Re: [Boost-users] transform_iterator causing strange problems

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
>    {
>        return 
> boost::make_transform_iterator(currgroups.begin(),GroupSelector());    }
>    const_iterator end() const
>    {
>        return 
> boost::make_transform_iterator(currgroups.end(),GroupSelector());  
>  }
>    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
> 0012F65C
> 1243176
> 
> 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?
>