Re: [boost] [vmd] Library Review

Subject: Re: [boost] [vmd] Library Review
From: Paul Mensonides (pmenso57_at_[hidden])
Date: 2014-09-14 18:49:01

On 9/14/2014 3:12 PM, Edward Diener wrote:
> On 9/14/2014 5:10 PM, pfultz2 wrote:

> I am not sure what you mean by namespaces ? Do you mean the type of the
> token parsed ?

I believe he means macro prefixes such as BOOST_PP_, BOOST_VMD_, or 
BOOST_VMD_NAMESPACE_.

>> Perhaps Paul can explain it better than me.
>
> The BOOST_PP_AUTO_REC macro is not documented so maybe you can
> understand my being puzzled by it and what it is used for. Other
> undocumented macros in Boost PP are generally easier for me to follow.

The BOOST_PP_AUTO_REC macro is used to find the first macro of a set of 
macros which has not been painted (i.e. will expand).  It requires a set 
of macros which are a power of 2 which are used in order.  For example:

#include <boost/preprocessor.hpp>
#include <boost/preprocessor/detail/is_nullary.hpp>

#define CAT_1(a, b) PRIMITIVE_CAT_1(a, b)
#define CAT_2(a, b) PRIMITIVE_CAT_2(a, b)
#define CAT_3(a, b) PRIMITIVE_CAT_3(a, b)
#define CAT_4(a, b) PRIMITIVE_CAT_4(a, b)

#define PRIMITIVE_CAT_1(a, b) a ## b
#define PRIMITIVE_CAT_2(a, b) a ## b
#define PRIMITIVE_CAT_3(a, b) a ## b
#define PRIMITIVE_CAT_4(a, b) a ## b

#define CAT \
     BOOST_PP_CAT( \
         CAT_, \
         BOOST_PP_AUTO_REC(PRIMITIVE_CAT_P, 4) \
     )
     /**/
#define PRIMITIVE_CAT \
     BOOST_PP_CAT( \
         PRIMITIVE_CAT_, \
         BOOST_PP_AUTO_REC(PRIMITIVE_CAT_P, 4) \
     ) \
     /**/
#define PRIMITIVE_CAT_P(n) \
     BOOST_PP_IS_NULLARY(PRIMITIVE_CAT_ ## n(,)()) \
     /**/

Here the CAT and PRIMITIVE_CAT macros are defined as object-like macros 
which expand to the next available CAT_n and PRIMITIVE_CAT_n macros 
which are found via the the BOOST_PP_AUTO_REC macro--which does a binary 
search from 1 to 4 in this case finding the first index for which the 
passed predicate PRIMITIVE_CAT_P expands to 1.

Essentially, it makes it look like CAT can be invoked recursively, but 
CAT itself is not really even a function-like macro.  E.g.

#define A1 CAT(B, 1) // here CAT expands to CAT_2
#define B1 CAT(C, 1) // here CAT expands to CAT_3
#define C1 CAT(D, 1) // here CAT expands to CAT_4

CAT(A, 1) // here CAT expands to CAT_1

> You could actually do something similar with VMD now but you would have
> to try each v-type separately to see if it begins the v-sequence. But
> after Paul Mensonides provided a better solution to identifying
> v-identifiers and v-numbers I can improve the parsing of VMD v-sequences
> greatly.
>
> In fact with VMD a v-sequence becomes another preprocessor "data type"
> similar to the high level Boost PP data types of array, list, seq, and
> tuple. I should really work to produce whatever operations with a
> v-sequence I can that would mimic some of those Boost PP data types. Its
> challenging to think about but it will be fun work except for the usual
> VC++isms.

The alternate registration mechanism (excluding the comma in the 
registration itself) that you guys realized is good and is obvious is 
retrospect.  That would make the registration much cleaner.

Regards,
Paul Mensonides