[Boost-users] [iterator_facade] Help with const iterator for binary file

From: nicola (vitacolo_at_[hidden])
Date: 2008-01-11 15:46:36

Hi,
I have a class modeling a record-based read-only binary file and I have 
written a random access iterator based on iterator_facade to iterate 
over the records. The class looks like this:

class my_binary_file {
public:
   typedef std::string                             key_type; 
   typedef unsigned int                            value_type;
   typedef std::pair<key_type, value_type>         record_type;   
private:
   class my_binary_file_iterator : public boost::iterator_facade<
      my_binary_file_iterator,
      my_binary_file::record_type const,
      boost::random_access_traversal_tag,
      my_binary_file::record_type&,
      int>
   {
   public:
      my_binary_file_iterator() : rec_num_(0), ff_ptr_(0) { }
      explicit my_binary_file_iterator(my_binary_file* ff, unsigned int 
n) : ff_ptr_(ff), rec_num_(n) { }
   private:
      friend class boost::iterator_core_access;
      
      unsigned int rec_num_;
      my_binary_file* ff_ptr_;
      mutable my_binary_file::record_type record;     
      
      void increment() { ++rec_num_; }
      
      void decrement() { --rec_num_; }
      
      void advance(int n) { rec_num_ += n; }
      
      int distance_to(const my_binary_file_iterator z) const {
         return z.rec_num_ - this->rec_num_;
      }
      
      bool equal(my_binary_file_iterator const& other) const {
         return this->ff_ptr_ == other.ff_ptr_ && this->rec_num_ == 
other.rec_num_;
      }
      
      const my_binary_file::record_type
      dereference() const {
         ff_ptr_->seekg(rec_num_);
         my_binary_file::key_type key;
         my_binary_file::value_type val;
         ff_ptr_->get_record(key, val);
         record = std::make_pair(key, val);
         return record;
      }
   };
   
public:
   typedef my_binary_file_iterator iterator;
   
   iterator begin() const { return iterator(this, 0); }
   iterator end() const { return iterator(this, num_records_); }
      
   // Opens a binary file for reading
   explicit my_binary_file(const std::string& path);
   
   ~my_binary_file();

   operator void*() const;
   bool operator!() const;
   
   // Reads the record at the current position
   bool get_record(key_type& k, value_type& v);

   std::ios::pos_type tellg() const { return fs_.tellg(); }
   void seekg(unsigned int n) {
      fs_.seekg(start_record_ + std::ios::off_type(n * record_size_), 
std::ios::beg);
   }
   
private:
   boost::filesystem::path pathname_;
   mutable boost::filesystem::fstream fs_;
   std::streampos start_record_;
   key_type::size_type key_size_;
   unsigned int record_size_, num_records_;

};

Unfortunately, when I try to compile my project, I get the following 
error:

error: invalid conversion from 'const frequency_file* const' to 
'frequency_file*'

I have tried several variants of my code, but without success.
I think I'm missing something pretty obvious in my definition of 
my_binary_file_iterator, but I can't get out of it. Anyone can help?

Regards
Nicola