Re: [Boost-users] Understanding Boost.Mpl Fold Algorithm

From: David Abrahams (dave_at_[hidden])
Date: 2006-01-08 12:00:27

Eduardo Bezerra <edubez_at_[hidden]> writes:

> Hi,
>
> I'm having a hard time trying to understand what the placeholders
> _1 and _2 mean in the context of the mpl::fold algorithm.
>
>>From the Boost.Mpl Reference manual:
>
>
> typedef vector<long,float,short,double,float,long,long double> types;
>
> typedef fold<
>     types
>   , int_<0>
>   , if_< is_float<_2>,next<_1>,_1 >
>>::type number_of_floats;
>
>
> BOOST_MPL_ASSERT_RELATION( number_of_floats::value, ==, 4 );
>
>
> Could someone give me some help on this ?

<shameless plug>
http://www.boost-consulting.com/mplbook contains a good explanation.
</shameless plug>

-- 
Dave Abrahams
Boost Consulting
www.boost-consulting.com