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

12 Jan 2008

      In article <478810E0.7030407@providere-consulting.com>,
 Steven Watanabe  wrote:
...
AMDG
nicola wrote:
...
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 {
<snip>
explicit my_binary_file_iterator(my_binary_file* ff, unsigned int n) 
      : ff_ptr_(ff), rec_num_(n) { }
my_binary_file* is *not* const here.
Thanks for pointing that out. In the tutorial example at

http://www.boost.org/libs/iterator/doc/iterator_facade.html#tutorial-exam
ple

sect. "A constant node_iterator", shouldn't the constructor's parameter 
of const_node_iterator be const? That is,

explicit const_node_iterator(const /* <== ? */ node_base* p)
      : m_node(p) {}
...
...
<snip>
};
public:
   typedef my_binary_file_iterator iterator;
iterator begin() *const* { return iterator(this, 0); }
this is a pointer to const.  The constructor of iterator requires a 
pointer to non-const.
There is more than that, however: as you can see in my original post, 
get_record() and seekg() are not const and that is a problem 
because they are invoked by dereference(). I can compile successfully if 
I const-qualify them, but this is not correct in my opinion.
The fact is, they do change the state of the object in a visible way. 
That's the implementation of get_record() that I had omitted in the 
original post (but that you could have easily guessed):

   bool get_record(key_type& k, value_type& v) /* const ??? */ { 
      if (fs_.read(reinterpret_cast(&k), key_size_)) {
         if (fs_.read(reinterpret_cast(&v), sizeof(value_type))) {
            return true;
         }
      }
      return false;
   }
So, for example, calling get_record() twice results in two consecutive 
records to be read — not the same record. Moreover, I will be 
interested, at a later stage, in extending the class so that it can read 
and *write* files.

Is there any better solution than using const member functions for my 
get_record() and seekg()? 

Nicola