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c++boostboost-lambdaboost-phoenix

How to use a phoenix expression with boost::transform_iterator?

<Update> As usual for me, the question was a wrong one. The actual question is: why doesn't transform_iterator use the conventional result_of<> metafunction to determine the return type, instead of accessing UnaryFunc::result_type directly. Posted an answer with a work around. </Update>

Specifically, is there a way to make a phoenix expression expose a result_type type as expected for the std::unary_function concept? boost::transform_iterator seems to expect this, and from looking at the src of it, I don't see a simple work around.

Here's some code that reproduces the problem I've been having:

#include <boost/iterator/transform_iterator.hpp>
#include <boost/spirit/home/phoenix.hpp>
#include <numeric>
#include <iostream>

using namespace boost::phoenix;
using namespace boost::phoenix::arg_names;

int main(void){
   int i[] = {4,2,5,3};

   std::cout <<
      std::accumulate(
         boost::make_transform_iterator(i,   _1*_1),
         boost::make_transform_iterator(i+4, _1*_1),
         0
      ) << std::endl;

   return 0;
}

The relavent portion of the error message from compiling this is (gcc 4.3.4, boost 1.43):

/usr/include/boost/iterator/transform_iterator.hpp:43: error: no type named ‘result_type’ in ‘struct boost::phoenix::actor<...

I have the same problem with boost::lambda (missing result_type). I thought that I had seen similar usage for make_transform_iterator and lambda in the past, now I'm wondering if I just imagined it.

Is there a provided wrapper or some other mechanism in phoenix or lambda to expose result_type?

Solution

  • It looks like this is fixed in the boost trunk (see line 51, result_of<> instead of an indirect UnaryFunc::result_type). So this shouldn't be an issue in 1.44 and above.

    Here's a workaround for boost < 1.44. The transform_iterator instantiation accesses UnaryFunc::result_type only if the Reference template parameter is not provided. So one trick is to replace make_transform_iterator with a version that calls the result_of<> meta function on the UnaryFunc and use the result for the Reference template parameter.

    #include <boost/iterator/transform_iterator.hpp>
#include <boost/utility.hpp>
#include <iterator>

template <class UnaryFunc, class Iterator>
boost::transform_iterator<
   UnaryFunc,
   Iterator,
   typename boost::result_of<
      UnaryFunc(typename std::iterator_traits<Iterator>::value_type)
   >::type
>
make_trans_it(Iterator it, UnaryFunc fun){
   return
      boost::transform_iterator<
         UnaryFunc,
         Iterator,
         typename boost::result_of<
            UnaryFunc(typename std::iterator_traits<Iterator>::value_type)
         >::type
      >(it, fun);
};