std::map<unsigned int, unsigned int>::iterator iterator;
for (iterator = access.begin(); iterator != access.end(); ++iterator)
{
access.erase((*iterator).first); // of course, the actual code is not this simple
}
[C++] Erasing std::map::iterators while walking the container [solved]
Author
Not too surprisingly the following code is crashing the executable:
I'm afraid I have this multiple times in my code, also for vectors. Is this code just as dangerous for vectors as well?
Also, what would be a better way of doing this? Should I remember the last iterator, then go to the next, and then erase the remembered iterator?
Thanks!
[Edited by - Decrius on May 19, 2009 1:55:31 AM]
For your code sample, you'd call std::map<>::clear().
If you want to conditionally erase iterators in an associative container you'd use:
If you want to conditionally erase iterators in an associative container you'd use:
for (iter = map.begin(); iter != map.end();) { if (condition(iter)) container.erase(iter++); else ++iter;}
Author
Again, thanks SiCrane ;)
Does this apply to vectors too, or is it harmless for vectors?
Does this apply to vectors too, or is it harmless for vectors?
Vectors you would use generally use the erase-remove idiom. If for some reason you had to manually loop you'd use:
for (iter = vec.begin(); iter != vec.end();) { if (condition(iter)) iter = vec.erase(iter); else ++iter;}
Author
Okay thanks, I guess you meant:
But I get the point, same as maps that is. Thanks.
for (iter = vec.begin(); iter != vec.end();) { if (condition(iter)) iter = vec.erase(iter++); else ++iter;}But I get the point, same as maps that is. Thanks.
Author
Quote:Original post by SiCrane
No, I didn't. The two loops are different.
Oops, just noticed ^^.
Quote:Original post by SiCrane
Vectors you would use generally use the erase-remove idiom. If for some reason you had to manually loop you'd use:for (iter = vec.begin(); iter != vec.end();) { if (condition(iter)) iter = vec.erase(iter); else ++iter;}
Hmmm... Wouldn't that cost O(n^2) time? What I have typically done is
for (read_iter = vec.begin(), write_iter = vec.begin(); read_iter != vec.end(); ++read_iter) { if (!condition(read_iter)) { *write_iter = *read_iter; ++write_iter; }}vec.resize(write_iter-vec.begin());
Author
Ah I see, is that because erase() makes the std::map reorder it's elements? And, wouldn't swapping elements of the std::map break the algorithm it uses to find elements?
Also, doesn't this remove-erase idiom make me copy the values hold by the std::map a lot of times? Hmm, or does std::swap not actually use the copy constructor / assignment operator?
Also, doesn't this remove-erase idiom make me copy the values hold by the std::map a lot of times? Hmm, or does std::swap not actually use the copy constructor / assignment operator?
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