Re: [Boost-users] BOOT_PP_REPEAT simple nesting problem

Subject: Re: [Boost-users] BOOT_PP_REPEAT simple nesting problem
From: Edward Diener (eldiener_at_[hidden])
Date: 2014-09-21 12:51:46

On 9/20/2014 9:35 AM, Colin Fowler wrote:
> Hi all, I'm trying to use BOOST_PP to generate some code.
>
> I can generate the sequence int x_0 to int x_4 using the code below.
>
> #include <boost/preprocessor/repetition/repeat.hpp>
> #define DECL1(z, n, text) text ## _ ## n = n;
> #define decl1(text, count) BOOST_PP_REPEAT(count, DECL1, text)
> decl1 (int x, 5)
>
>
> What I'd like to do is write a decl2 that can be used something like this:
>
> decl2 (int x, 5, 2)
>
> to generate int x_0_0 to int x_4_1
>
> I've tried nesting BOOST_PP_REPEAT but so far I've been unable to
> produce the desired behaviour unless I hardcode one of the iteration
> counts.
>
> I'd like to also eventually write a decl3(int x, 5, 3, 4) when I
> understand how a decl2 would work. Any help much appreciated!

The BOOST_PP_REPEAT macro takes a single count, so you would need to 
pass into the macro a single repetition count while preserving the 
individual counts in some way for BOOST_PP_REPEAT. The only way you 
could do this is by having the 'data' field of BOOST_PP_REPEAT be more 
complicated, ie. by having the 'data' field include both your 'text' and 
the individual count numbers. When you need a single input to include 
complicated data you should look at how to work with Boost PP arrays, 
lists, tuples, and/or seqs; all of which let you incorporate a number of 
pieces of data in a single input.

So given:

decl2 ( int x, 5, 2 )

and using a Boost PP tuple,

you want to pass to initially pass to BOOST_PP_REPEAT something like
(10, DECL2, (text,(5,2))) and then you can work out how to generate your 
output for each of the 10 repetitions of DECL2.

You can recursively call BOOST_PP_REPEAT from within BOOST_PP_REPEAT 
processing, and while it is advantageous to use the BOOST_PP_REPEAT_z 
form it is not necessary.

I hope this helps you to figure out how to do what you want.