Re: [Boost-users] preprocessor metaprogramming question

Subject: Re: [Boost-users] preprocessor metaprogramming question
From: Nathan Ridge (zeratul976_at_[hidden])
Date: 2010-06-20 21:33:27

> Date: Sun, 20 Jun 2010 18:02:16 -0700
> From: watanabesj_at_[hidden]
> To: boost-users_at_[hidden]
> Subject: Re: [Boost-users] preprocessor metaprogramming question
> 
> AMDG
> 
> Nathan Ridge wrote:
> >> Nathan Ridge wrote:
> >> 
> >>>> MEMBERS((a)(b)(c))
> >>>>
> >>>> 
> >>> I don't suppose there's any way to have the syntax be MEMBERS(a, b, c)?
> >>>
> >>> 
> >> Not without variadic macros.
> >> 
> >
> >
> > How would one do it using variadic macros?
> > 
> 
> I don't know, as I've never tried to use variadic macros.
> There ought to be some way to figure out how many
> argument you have. Once you have that, you can
> use Boost.PP tuples or arrays.

There appears to be no builtin way, but it turns out (http://groups.google.com/group/comp.std.c/browse_thread/thread/77ee8c8f92e4a3fb/346fc464319b1ee5)

you can write a macro that counts the number of arguments:

 

#define PP_NARG(...) \ 
         PP_NARG_(__VA_ARGS__,PP_RSEQ_N()) 
#define PP_NARG_(...) \ 
         PP_ARG_N(__VA_ARGS__) 
#define PP_ARG_N( \ 
          _1, _2, _3, _4, _5, _6, _7, _8, _9,_10, \ 
         _11,_12,_13,_14,_15,_16,_17,_18,_19,_20, \ 
         _21,_22,_23,_24,_25,_26,_27,_28,_29,_30, \ 
         _31,_32,_33,_34,_35,_36,_37,_38,_39,_40, \ 
         _41,_42,_43,_44,_45,_46,_47,_48,_49,_50, \ 
         _51,_52,_53,_54,_55,_56,_57,_58,_59,_60, \ 
         _61,_62,_63,N,...) N 
#define PP_RSEQ_N() \ 
         63,62,61,60,                   \ 
         59,58,57,56,55,54,53,52,51,50, \ 
         49,48,47,46,45,44,43,42,41,40, \ 
         39,38,37,36,35,34,33,32,31,30, \ 
         29,28,27,26,25,24,23,22,21,20, \ 
         19,18,17,16,15,14,13,12,11,10, \ 
         9,8,7,6,5,4,3,2,1,0 

 

 

Is it possible to improve on this macro and write it in constant space with Boost.PP?

 

Thanks,

Nate.
                                               
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