Boost Users :
Subject: Re: [Boost-users] transform_iterator causing strange problems
From: Lindley M French (lfrench1_at_[hidden])
Date: 2009-05-26 19:13:48
I made a mistake in copying the code----"segs" and "vec" are actually the same thing. Just forgot to change it one place. So that's not the problem, just FYI.
----- Original Message -----
From: Lindley M French <lfrench1_at_[hidden]>
Date: Tuesday, May 26, 2009 7:09 pm
Subject: transform_iterator causing strange problems
> I'm not sure if this is the fault of transform_iterator----I
> really don't see how it could be----but I'm really confused here.
> I've got an unordered_map<GroupID, Group> (typedefed to GroupMap)
> in my class. The specifics of those types don't matter except that
> GroupID is an integral type, and Group contains a std::vector vec.
> I'm trying to use a transform_iterator so that my class may be
> viewed as a group container:
> typedef boost::transform_iterator<GroupSelector,
> GroupMap::const_iterator> const_iterator;
> const_iterator begin() const
> boost::make_transform_iterator(currgroups.begin(),GroupSelector()); }
> const_iterator end() const
> std::pair<const_iterator, const_iterator> groups() const
> const Group &grp = *(begin());
> return std::make_pair(begin(),end());
> Where my GroupSelector class is defined as
> struct GroupSelector
> typedef const Group& result_type;
> const Group& operator()(const std::pair<GroupID, Group> &p) const
> std::cout << p.second.vec.size() << " " << &p.second <<
> std::endl; return p.second;
> Note the output statement. Now, the usage in my main function is
> for (tie(iter,end) = final.groups(); iter != end; ++iter)
> const Group& grp = *iter;
> cerr << &grp << endl;
> cerr << grp.segs.size() << endl;
> My output is
> 10 0012F50C
> 10 0012F65C
> which is strange for two reasons. The first line comes from
> GroupSelector when I dereference begin() in the groups() function.
> The second comes from GroupSelector when I dereference iter in
> main. As these should both correspond to the same iterator, I
> can't understand why the address of the returned Group reference
> is different.
> The 3rd and 4th lines are outputted in main. Here's the second
> confusion: The address for the Group object is the same as in the
> most recent GroupSelector output, yet now the size of the vector
> within the Group comes out wrong. HUH?
Boost-users list run by williamkempf at hotmail.com, kalb at libertysoft.com, bjorn.karlsson at readsoft.com, gregod at cs.rpi.edu, wekempf at cox.net