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 (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 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