Re: [boost] Alternative implementation for BOOST_PP_VARIADIC_SIZE

Subject: Re: [boost] Alternative implementation for BOOST_PP_VARIADIC_SIZE
From: Edward Diener (eldiener_at_[hidden])
Date: 2011-11-12 14:17:33

On 11/10/2011 6:43 PM, Gennadiy Rozental wrote:
> Hi,
>
> Current implementation of subject macro have a very serious shortcoming - it
> does not work for empty __VA_ARGS__. After googling around a bit for some ideas
> I came up with the following alternative:
>
> #include<boost/preprocessor/arithmetic/mul.hpp>
> #include<boost/preprocessor/arithmetic/sub.hpp>
> #include<boost/preprocessor/punctuation/comma.hpp>
>
> #define MY_VA_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 MY_VA_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
>
> #define MY_DO_VA_NARG2(...) MY_VA_ARG_N __VA_ARGS__
> #define MY_DO_VA_NARG(...) MY_DO_VA_NARG2(( __VA_ARGS__ ))
>
> // this version produces incorrect value (1) for empty __VA_ARGS__
> #define MY_VA_NARG_(...) MY_DO_VA_NARG(__VA_ARGS__, MY_VA_RSEQ_N())
>
> // this version fixes it
> #define MY_VA_NARG(...)  BOOST_PP_SUB( BOOST_PP_MUL( MY_VA_NARG_( __VA_ARGS__ ),
> 2 ), MY_VA_NARG_( BOOST_PP_COMMA __VA_ARGS__ () ) )
>
> This version work for zero sized __VA_ARGS__ and also works for MSVC-9,10 ,
> gcc-4.1 gcc 3.2 (did not test with other compilers).
>
> Does it make sense to improve trunk version?

This has the same limitation as the BOOST_VMD_IS_EMPTY(...) macro in my 
own latest version of the variadic macro data library, which itself was 
adapted from a solution Paul Mensonides showed on the Internet. It also 
uses much the same technique, although I find Paul's solution more 
elegant ( involves no PP arithmetic ). The limitation is that if the 
variadic data ends with a function-like macro the '__VA_ARGS__ ()' could 
cause function-like macro expansion.

Eddie