why can't i return a non-const string from line_iterator::dereference() (line 32). if i change it to `const string& dereference() const` it compiles.

error.txt

astacy∂ʇoqǝɔɐʇs ☠ ~/Downloads/11219461  12:54:30 Apr23
§ g++-4.8 -Wall -Werror -g -L/Users/astacy/boost/lib -I/Users/astacy/boost/include -o test test.cpp
test.cpp: In member function 'std::string& line_iterator::dereference() const':
test.cpp:33:12: error: invalid initialization of reference of type 'std::string& {aka std::basic_string<char>&}' from expression of type 'const string {aka const std::basic_string<char>}'
     return line;
            ^

test.cpp

#include <fstream>
#include <istream>
#include <string>

#include <assert.h>

#include <boost/iterator/iterator_facade.hpp>

using namespace std;

class line_iterator :
  public boost::iterator_facade<
    line_iterator,                    // this class
    istream,                          // the thing over which we're iterating
    input_iterator_tag,               // the iterator concept
    string> {                         // the deref type

  istream* is;
  string line;

  friend class boost::iterator_core_access;

  void increment() {
    if (is && !getline(*is, line))
      is = NULL;
  }

  bool equal(const line_iterator& other) const {
    return this->is == other.is;
  }

  string& dereference() const {
    return line;
  }

  public:
    line_iterator() : is(NULL), line() {}

    explicit line_iterator(istream* is) : is(is), line() {
      increment();
    }
};

int main(int argc, char* argv[]) {
  ifstream f3("test/threelines.txt");
  ifstream f4("test/fourlines.txt");

  assert(distance(line_iterator(&f3), line_iterator()) == 3);
  assert(distance(line_iterator(&f4), line_iterator()) == 4);

  return 0;
}