Re: pointer iterator interaction
"silversurfer" <kietzi@web.de> wrote:
Ok, this should be fairly easy for most of you (at least I hope so),
but not for me:
Let us say we have got the following elements:
std::vector<Entry> models; //Entry is a struct
std::vector<Entry>::iterator modelIterator;
In a method, I am currently writing, I need to get a pointer to an
entry in the vector.
No you don't. Don't use pointers to elements in Vectors.
Pointers to elements in vectors tend to get "stale", because
vectors reallocate themselves if they grow, so you'd eventually
end up dereferencing wild pointers and crashing your program.
I thought of the following:
Entry* MyObject::getNextEntry() {
Entry* tmpEntry;
if (modelIterator == models.end()) {
return NULL;
} else {
tmpEntry = modelIterator;
modelIterator++;
return tmpEntry;
}
}
Unfortunately, this does not work
Of course it doesn't work. The return type of your function is:
Entry*
but the type of tmpEntry is:
std::vector<Entry>::iterator
The two types are not even remotely similar.
I get the error message:
error: cannot convert `
__gnu_cxx::__normal_iterator<Entry*, std::vector<Entry,
std::allocator<Entry> > >' to `Entry*' in assignment
Yes, as I said, "not even remotely similar". You're asking the
compiler to convert apples to pipedreams. The compiler is
doubting your sanity, and rightly so.
I do not really understand this: As far as I understood it,
an iterator should be a pointer to the elements of the vector,
An iterator is not a pointer. It might (or might not) be
IMPLIMENTED in terms of a pointer, but unless you're writing
a compiler, that's none of your business.
An iterator can (and should) be USED LIKE a pointer, yes.
But it's not a pointer. It's an iterator. Think of it as
"one level higher of abstraction" compared to a pointer.
How can I get a pointer to the element to which the iterator
is currently pointing?
If you absolutely MUST do that, then you can, easily, like so:
Entry* DangerousPointer = &(*modelIterator);
But that is very dangerous. For iterating through the elements
of a Vector, use real iterators. You can use iterators for (almost)
anything you could use pointers for.
Or use integer subscripting. With vectors, that's often easier than
using iterators, and you don't need to worry about whether an old
iterator still points to something valid.
--
Cheers,
Robbie Hatley
Tustin, CA, USA
lonewolfintj at pacbell dot net (put "[ciao]" in subject to bypass spam filter)
http://home.pacbell.net/earnur/