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

From: Karl Friedrich Walkow (walkow_at_[hidden])
Date: 2005-12-23 08:40:59

Hi Eduardo,

the fold algorithm iterates over a mpl-sequence. the function needs 
three arguments as input: the sequence to iterate over, a "starting 
condition" and a binary operation.
at every step of the iteration the algorithm applies two arguments to 
the binary operation: the first argument is the output of the previous 
invocation of the operation (at the first step that is the starting 
condition). the second argument is the currently processed element from 
your input sequence. the "starting condition" also determines the return 
type of your fold-operation.
about the placeholders you only have to know that "_1" means the first 
and "_2" stands for the second argument. if you don't have the line 
"using namespace boost::mpl" in your code, the placeholders are 
boost::mpl::_1 and boost::mpl::_2.
that should be all!

the algorithm from the example returns an integral constant with the 
number of elements in the input sequence that are float types.
if is_float applied to the current element returns true, than the 
integral constant will be incremented by one ("next<_1>"), otherwise it 
will not be changed and so on.

Regards,
Karl