Re: [Boost-users] Preprocessor Question

From: Ed Keith (e_d_k_at_[hidden])
Date: 2008-05-06 12:00:21

--- Joel Falcou <joel.falcou_at_[hidden]> wrote:

> > I have two sequences. Both represent sets, one is
> a
> > subset of the other. What I want to do is for each
> > element in the superset, generate one value if it
> is
> > in the subset, and another value if it is not in
> the
> > subset. The problem is that the elements are not
> > numeric (they are just tokens), so I can not seem
> to
> > compare them to determine if they are the same.
> > 
> > Any sugestions?
> 
> What I woukld write is borrowed from the inner
> mechanics of Boost::PP.
> let's say you have two token FOO & BAR.
> 
> #define IS_SAME_TOKEN_FOO_FOO 1
> #define IS_SAME_TOKEN_FOO_BAR 0
> #define IS_SAME_TOKEN_BAR_FOO 0
> #define IS_SAME_TOKEN_BAR_BAR 0
> 
> #define IS_SAME_TOKEN(X,Y) 
>
BOOST_PP_CAT(BOOST_PP_CAT(BOOST_PP_CAT(IS_SAME_TOKEN_,X),_),Y)
> 
> then use IS_SAME_TOKEN in BOOST_PP_IF
> 
> I knwo it gets cumbersome as the number of tokens
> increase but I can't 
> see hwo to do it better.

I have not got it working yet, but I think I can avoid
the combinatorial explosion.

For each token 'foo', I define 'X_foo' and give it a
uneque numeric value. Then I can use the following to
compare them :

#define MY_COMPARE(x,y)
BOOST_PP_EQUAL(BOOST_PP_CAT(X_, x),
BOOST_PP_CAT(X_,y))

Still not a elegant as I'd like, but no combinatorial
explosion.

Thanks a lot for putting me on the right track.

   -EdK

Ed Keith
e_d_k_at_[hidden]

Blog: edkeith.blogspot.com

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