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: 2009-05-27 00:47:18

Okay, that makes sense as a cause, anyway. I'm not clear on how to fix it. Does this mean that transform_iterator can't be used to convert a map iterator into a value or key iterator? I've done that before and it seemed to work, but that may have been a std::map rather than a boost::unordered_map.....or perhaps it was the fact that I was returning by value before.

If I have to return by value for this to be feasible, it's not worth it. I'll just go with the slightly messier interface.

----- Original Message -----
From: Steven Watanabe <watanabesj_at_[hidden]>
Date: Tuesday, May 26, 2009 7:22 pm
Subject: Re: [Boost-users] transform_iterator causing strange problems

> AMDG
> 
> Lindley M French wrote:
> > 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;
> > <snip>
> >
> > 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?
> >   
> 
> * The value_type of unordered_map<GroupID, Group>
>  is pair<const GroupID, Group>
> * std::pair has a converting constructor.
> * Therefore, when transorm_iterator calls your function object,
>    it creates a temporary std::pair<GroupID, Group> which
>    goes out of scope when operator* returns.
> 
> In Christ,
> Steven Watanabe
> 
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