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

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