CODEWITHSUNDEEP
c++qtiterator
Ashley Phillips
Ashley Phillips

Reputation: 3

Is it possible to store an iterator?

For example, say I have a const_iterator:

QHash<const QString, QPair<const Node, double> >::const_iterator citer =  adjNodeHash.begin();

Can I then store citer in a data structure (containing many iterators) and re-use it later, with it still referring to the same place I left off the next time I use it? (assuming I update it accordingly/use a reference to it when I am incrementing it)

I ask this because I have used this approach yet am getting some undefined bahaviour and am wondering if this is the culprit.

Any help would be much appreciated.

Upvotes: 0

Views: 1162

Answers (1)

Yakk - Adam Nevraumont
Yakk - Adam Nevraumont

Reputation: 275896

Iteration invalidation rules for std containers are described in the standard. QHash will also have some iterator invalidation rules in its documentation (hopefully!).

A stored iterator remains valid until invalidated. Most hash maps invalidate their iterators when they "rehash", which happens when they grow past a certain bound.

In practice, it is probably a bad idea to store an iterator into a hash map over a period in which elements are added or removed from it. Maintaining that iterator as valid will take constant maintenance and error checking, adding overhead to every use of that hash map, and any errors developing may not immediately show up, and the error that happens won't occur near the spot where the mistake is made.

On top of that, if you ever swap out what hash container you are using, the details of the iterator invalidation rules are going to be different. This makes refactoring in the future more painful.

Upvotes: 6

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