projection iterator onto the k-th dimension

From: Antonio Piccolboni (antonio_piccolboni_at_[hidden])
Date: 2002-09-24 17:25:07

Reporting some progress wrt my previous post and adding a meaningful 
title. The code doesn't break on the iterator def but at the first 
usage. Error is (translated) can't convert return value of unary func 
obj to float &. Note that () returns float &

Current code

#ifndef GRAPH_H
#define GRAPH_H

#include <boost/config.hpp>
#include <vector>
#include <iostream>
#include <iterator>
#include <algorithm>
#include <boost/iterator_adaptors.hpp>

typedef vector<float> graph_element;

template <int index>
struct select_component {
  typedef pair < long, graph_element > argument_type;
  typedef float result_type;
  const float & operator()(const argument_type & r) const {
    return r.second[index];
  }
  float & operator()(argument_type & r) const {
    return r.second[index];
  }
};

#endif

Use case

#include "graph.h"
#include <map>
#include<vector>

int main(int, char*[])
{
  typedef map < long, graph_element > graph;

  graph aGraph;

  aGraph[0].push_back(0.00);
  aGraph[0].push_back(0.01);
  aGraph[0].push_back(0.02);
  aGraph[1].push_back(0.10);
  aGraph[1].push_back(0.11);
  aGraph[1].push_back(0.12);
  aGraph[2].push_back(0.20);
  aGraph[2].push_back(0.21);
  aGraph[2].push_back(0.22);
  aGraph[3].push_back(0.30);
  aGraph[3].push_back(0.31);
  aGraph[3].push_back(0.32);

  boost::projection_iterator_generator < select_component < 0 >,
    graph::iterator >::type
    graph_first(aGraph.begin()),
    graph_last(aGraph.end());
  
  //  copy(graph_first, graph_last,
  //   ostream_iterator<float>(cout, "\n"));
  cout << *graph_first;  
  cout << endl;
}

Postmodern error message:
cd /home/apicco/src/analysis/transcriptome/transmap/
g++ -g -I/home/apicco/install/boost_1_28_0 -o graph-test graph-test.C
/home/apicco/install/boost_1_28_0/boost/iterator_adaptors.hpp: In 
method `typename 
IteratorAdaptor::reference 
boost::projection_iterator_policies<AdaptableUnaryFunction>::dereferen
ce 
(const IteratorAdaptor &) const [with IteratorAdaptor = 
boost::iterator_adaptor<_Rb_tree_iterator<pair<const long int, 
graph_element>, pair<const long int, 
graph_element> &, pair<const long int, graph_element> *>, 
boost::projection_iterator_policies<select_component<0> >, float, 
float &, float *, 
boost::detail::default_argument, boost::detail::default_argument>, 
AdaptableUnaryFunction = 
select_component<0>]':
/home/apicco/install/boost_1_28_0/boost/iterator_adaptors.hpp:867:   
instantiated from `boost::iterator_adaptor<Base, Policies, Value, 
Reference, Pointer, Category, Distance>::operator* () const [with 
Base = _Rb_tree_iterator<pair<const long int, graph_element>, 
pair<const long int, graph_element> &, pair<const long int, 
graph_element> *>, Policies = 
boost::projection_iterator_policies<select_component<0> >, Value = 
float, Reference = float &, Pointer = float *, Category = 
boost::detail::default_argument, Distance = 
boost::detail::default_argument]'
graph-test.C:31:   instantiated from here
/home/apicco/install/boost_1_28_0/boost/iterator_adaptors.hpp:1247: 
could not convert `(this + 
1)->select_component<index>::operator()((&pair<long int,vector<float, 
allocator<float> > 
>((+(+(+iter)->boost::iterator_adaptor<Base, Policies, Value, 
Reference, Pointer, Category, 
Distance>::base())->_Rb_tree_iterator<_Value, _Ref, _Ptr>::operator*
()))))' to `float &'

Compilation exited abnormally with code 1 at Tue Sep 24 14:52:21