[Boost-users] Iterators and maps [was Re: Where should STL suggestions go?]

From: Jim Lear (jim.lear_at_[hidden])
Date: 2004-12-10 11:07:55

The map iterators are really what tripped me up as an STL newbie.  One must
understand the iterators are implemented as pairs for maps, and pointers
for vectors and other things.  By providing an "T operator[](iterator)" function
for random access and associative containers (and for sequences maybe), one
could achieve a consistant interface.  So, a better example of what tripped
me up is multidimensional associative arrays:

    #include <map>
    #include <string>
    #include <iostream>
    
    void main() {
      typedef map<string, int> si_map_t;
      typedef map<string, si_map_t> ssi_map_t;
      ssi_map_t table;
      table["0"]["0"]=20;
      table["0"]["1"]=21;
      table["1"]["0"]=10;
      table["1"]["1"]=11;
    
      for (ssi_map_t::iterator i=table.begin(); i!=table.end(); i++)
        for (si_map_t::iterator j=i->second.begin(); j!=i->second.end();
j++)
           cout << j->second << endl;
      // Blech! :-)  This is quite obscure for newbies.

    }

Instead, the following is how I felt the inner loop should be implemented
(as it is in other languages with associative arrays):

    for (si_map_t::iterator j=table[i].begin(); j!=table[i].end(); j++)
       cout << table[i][j] << endl;

Granted, it's more inneficient with the two operators, but the code is easy
to read and write and consistant with the map's common semantics.


David Abrahams wrote:
> goochrules! wrote:
> > > I know this is not the correct place to suggest new features to the STL,
> > > but I can't find a similar group for STL.  Any suggestions?
> > >
> > > Sorry for the interruption.
> > >
> > > P.S. In case you're wondering what the suggestion is: Add ability to use
> > > random access operators with iterators.  E.g.
> > >
> > > vector<int> intv;
> > > ...
> > > vector<int>::iterator i=intv.begin();
> > > intv[i]++;
> > >     
> > What do you want that last line to do?  To me it looks like you could
> > be using "(*i)++;" instead to achieve the same result.
> >
> > > It seems odd and confusing that iterators have pointer semantics but not
> > > random access semantics.
> > >     
> > they do already, well, some of them at least do (of which, vector is
> > one): http://www.sgi.com/tech/stl/RandomAccessIterator.html.  
> >   
>
> Random access iterators do have random access semantics, but that
> particular expression won't compile with some (mutable) random access
> iterators.  The problem is that the result of p[i] is only required to
> be convertible to the iterator's value_type.
>
> Cheers,

-- 
Jim Lear

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