[Boost-users] Understanding Boost.Mpl Fold Algorithm

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