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Hi,

I suggest to change the Mix_ChannelFinished callback (or add a new appropriate one if API compatibility is a must). The patch below changes the callback to also carry a void* argument, so that C++ objects can be supplied, and thus C++ can utilize the callback.

--- SDL_mixer/SDL_mixer.h	2014-03-23 14:04:43.089323652 +0100
+++ SDL_mixer_new/SDL_mixer.h	2014-03-23 11:04:40.000000000 +0100
@@ -223,7 +223,7 @@
  *  inside the audio callback, or SDL_mixer will explicitly lock the audio
  *  before calling your callback.
  */
-extern DECLSPEC void SDLCALL Mix_ChannelFinished(void (*channel_finished)(int channel));
+extern DECLSPEC void SDLCALL Mix_ChannelFinished(void *userdata, void (*channel_finished)(void *userdata, int channel));
 
 
 /* Special Effects API by ryan c. gordon. (icculus@icculus.org) */

--- SDL_mixer/mixer.c	2014-03-23 14:04:46.421417431 +0100
+++ SDL_mixer_new/mixer.c	2014-03-23 11:17:49.000000000 +0100
@@ -93,7 +94,8 @@
 static void *mix_postmix_data = NULL;
 
 /* rcg07062001 callback to alert when channels are done playing. */
-static void (*channel_done_callback)(int channel) = NULL;
+static void (*channel_done_callback)(void *userdata, int channel) = NULL;
+static void *channel_done_callback_userdata;
 
 /* Music function declarations */
 extern int open_music(SDL_AudioSpec *mixer);
@@ -261,7 +317,7 @@
 static void _Mix_channel_done_playing(int channel)
 {
     if (channel_done_callback) {
-        channel_done_callback(channel);
+        channel_done_callback(channel_done_callback_userdata, channel);
     }
 
     /*
@@ -842,10 +903,11 @@
     return(music_data);
 }
 
-void Mix_ChannelFinished(void (*channel_finished)(int channel))
+void Mix_ChannelFinished(void *userdata, void (*channel_finished)(void *userdata, int channel))
 {
     SDL_LockAudio();
     channel_done_callback = channel_finished;
+    channel_done_callback_userdata = userdata;
     SDL_UnlockAudio();
 }