Re: how to iterator and delete elements in std::set
On 23/11/2010 11:43, Bogdan wrote:
On Nov 22, 4:18 pm, Leigh Johnston<le...@i42.co.uk> wrote:
On 22/11/2010 13:49, Bogdan wrote:
On Nov 22, 1:12 am, zl2k<kdsfin...@gmail.com> wrote:
hi, there
Here is what I want to do: I have a set of objects. I need to iterate
each of them to ask if it needs to be eliminated. If yes, I'll erase
it from the set. I don't know which needs to be erased before I touch
it.
std::set<Obj> objs;
...
for (auto obj_iterator = objs.begin(); obj_iterator != objs.end(); +
+obj_iterator){
if (obj_iterator.do_you_want_to_be_erased() == true){
objs.erase(*obj_iterator);
}
}
Now I have the trouble since the iterator is destroyed after the first
erase. What is the proper way to do the job? Thanks.
zl2k
It was some time ago a discussion on this list about this issue.
Indeed, it seems strange to erase the object to which the iterator
points to, but running your snippet exactly as is still produces the
right answer.
No it doesn't. See else-thread for the right answer.
/Leigh
Maybe I am wrong, but erasing one element from the set doesn't mean
that the heap memory is freed ? That location though remains unused
till some other object is inserted into the set. So I really don't see
what's wrong with this snippet:
for (auto it = my_set.begin(); it != my_set.end();++it)
{
if (it->IsDeletable())
{
my_set.erase(*it);
}
}
as long as one is trying to iterate over the entire set without
inserting a new element, only erasing the existing ones.
std::set is a node based container so the node will in all likelyhood be
freed when the associated element is erased. The actual problem is that
the erase will invalidate the iterator "it" making "++it" undefined
behaviour (a bug).
/Leigh
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