Re: [Boost-users] mpl::fold problems

Subject: Re: [Boost-users] mpl::fold problems
From: sergey kostanbaev (sergey.kostanbaev_at_[hidden])
Date: 2010-08-23 10:24:13

Thanks! It really helps!

On Mon, Aug 23, 2010 at 5:47 PM, Stefan Strasser <strasser_at_[hidden]>wrote:

> Zitat von sergey kostanbaev <sergey.kostanbaev_at_[hidden]>:
>
>  template <class T, unsigned space=1> struct tr_field2
>> {
>>   static const unsigned count = space;
>> }
>>
>> typedef boost::mpl::vector4<
>>   tr_field2<int,  4>,
>>   tr_field2<int,  7>,
>>   tr_field2<char, 1>,
>>   tr_field2<char, 1>
>>
>>> Test_description;
>>>
>>
>>
>> I want to calculate a sum of all tr_field2::count
>> I've read in the docs mpl::fold is for the case.
>>
>> I tried to compose
>>
>>   typedef boost::mpl::fold<Test_description,
>>      boost::mpl::int_<0>,
>>      boost::mpl::plus<boost::mpl::_, boost::mpl::_::count > >
>>   >::type fcount;
>>
>> but it won't compile. It seems I misundestood something.
>>
>
> template<typename T>
> struct get_count{
>    typedef mpl::int_<T::count> type;
>
> };
>
> typedef boost::mpl::fold<Test_description,
>  boost::mpl::int_<0>,
>  boost::mpl::plus<boost::mpl::_1, get_count<mpl::_2> >
>
>> ::type fcount;
>>
>
> the placeholders are not evaluated by mpl::_::count.
> and mpl::plus expects a mpl integral constant, not a value.
>
>
>
>
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