Re: [boost] [preprocessor] Lists vs. nil-sequences?

Subject: Re: [boost] [preprocessor] Lists vs. nil-sequences?
From: Jeffrey Lee Hellrung, Jr. (jhellrung_at_[hidden])
Date: 2011-03-02 11:54:04

On 3/2/2011 6:32 AM, Lorenzo Caminiti wrote:
> Hello all,
>
> I have a curiosity about Boost.Preprocessor: What is the difference
> between pp-lists and pp-sequences starting with a NIL element?
>
> PP-Lists can be empty or nil (e.g., have 0 size, etc) while
> pp-sequences cannot. However, I could use sequences that always start
> with a nil element `(NIL) ...` and write macros that ignore this 1st
> element so to handle empty pp-sequences:
>
> #define NILSEQ_SIZE(nilseq) BOOST_PP_DEC(BOOST_PP_SEQ_SIZE(nilseq))
>
> NILSEQ_SIZE( (NIL) ) // 0
> NILSEQ_SIZE( (NIL)(a) ) // 1
> NILSEQ_SIZE( (NIL)(a)(b) ) // 2
>
> BOOST_PP_LIST_SIZE( BOOST_PP_NIL ) // 0
> BOOST_PP_LIST_SIZE( (a, BOOST_PP_NIL) ) // 1
> BOOST_PP_LIST_SIZE( (a, (b, BOOST_PP_NIL)) ) // 1
>
> Similarly, I could program NILSEQ_FOR_EACH, NILSEQ_ENUM, etc.
>
> What are the pros and cons of using pp-nil-sequences vs pp-lists given
> that both allow for emptiness?
>
> Thank you very much.

I don't know whether this is necessarily good practice or not, but I've 
created macros that allow BOOST_PP_SEQ_NIL to be passed as a parameter 
where a Boost.PP seq is intended, and within the macro implementation I 
dispatch appropriately.  From an interface-design point-of-view, that 
would seem to be preferable to prepending all non-empty Boost.PP seq's 
with "(NIL)".  However, I would believe it's easy to sacrifice 
efficiency depending on how you do the dispatch...

- Jeff