Re: [Boost-users] [MPL/Fusion?]Call a 0-args template function on members of a sequence

From: David Abrahams (dave_at_[hidden])
Date: 2006-08-21 22:31:21

Edd Dawson <edd_at_[hidden]> writes:

>>>> I'm looking for a way to call f<T>() for all types T in my_vec. I initially 
>>>> hoped that either an mpl algorithm or fusion::for_each might be able to 
>>>> accomplish this for me, but after reading through their docs this doesn't appear 
>>>> to be the case.
>>>   struct call_f
>>>   {
>>>      template <class Wrapper>
>>>      void operator()(Wrapper)
>> Sorry, it's missing a "const" right here--^^^^
>>>      {
>>>          f<typename Wrapper::type>();
>>>      }
>>>   };
>>>
>>>   mpl::for_each<my_vec, mpl::identity<_> >(call_f());
>> 
>
> I didn't find this in the docs. Is it meant to be there?

The specific answer to your specific problem?  No.  Oh yeah, for_each
wasn't documented in 1.33.1... and apparently it isn't documented in
the CVS head either (still?  Aleksey?)

> Unfortunately, that didn't quite work for me, anyway. A requirement
> of for_each (according to my MSVC 8 diagnostics) is that each type
> in the sequence must be default constructible.

In the case above, only mpl::identity<T> needs to be
default-constructible for each type in the list... or that was my
intention Ach!  My mistake...

    template <class T> 
    struct make_identity { typedef mpl::identity<T> type; };

    mpl::for_each<my_vec, make_identity<_> >(call_f());

should work.

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