Re: [Boost-users] transform_iterator causing strange problems

27 May 2009

      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 
Date: Tuesday, May 26, 2009 7:22 pm
Subject: Re: [Boost-users] transform_iterator causing strange problems
...
AMDG
...
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 (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 const_iterator;
<snip>
Where my GroupSelector class is defined as
struct GroupSelector
{
    typedef const Group& result_type;
    const Group& operator()(const std::pair &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
Lindley M French wrote:
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
 is pair
* std::pair has a converting constructor.
* Therefore, when transorm_iterator calls your function object,
   it creates a temporary std::pair which
   goes out of scope when operator* returns.
In Christ,
Steven Watanabe
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