output/Filtered: add method NeedFullyDefinedAudioFormat()

src/output/Filtered.hxx

@@ -125,6 +125,13 @@ public:
 
 	~FilteredAudioOutput();
 
+	/**
+	 * Plugins shall call this method if they require an
+	 * "audio_format" setting which evaluates
+	 * AudioFormat::IsFullyDefined().
+	 */
+	void NeedFullyDefinedAudioFormat();
+
 private:
 	void Configure(const ConfigBlock &block);
 

src/output/Init.cxx

@@ -62,6 +62,13 @@ FilteredAudioOutput::FilteredAudioOutput(const AudioOutputPlugin &_plugin,
 	Configure(block);
 }
 
+void
+FilteredAudioOutput::NeedFullyDefinedAudioFormat()
+{
+	if (!config_audio_format.IsFullyDefined())
+		throw std::runtime_error("Need full audio format specification");
+}
+
 static const AudioOutputPlugin *
 audio_output_detect()
 {

src/output/plugins/ShoutOutputPlugin.cxx

@@ -117,9 +117,7 @@ ShoutOutput::ShoutOutput(const ConfigBlock &block)
 	 shout_conn(shout_new()),
 	 shout_meta(shout_metadata_new())
 {
-	const AudioFormat audio_format = base.config_audio_format;
-	if (!audio_format.IsFullyDefined())
-		throw std::runtime_error("Need full audio format specification");
+	base.NeedFullyDefinedAudioFormat();
 
 	const char *host = require_block_string(block, "host");
 	const char *mount = require_block_string(block, "mount");