[Boost-users] Understanding Boost.Mpl Fold Algorithm

From: Eduardo Bezerra (edubez_at_[hidden])
Date: 2005-12-23 07:36:14

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 ?

Best regards,
Eduardo