[Boost-users] fusion::result_of::invoke_function_object and references

Subject: [Boost-users] fusion::result_of::invoke_function_object and references
From: Mike Tegtmeyer (tegtmeye_at_[hidden])
Date: 2009-06-26 17:48:05

I have another question regarding compilation failures with fusion. If 
this has been discussed before, can someone please point me to the right 
location. If not...

Is there some reason the application of result_of::invoke_function_object 
fails to compile for sequence argument references?

ie: result_of::invoke_function_object<doit, fusion::vector<int>&>::type

Digging a little, it seems that the failure is due to the size<> operator 
doesn't work with reference types (or is expecting the reference to be 
stripped prior to this point). I understand that to avoid unnecessary 
copies, tiers are typically used but in the case where chains of 
invoke_function_object would like to be strung together where there is an 
assignment or construction from the actual return value to a variable 
declared as the type obtained from result_of (in a fold for example), it 
seems that the vector copy is unavoidable (at least in my reading) unless 
the argument vector is a const reference.

Is this an oversight or am I missing something?

Basically I am trying to string together N function objects in the 
sequence where the ith object gets called with the result of the i-1th 
object (where each has a different argument list) using fold and I am 
running into extraneous copies of the return value. I can post a full 
example if needed.

Thanks in advance,
Mike

error message:
/opt/local/include/boost/fusion/sequence/intrinsic/size.hpp:28: error: 
const boost::fusion::vector<int, boost::fusion::void_, 
boost::fusion::void_, boost::fusion::void_, boost::fusion::void_, 
boost::fusion::void_, boost::fusion::void_, boost::fusion::void_, 
boost::fusion::void_, boost::fusion::void_>& is not a class, struct, or 
union type

See code below.

#include <boost/fusion/container/vector.hpp>
#include <boost/fusion/functional/invocation/invoke_function_object.hpp>

namespace b = boost;
namespace bf = boost::fusion;

struct doit {
   template <typename Sig>
   struct result;

   template <typename Self, typename T>
   struct result< Self(const T &) >
   {
     typedef int type;
   };

   template <typename T>
   int operator()(const T &t) const
   {
     return 1;
   }
};

#if !OK
   typedef bf::result_of::
     invoke_function_object<doit,const bf::vector<int>&>::type 
result_type2;
#else
   typedef bf::result_of::
     invoke_function_object<doit,const bf::vector<int> >::type 
result_type2;
#endif

int main() {return 0;}