Re: [Boost-users] A MPL loop..

Subject: Re: [Boost-users] A MPL loop..
From: Larry Evans (cppljevans_at_[hidden])
Date: 2010-07-11 11:39:31

On 07/11/10 02:18, Frank Winter wrote:
> I am trying to translate this runtime loop
> 
> while( totalsize * maxelem * 3 > poolsize )
>   maxelem >>= 1;
> 
> into MPL. I got so far:
> 
> template< typename totalsize , typename maxelement , typename poolsize >
> struct loop
> {
>   typedef mpl::times< totalsize , maxelement > part0;
>   typedef mpl::times< part0 , mpl::int_< 3 > > part1;
>   typedef mpl::greater< part1 , poolsize > gr;
>   typedef mpl::if_< gr , typename loop< totalsize , mpl::divides< 
> maxelement , mpl::int_< 2 > > , poolsize >::value , maxelement > value;
> };
> 
> template< class totalsize , class poolsize >
> struct loop< totalsize , mpl::int_< 1 > , poolsize >
> {
>   typedef mpl::int_< 1 > value;
> };
> 
> 
> typename loop< mpl::int_< 144 > , mpl::int_< 1024 > , mpl::int_< 140000 >
>> ::value max;
> 
> g++43 (GCC) 4.3.2 says:
> error: template instantiation depth exceeds maximum of 500
> 
> If coded it correctly that should not happen. Since maxelement is 
> already after the 1st interation mpl::int_< 512 > and therefor the 
> product with mpl::int_< 144 > smaller than mpl::int_< 140000 >
> 
> I could imagine:
>  - that I need a constructor that converts the divided types, correct?
>  - that I need to use a vector< sequence > like in the physics dimension
>    example, correct?
> 
> Can somebody fix this or point me to a working loop example?
> 
> 
> Regards,
> Frank
You might try using:

http://svn.boost.org/svn/boost/sandbox/variadic_templates/boost/mpl/while.hpp

or maybe the while_recur template used here:

https://svn.boost.org/trac/boost/attachment/ticket/3044/while_recur.cpp