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

From: Sebastian Redl (sebastian.redl_at_[hidden])
Date: 2005-12-23 08:26:22

Eduardo Bezerra wrote:

>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 );
>  
>
_1 is the result holder, _2 is the current element.

Let's for a moment translate the thing to some pseudo-code:

number_of_floats = fold(types, 0, lambda(value, element){return 
is_float(element) ? value+1, value; });

Where fold is
function fold(sequence, initial_value, func)
{
  value = initial_value;
  for each element in sequence {
    value = func(value, element);
  }
}

_1 is value, _2 is element.

Sebastian Redl