Re: STL Vector: Unexpected behavior

bob wrote:
> for (localIterator = localVector.begin(); localIterator !=
> localVector.end(); ++localIterator) {
> MyClass* obj_ptr = *localIterator; // dereference to get element
> if(obj_ptr->some_field == true) do_stuff();
> else do_different_stuff();
> // now remove element from list
> localVector.erase(remove(localVector.begin(), localVector.end(),
> *localIterator), localVector.end());
> } // end of for loop

Erasing an element from a container always invalidates the iterator used for
the deletion, i.e. you can't use it afterwards, not even increment it to
the next element. For a vector, you should expect it to invalidate every
iterator of that container. Also, removing elements from the middle of a
vector is a slow operation, you should use list<> instead, then you can use
this simple algorithm:

iterator it = the_list.begin();
while(it!=the_list.end)
{
if(some_condition(*it))
the_list.erase(it++);
else
++it;
}

Other than that, you could move all pointers you want to keep to a new
vector and then swap the former one with it.

> This results in an error since there is nothing
> in the vector to dereference!

Yes, you also omit checking if the pointer is zero in your loop. I'd
consider using boost::shared_ptr<>, btw.

> for (unsigned index = 0; index < localVector.size; ++index)
>
> Whilst that _may_ solve the problem, it won't allow me to _understand_
> why the other approach doesn't work.

No, this also doesn't work, because when you erase an element from the
middle and then increment 'index' you skipped an element.

Uli

.

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