Re: [Boost-users] BOOST_FOREACH and maps

From: Shunsuke Sogame (mb2act_at_[hidden])
Date: 2006-09-18 19:56:58

Dave Steffen wrote:
> Sebastian Redl writes:
>  > 
>  > On Mon, September 18, 2006 12:52 pm, Rob Caldecott wrote:
>  > 
>  > > Can the pair be declared without a typedef?  Is this a limitation of the
>  > > MS
>  > > compiler?
>  > 
>  > It is a limitation of the preprocessor. It does not recognize <> as any
>  > sort of parentheses, so it thinks the , separating the template parameters
>  > is a macro argument separator.
>  > 
>  > You can try wrapping the whole argument in parentheses:
>  > BOOST_FOREACH((std::pair<int, int> p), m)
>  > 
>  > But I'm not sure if the PP passes the parentheses on - in that case, you
>  > might get syntax errors in the generated code. If that's the case, you'll
>  > have to use the typedef.
> 
>  I've done something like this:
> 
> #include "boost/tuple/tuple.hpp"
> 
> #define FOREACH_PAIR( KEY, VAL, COL) FOREACH (boost::tie(KEY,VAL),COL)
> 
> // and then, for example
> 
> int key, value;
> 
> FOREACH_PAIR(key, value m);
> 
> It's not perfect; specfically, I don't think that you can write into
> the map this way.  I suspect there _is_ a way for that to work, I just
> haven't figured it out yet. :-)
> 
> I'd like the eventual "official" FOREACH to have some additional
> variations like this...

Boost.Parameter(cvs head) seems to already
have a workaround for it.

#include <boost/parameter/aux_/parenthesized_type.hpp>
#define UNPARENTHESIZE BOOST_PARAMETER_PARENTHESIZED_TYPE

int main()
{
     std::map<int, int> m;
     BOOST_FOREACH (
         UNPARENTHESIZE((std::pair<int, int>)) p,
         m )
     {
     }
}

-- 
Shunsuke Sogame