					       type, (const char*)value);
+			handler.OnTag(type, (const char*)value);
 	}
 }
 
@@ -257,7 +251,7 @@ tag_id3_import_musicbrainz(struct id3_tag *id3_tag,
  */
 static void
 tag_id3_import_ufid(struct id3_tag *id3_tag,
-		    const TagHandler &handler, void *handler_ctx)
+		    TagHandler &handler) noexcept
 {
 	for (unsigned i = 0;; ++i) {
 		const id3_frame *frame = id3_tag_findframe(id3_tag, "UFID", i);
@@ -284,65 +278,63 @@ tag_id3_import_ufid(struct id3_tag *id3_tag,
 			continue;
 
 		std::string p((const char *)value, length);
-		tag_handler_invoke_tag(handler, handler_ctx,
-				       TAG_MUSICBRAINZ_TRACKID, p.c_str());
+		handler.OnTag(TAG_MUSICBRAINZ_TRACKID, p.c_str());
 	}
 }
 
 void
-scan_id3_tag(struct id3_tag *tag,
-	     const TagHandler &handler, void *handler_ctx)
+scan_id3_tag(struct id3_tag *tag, TagHandler &handler) noexcept
 {
 	tag_id3_import_text(tag, ID3_FRAME_ARTIST, TAG_ARTIST,
-			    handler, handler_ctx);
+			    handler);
 	tag_id3_import_text(tag, ID3_FRAME_ALBUM_ARTIST,
-			    TAG_ALBUM_ARTIST, handler, handler_ctx);
+			    TAG_ALBUM_ARTIST, handler);
 	tag_id3_import_text(tag, ID3_FRAME_ARTIST_SORT,
-			    TAG_ARTIST_SORT, handler, handler_ctx);
+			    TAG_ARTIST_SORT, handler);
 
-	tag_id3_import_text(tag, "TSOA", TAG_ALBUM_SORT, handler, handler_ctx);
+	tag_id3_import_text(tag, "TSOA", TAG_ALBUM_SORT, handler);
 
 	tag_id3_import_text(tag, ID3_FRAME_ALBUM_ARTIST_SORT,
-			    TAG_ALBUM_ARTIST_SORT, handler, handler_ctx);
+			    TAG_ALBUM_ARTIST_SORT, handler);
 	tag_id3_import_text(tag, ID3_FRAME_TITLE, TAG_TITLE,
-			    handler, handler_ctx);
+			    handler);
 	tag_id3_import_text(tag, ID3_FRAME_ALBUM, TAG_ALBUM,
-			    handler, handler_ctx);
+			    handler);
 	tag_id3_import_text(tag, ID3_FRAME_TRACK, TAG_TRACK,
-			    handler, handler_ctx);
+			    handler);
 	tag_id3_import_text(tag, ID3_FRAME_YEAR, TAG_DATE,
-			    handler, handler_ctx);
+			    handler);
 	tag_id3_import_text(tag, ID3_FRAME_ORIGINAL_RELEASE_DATE, TAG_ORIGINAL_DATE,
-			    handler, handler_ctx);
+			    handler);
 	tag_id3_import_text(tag, ID3_FRAME_GENRE, TAG_GENRE,
-			    handler, handler_ctx);
+			    handler);
 	tag_id3_import_text(tag, ID3_FRAME_COMPOSER, TAG_COMPOSER,
-			    handler, handler_ctx);
+			    handler);
 	tag_id3_import_text(tag, "TPE3", TAG_PERFORMER,
-			    handler, handler_ctx);
-	tag_id3_import_text(tag, "TPE4", TAG_PERFORMER, handler, handler_ctx);
+			    handler);
+	tag_id3_import_text(tag, "TPE4", TAG_PERFORMER, handler);
 	tag_id3_import_comment(tag, ID3_FRAME_COMMENT, TAG_COMMENT,
-			       handler, handler_ctx);
+			       handler);
 	tag_id3_import_text(tag, ID3_FRAME_DISC, TAG_DISC,
-			    handler, handler_ctx);
+			    handler);
 
