[SDL] Compile-time error when forward declaring Mix_Chunk

[Vassilis Virvilis]

Gianfranco Berardi wrote:
> I recently started to learn how to use SDL_mixer, and so I followed a 
> tutorial to get up to speed.  It seems most tutorials assume that you'd 
> want to declare the pointer to Mix_Chunk right before the code you use 
> it with, but I wanted to add a Mix_Chunk pointer as a member of a class.
> 
> Like I usually do when I add a pointer as a member, I forward declared 
> the type and included the relevant header in the .cpp file for the 
> class.    Specifically, I have a Game class, and I want to add a member:
> 
> Mix_Chunk * sound_;
> 
> So at the top of the header, I wrote:
> 
> struct Mix_Chunk;  // It could be class Mix_Chunk; The same error occurs
> 
> And in the game.cpp file, I include SDL.h and SDL_mixer.h.
> 
> When I try to compile, I get the following error:
> 
> g++ -c game.cpp -Wall -Wno-unknown-pragmas -Wno-format -g -DDEBUG 
> -I/usr/include/SDL -D_GNU_SOURCE=1 -D_REENTRANT
> /usr/include/SDL/SDL_mixer.h:90: error: conflicting declaration 'typedef 
> struct Mix_Chunk Mix_Chunk'
> game.h:36: error: 'Mix_Chunk' has a previous declaration as 'typedef 
> struct Mix_Chunk Mix_Chunk'
> 
> Any idea why I can't forward declare Mix_Chunk?  I imagine it is 
> something simple that I've missed, but I'm at a loss so far.
> 
> Thank for your time,
> Gianfranco
> 

If it is typedefed you can't do that. You have to include the SDL_Mixer.h
header in your class definition header. I am not sure why. Sorry...

The point is with
	-my SDL_Mixer 1.2.6 and SDL 1.2.11 in debian unstable
         -and SDL_Mixer from svn checkout tree
I can't see this. What I see is in SDL_mixer.h

struct Mix_Chunk {
... /* the actual declaration */
} Mix_Chunk;

Notice that Mix_Chunk is in both tag and name. In C++ struct tags are
first class citizens and do not need a typedef. In C there are two
completely different namespaces. This is why writing code like that
is a good practice if you want to be C++ friendly.

Now on the other hand The Mix_Music is declared like this

typedef struct _Mix_Music Mix_Music;

In my setup I can't forward declare Mix_Music;

The reason people are using this typedef trick is to provide an
opaque pointer. This is a  pointer where the programmer cannot reach
the internal variables freely. Unfortunately in C++ which
has proper access mechanisms (private, protected) that means
we have to drag the whole header file in our headers.

      .bill