Re: [boost] [preprocessor] nested for_each not working like expected

Subject: Re: [boost] [preprocessor] nested for_each not working like expected
From: Mathias Gaunard (mathias.gaunard_at_[hidden])
Date: 2013-08-09 07:40:56

On 07/08/13 22:25, Eric Niebler wrote:
> On 13-08-07 12:58 PM, paul Fultz wrote:
>> As others have stated, `BOOST_PP_SEQ_FOR_EACH` is not reentrant. One simple
>> workaround is to use deferred expressions, like this:
>>
>>      #define BOOST_PP_SEQ_FOR_EACH_R_ID() BOOST_PP_SEQ_FOR_EACH_R
>>      #define DEFER(x) x BOOST_PP_EMPTY()
>>
>>      #define S0 (0)(1)(2)(3)
>>      #define S1 (5)(6)(7)(8)
>>      #define M4(R, DATA, ELEM) (DATA,ELEM)
>>      #define M2(R, DATA, ELEM) DEFER(BOOST_PP_SEQ_FOR_EACH_R_ID)()(R, M4, ELEM, S1);
>>      BOOST_PP_EXPAND(BOOST_PP_SEQ_FOR_EACH_R(1, M2, ~, S0))
>>
>> If more depths are needed, just add more `BOOST_PP_EXPAND` macros. Also, you
>> can reuse the recursion state so you could write `M2` like this:
>>
>>      #define M2(R, DATA, ELEM) DEFER(BOOST_PP_SEQ_FOR_EACH_R_ID)()(1, M4, ELEM, S1);
>>
>> or like this, if you don't know the original recursion state:
>>
>>      #define M2(R, DATA, ELEM) DEFER(BOOST_PP_SEQ_FOR_EACH_R_ID)()(BOOST_PP_DEC(R), M4, ELEM, S1);
>
> I won't pretend to understand how this works, but it does. Thanks!

Why not just use BOOST_PP_REPEAT otherwise? It's simple and it always works.

Just replace ELEM by BOOST_PP_SEQ_ELEM(N, S0)