db/simple/Song: move struct Disposer to separate header

Allow forward-declaring it.

src/db/plugins/simple/Directory.cxx

@@ -20,6 +20,7 @@
 #include "Directory.hxx"
 #include "SongSort.hxx"
 #include "Song.hxx"
+#include "Disposer.hxx"
 #include "Mount.hxx"
 #include "db/LightDirectory.hxx"
 #include "song/LightSong.hxx"
@@ -50,7 +51,7 @@ Directory::~Directory() noexcept
 		mounted_database.reset();
 	}
 
-	songs.clear_and_dispose(Song::Disposer());
+	songs.clear_and_dispose(SongDisposer());
 	children.clear_and_dispose(DeleteDisposer());
 }
 

src/db/plugins/simple/Disposer.hxx

@@ -0,0 +1,29 @@
+/*
+ * Copyright 2003-2019 The Music Player Daemon Project
+ * http://www.musicpd.org
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License along
+ * with this program; if not, write to the Free Software Foundation, Inc.,
+ * 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
+ */
+
+#ifndef MPD_SONG_DISPOSER_HXX
+#define MPD_SONG_DISPOSER_HXX
+
+struct Song;
+
+struct SongDisposer {
+	void operator()(Song *song) const noexcept;
+};
+
+#endif

src/db/plugins/simple/Song.cxx

@@ -18,6 +18,7 @@
  */
 
 #include "Song.hxx"
+#include "Disposer.hxx"
 #include "Directory.hxx"
 #include "tag/Tag.hxx"
 #include "util/VarSize.hxx"
@@ -76,6 +77,12 @@ Song::Free() noexcept
 	DeleteVarSize(this);
 }
 
+void
+SongDisposer::operator()(Song *song) const noexcept
+{
+	song->Free();
+}
+
 std::string
 Song::GetURI() const noexcept
 {

src/db/plugins/simple/Song.hxx

@@ -45,12 +45,6 @@ struct Song {
 	typedef boost::intrusive::link_mode<link_mode> LinkMode;
 	typedef boost::intrusive::list_member_hook<LinkMode> Hook;
 
-	struct Disposer {
-		void operator()(Song *song) const noexcept {
-			song->Free();
-		}
-	};
-
 	/**
 	 * Pointers to the siblings of this directory within the
 	 * parent directory.  It is unused (undefined) if this song is