[Audacity-devel] Re: commands.h leads to compile errors on linux

[Hermann <her...@ui...>]

hello

> From: Shane Mueller <smu...@um...>

> I'm having trouble compiling a clean cvs checkout on linux. Using gcc
> 2.95.4, I get the following errors when it reaches commands.h:
>  
> commands.h: In method `void AudacityProject::CreateMenuBar()':
> commands.h:132: non-lvalue in unary `&'
> commands.h:138: non-lvalue in unary `&'
> commands.h:144: non-lvalue in unary `&'
> .....<output truncated>
> 
> These problem lines all use the following macro:
> 
>  #define CMD_CACFUNC(a) ( (wxObjectEventFunction
> *)memcpy(malloc(sizeof(wxObjectEventFunction)), &(&a),
> sizeof(wxObjectEventFunction)) )
> 
> I presume this has something to do with something that VC++ is forgiving
> about but gcc isn't, maybe in relation to wxWindows
> wxObjectEventFunction.  Has anyone else seen this problem (the file has
> been around about 3 days)?

Yes I had the very same problem.

> I played with the code a little but couldn't figure out a fix, although
> I think it probably stems from the &(&a) part: 

It is definitely this part.
I searched the web and found something in the line of: unary operator & 
requires an lvalue. but (&a) is not an lvalue (at least not for g++ :-)

My solution was to "craft" a function that replaces "&(&a)" by producing an 
lvalue inside of the function an returning it.
this is what I added at the top of Menu.cpp

void *pmf_to_vp(void (AudacityProject::* pmf)(wxCommandEvent &)) {
    void *vp = &pmf;
    return vp;
}

in commands.h I replaced "&(&a)" in the CMD_CACFUNC-definition with 
"pmf_to_vp(&a)".

:-(
this is ugly. I guess there is quite a simple solution to this problem, but I 
did not manage to find one...


hermann