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From: Paul Mensonides (pmenso57_at_[hidden])
Date: 2003-01-01 05:28:54

----- Original Message -----
From: "Aleksey Gurtovoy" <agurtovoy_at_[hidden]>

Hi Aleksey,

> I am going to use our wonderful Preprocessor library to generate a
> metafunction that basically looks like this:


> I love everything about it except for the "(0, (1, (2, (3, (4,
> BOOST_PP_NIL)))))" part. I would like the above to become something along
> these lines:
> , -1
> , BOOST_PP_RANGE(0, 4)
> )

This one is certainly possible. However, "BOOST_PP_RANGE_FOLD_LEFT" would
just be a synonym for whatever folding macro operates on the type generated
by "BOOST_PP_RANGE." It would be to difficult to make a parametizable macro
that yields a sequence of numbers:

    , -1
    , BOOST_PP_SEQ_RANGE(0, 4)

Which is close to what you want. In fact, it is almost already there:

#include <boost/preprocessor/seq/fold_left.hpp>
#include <boost/preprocessor/seq/subseq.hpp>

#define NUMBERS \
    (0)(1)(2)(3)(4)(5)(6)(7)(8)(9) \
    (10)(11)(12)(13)(14)(15)(16)(17)(18)(19) \
    /* ... */

#define RANGE(first, length) \
    BOOST_PP_SEQ_SUBSEQ( NUMBERS, first, length ) \


The only major difference is that BOOST_PP_SEQ_SUBSEQ takes first and length
operands, rather than first and last. Which, for the zero-case is almost
the same: first -> BOOST_PP_INC(last). It wouldn't be too difficult to
bang out a variant that goes from first -> last though.

> or, better yet,
> , -1
> , BOOST_PP_RANGE(0, 4)
> )
> where 'BOOST_PP_FOLD_LEFT' is a generic algorithm that can be used on all
> sequences.
> How hard would it be to have something like this?

With C99's variadic macros, fairly easy with lists vs. sequences. Without
them, impossible. The reason is simple, without variadics I have no way of
telling the difference between (a) and (a, b), etc.... With variadics I can
count the arguments.

> I suppose it's possible to generate the code I need using BOOST_PP_WHILE,
> but IMO that solution wouldn't be as conceptually nice and intuitive as
> above.

Paul Mensonides

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