CODEWITHSUNDEEP
c++stl
zer0stimulus
zer0stimulus

Reputation: 23606

C++: how to track a pointer to a STL list element?

I would like to track a pointer to a list element for the next read access. This pointer would be advanced every time the list is read. Would it be bad practice to cache an iterator to the list as a member variable, assuming I take proper precautions when deleting list elements?

Upvotes: 3

Views: 2132

Answers (3)

Nicol Bolas
Nicol Bolas

Reputation: 473407

Iterators are meant to be used. They aren't just for looping over each element in a for-loop. All you have to take into account is the rules for iterator invalidation.

std::vector iterators can be invalidated by any operation that inserts elements into the list. The iterators for all elements beyond the point of insertion are invalidated, and all iterators are invalidated if the insertion operation causes an increase in capacity. An operation that removes an element from the vector will invalidate any iterator after the point of removal.

std::deque iterators are invalidated by any operation that adds or removes elements from anywhere in the deque. So it's probably not a good idea to keep these around very long.

std::list, std::set, and std::map iterators are only invalidated by the specific removal of the particular element that the iterator refers to. These are the longest-lived of the iterator types.

As long as you keep these rules in mind, feel free to store these iterators all you want. It certainly isn't bad form to store std::list iterators, as long as you can be sure that that particular element isn't going anywhere.

Upvotes: 3

Jason
Jason

Reputation: 32510

The only way you're going to be able to properly advance though an STL std::list<T> in a platform and compiler independent way is through the use of a std::list<T>::iterator or std::list<T>::const_iterator, so that's really your only option unless you're planning on implementing your own linked-list. Per the standard, as others here have posted, an iterator to a std::list element will remain valid until that element is removed from the list.

Upvotes: 2

Sven
Sven

Reputation: 22683

An iterator to a list element remains valid until the element it refers to is removed. This is guaranteed by the standard.

There is no problem with caching an iterator as long as you make sure you don't remove elements from the list or refresh the cached iterator when you do.

Upvotes: 4

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