Re: [Boost-users] [iterator || function_types] zip_iterator ->unpack and call?

From: shunsuke (pstade.mb_at_[hidden])
Date: 2007-02-16 11:44:56

Peter Dimov wrote:
> shunsuke wrote:
>> Sohail Somani wrote:
>>> Hi,
>>>
>>> Say I have a function "int add(int, int)" and want to use it in a
>>> call to std::transform with zip_iterators. Is there a way to do it
>>> without writing an intermediate function object?
>>>
>>> If not, would it be sensible to write a generic one using the
>>> function_types library? I can't imagine this would be too hard but
>>> just wanted to know if there is anything I can leverage before
>>> writing it myself.
>> I guess what you're looking for is...
>>
>> http://tinyurl.com/26t9dd
>> =http://lists.boost.org/Archives/boost/2007/01/115135.php
>>
>> http://tinyurl.com/y9ax7t
>> =http://boost.cvs.sourceforge.net/boost/boost/boost/iterator/zip_iterator.hpp?view=log&pathrev=zip_iterator_fusion
>>
>> It seems not so easy because of the Forwarding Problem.
> 
> Forwarding doesn't seem a problem to me since you already have the argument 
> tuple. The return type inference is painful to get right, though. On the 
> surface it seems as easy as
> 
> template<class F, class Tp> ... unpack_and_call( F f, Tp const & tp )
> {
>     return unpack_and_call_( tuple_size<Tp>(), f, tp );
> }
> 
> template<class F, class Tp> ... unpack_and_call_( mpl::int_<0>, F f, Tp 
> const & tp )
> {
>     return f();
> }
> 
> template<class F, class Tp> ... unpack_and_call_( mpl::int_<1>, F f, Tp 
> const & tp )
> {
>     return f( get<0>(tp) );
> }
> 
> // and so on
> 
> but the ... part is more involved if one decides to use result_of. 
> boost::bind-style inference is of course easier since it only needs to look 
> at F. 

Right. I have ever written such codes :-)
I should say " *pack*_and_call is not so easy. ".

Regards,

-- 
Shunsuke Sogame