[C++] Funzioni "friend" ed errore

[jobv]

Salve forum!

Non riesco a capire perché compilando il seguente codice ottengo questo errore:

giovanni@Ubuntu10:~/Scrivania$ gcc a.cpp 
a.cpp: In member function ‘bool MPEGaudio::WantedSpec(SDL_AudioSpec*)’:
a.cpp:20: error: ‘Play_MPEGaudioSDL’ was not declared in this scope

Ecco il codice:

//a.h 
#include <SDL/SDL.h>

class MPEGaudio  {

    friend void Play_MPEGaudioSDL(void *udata, Uint8 *stream, int len);
    friend int Play_MPEGaudio(MPEGaudio *audio, Uint8 *stream, int len);
#ifdef THREADED_AUDIO
    friend int Decode_MPEGaudio(void *udata);
#endif

public:
  
    bool WantedSpec(SDL_AudioSpec *wanted);

private:
  static const int bitrate[2][3][15],frequencies[2][3];
  int stereobound,subbandnumber,inputstereo,outputstereo;
  int layer,protection,bitrateindex,padding,extendedmode;
  enum _mpegversion  {mpeg1,mpeg2}                               version;
  enum _mode    {fullstereo,joint,dual,single}                   mode;
  enum _frequency {frequency44100,frequency48000,frequency32000} frequency;
};

//a.cpp
#include <SDL/SDL.h>

#include "a.h"

bool
MPEGaudio:: WantedSpec(SDL_AudioSpec *wanted)
{
    wanted->freq = frequencies[version][frequency];
#if SDL_BYTEORDER == SDL_LIL_ENDIAN
    wanted->format = AUDIO_S16LSB;
#else
    wanted->format = AUDIO_S16MSB;
#endif
    if ( outputstereo ) {
        wanted->channels = 2;
    } else {
        wanted->channels = 1;
    }
    wanted->samples = 4096;
    wanted->callback = Play_MPEGaudioSDL;
    wanted->userdata = this;
    return true;
}

Grazie!

[jobv]

Ho semplificato tutto per individuare l'errore...sicuramente è una cavolata ma proprio non capisco:

//a2.h

typedef struct SDL_AudioSpec {
	//void (SDLCALL *callback)(void *userdata, uint8_t *stream, int len);
	void (*callback)(void);
	
} SDL_AudioSpec;

class MPEGaudio  {

    //friend void Play_MPEGaudioSDL(void *udata, uint8_t *stream, int len);
    friend void call(void);

public:
  
    bool WantedSpec(SDL_AudioSpec *wanted);


};

//a2.cpp
#include "a2.h"

bool
MPEGaudio:: WantedSpec(SDL_AudioSpec *wanted)
{
    wanted->callback = call;//Play_MPEGaudioSDL;

    return true;
}

void call(void)
{}

[shodan]

Prova a mettere il prototipo prima della classe.

void Play_MPEGaudioSDL(void *udata, Uint8 *stream, int len);
class MPEGaudio  { // etc...

[jobv]

Grazie mille!!!

é andata!