RE: [boost] mpl::fold

From: Aleksey Gurtovoy (agurtovoy_at_[hidden])
Date: 2002-12-12 17:52:47

Iain K.Hanson wrote:
> I'm having a problem using fold with the example from the reference
> page. I can get the following example from copy to comile as fold
> 
>     namespace mpl = boost::mpl;
>     using mpl::_;
> 
>     typedef mpl::vector10_c<int,9,8,7,6,5,4,3,2,1,0> answer;
> 
>     typedef mpl::fold<
>           mpl::range_c<int,0,10>
>         , mpl::vector<>
>         , mpl::push_front<_,_>
>         >::type result;
> 
>     BOOST_STATIC_ASSERT(mpl::size<result>::value == 10);
>     BOOST_STATIC_ASSERT((mpl::equal< result,answer >::type::value));
> 
> 
> but this won't:
> 
> 
>     namespace mpl = boost::mpl;
>     using mpl::_1;
>     using mpl::_2;
> 
>     typedef 
> mpl::vector<long,float,short,double,float,long,long double>
> types;
> 
>     typedef mpl::fold<
>         types
>         , mpl::integral_c<long, 0>
>         , mpl::if_< boost::is_float<_2>, mpl::next<_1>,_1 >
>         >::type number_of_floats;
> 
> typedef mpl::lambda< number_of_floats >::type func;

Just a quibble - the above line is unnecessary.

> 
>     BOOST_STATIC_ASSERT(( number_of_floats::value == 3 ));
                                                      ^^^
An error in the docs, should be 4. Sorry!

> 
> I presume that the problem is here
> 
>         , mpl::if_< boost::is_float<_2>, mpl::next<_1>,_1 >
>                     ^^^^^^^^^^^^^^^^^^^
> 
> as this is a non-mpl metafunction. 

Actually, it is! Try this one:

    typedef lambda< boost::is_float<_1> >::type is_float;
    typedef apply1< is_float, float >::type res;
    BOOST_STATIC_ASSERT(res::value == true);

> Can any one tell me how to fix the above please?

    BOOST_STATIC_ASSERT(number_of_floats::value == 4);

Fixed in the reference docs' sources, too.

> 
> Also, what are the requirement on a metafunction to be used with
> lambda. A simple example would be useful.

On a conforming compiler, the simple form
(http://www.mywikinet.com/mpl/paper/mpl_paper.html#metafunctions.simple)

    template< typename T1, ..., typename Tn >
    struct f
    {
        typedef ... type;
    };

just works "out-of-box":

    template< typename T > struct identity
    {
        typedef T type;
    };

    typedef lambda< identity<_1> >::type f;
    typedef apply1< f, int >::type res;
    BOOST_MPL_ASSERT_IS_SAME(res, int);

HTH,
Aleksey