-	tag_id3_import_musicbrainz(tag, handler, handler_ctx);
-	tag_id3_import_ufid(tag, handler, handler_ctx);
+	tag_id3_import_musicbrainz(tag, handler);
+	tag_id3_import_ufid(tag, handler);
 }
 
 std::unique_ptr<Tag>
 tag_id3_import(struct id3_tag *tag)
 {
 	TagBuilder tag_builder;
-	scan_id3_tag(tag, add_tag_handler, &tag_builder);
+	AddTagHandler h(tag_builder);
+	scan_id3_tag(tag, h);
 	return tag_builder.empty()
 		? nullptr
 		: tag_builder.CommitNew();
 }
 
 bool
-tag_id3_scan(InputStream &is,
-	     const TagHandler &handler, void *handler_ctx)
+tag_id3_scan(InputStream &is, TagHandler &handler) noexcept
 {
 	UniqueId3Tag tag;
 
@@ -355,6 +347,6 @@ tag_id3_scan(InputStream &is,
 		return false;
 	}
 
-	scan_id3_tag(tag.get(), handler, handler_ctx);
+	scan_id3_tag(tag.get(), handler);
 	return true;
 }

src/tag/Id3Scan.hxx

@@ -25,13 +25,12 @@
 #include <memory>
 
 class InputStream;
-struct TagHandler;
+class TagHandler;
 struct Tag;
 struct id3_tag;
 
 bool
-tag_id3_scan(InputStream &is,
-	     const TagHandler &handler, void *handler_ctx);
+tag_id3_scan(InputStream &is, TagHandler &handler) noexcept;
 
 std::unique_ptr<Tag>
 tag_id3_import(id3_tag *);
@@ -41,7 +40,6 @@ tag_id3_import(id3_tag *);
  *
  */
 void
-scan_id3_tag(id3_tag *tag,
-	     const TagHandler &handler, void *handler_ctx);
+scan_id3_tag(id3_tag *tag, TagHandler &handler) noexcept;
 
 #endif

test/read_tags.cxx

@@ -40,31 +40,29 @@
 #include <locale.h>
 #endif
 
-static bool empty = true;
+class DumpTagHandler final : public NullTagHandler {
+	bool empty = true;
 
-static void
-print_duration(SongTime duration, gcc_unused void *ctx)
-{
-	printf("duration=%f\n", duration.ToDoubleS());
-}
+public:
+	DumpTagHandler() noexcept
+		:NullTagHandler(WANT_DURATION|WANT_TAG|WANT_PAIR) {}
 
-static void
-print_tag(TagType type, const char *value, gcc_unused void *ctx)
-{
-	printf("[%s]=%s\n", tag_item_names[type], value);
-	empty = false;
-}
+	bool IsEmpty() const noexcept {
+		return empty;
+	}
 
-static void
-print_pair(const char *name, const char *value, gcc_unused void *ctx)
-{
-	printf("\"%s\"=%s\n", name, value);
-}
+	void OnDuration(SongTime duration) noexcept override {
+		printf("duration=%f\n", duration.ToDoubleS());
+	}
+
+	void OnTag(TagType type, const char *value) noexcept override {
+		printf("[%s]=%s\n", tag_item_names[type], value);
+		empty = false;
+	}
 
-static constexpr TagHandler print_handler = {
-	print_duration,
-	print_tag,
-	print_pair,
+	void OnPair(const char *key, const char *value) noexcept override {
+		printf("\"%s\"=%s\n", key, value);
+	}
 };
 
 int main(int argc, char **argv)
@@ -100,9 +98,10 @@ try {
 		return EXIT_FAILURE;
 	}
 
+	DumpTagHandler h;
 	bool success;
 	try {
-		success = plugin->ScanFile(path, print_handler, nullptr);
+		success = plugin->ScanFile(path, h);
 	} catch (const std::exception &e) {
 		LogError(e);
 		success = false;
@@ -113,7 +112,7 @@ try {
 
 	if (!success && plugin->scan_stream != NULL) {
 		is = InputStream::OpenReady(path.c_str(), mutex);
-		success = plugin->ScanStream(*is, print_handler, nullptr);
+		success = plugin->ScanStream(*is, h);
 	}
 
 	if (!success) {
@@ -121,11 +120,11 @@ try {
 		return EXIT_FAILURE;
 	}
 
-	if (empty) {
+	if (h.IsEmpty()) {
 		if (is)
-			ScanGenericTags(*is, print_handler, nullptr);
+			ScanGenericTags(*is, h);
 		else
-			ScanGenericTags(path, print_handler, nullptr);
+			ScanGenericTags(path, h);
 	}
 
 	return 0;