Re: [Boost-users] variadic template function using fusion vector

Subject: Re: [Boost-users] variadic template function using fusion vector
From: Tobias Schwinger (tschwinger_at_[hidden])
Date: 2010-01-14 13:55:20

Roman Perepelitsa wrote:
> 2010/1/13 noman javed <noman_bwp_at_[hidden] <mailto:noman_bwp_at_[hidden]>>
> 
> 
> You can do that, but execute must be a function object instead of function.
> 
> #include <boost/fusion/functional/adapter/unfused.hpp>
> 
> struct execute_impl
> {

>     template <class Seq>
>     struct result
>     {
>         typedef void type;
>     };
> 

Yeah, used to be that way. And works (by "coincidence" :-).

Fusion has been changed at some point to use the
boost::result_of protocol for result type computation.
Thus, to really get to the argument vector you'll have to
write:

     template< typename Signature >
     struct result;

     template< class Self, class ArgSeq >
     struct result< Self(ArgSeq&) > // <- NOTE: deducing
     // [... defines type memember 'type']

Anyway, in this very case the result type does not depend
on the arguments and so it's sufficient to just write:

     typedef void result_type;

>     template <class Seq>
>     void operator()(Seq const & s) const
>     {
>         // s contains all arguments passed to execute.
>     }
> };
> 
> boost::fusion::unfused<execute_impl> execute;
> 
> int main()
> {
>     const char* s = "hello";
>     int n = 42;
>     execute(s, n);
> }
> 
> Here "execute" suffers from perfect forwarding problem, which you can 
> solve by using perfect forwarding adaptor. There is one in p-stade 
> <http://p-stade.sourceforge.net/egg/doc/html/egg/function_adaptors.html#egg.function_adaptors.perfect> 
> and also I remember seeing one being proposed for boost, but I can't 
> find it.
> 

Once Fusion's Functional adapters used to attack the Forwarding Problem
themselves. Today, there's Boost.Functional.Forward (and .Factory in
case u want to e.g. fusion::invoke constructors).

Regards,
Tobias