Deleting elements from containers while iterating through them

Question

Ok, I've run into this issue multiple times and thought it would be nice to throw this out to the good people on SO.

Say I have made a class, lets call it Resource. So the Resource class has a boolean variable which indicates weather any instance is active or not.

Now, I create a container which holds references to objects of the Resource type.

Over time , some of them get deactivated and I want to delete these deactivated objects so that they free up memory. Invariably I try to do this by : trying to iterate over the elements in the container and deleting the ones flagged as inactive. This , obviously leads to problems with iterator invalidation and the program starts throwing runtime errors.

So finally, my question is , what is the best method to safely delete objects depending on some condition that can only be evaluated by looking into the contents of the object.

possible duplicate of iterate vector, remove certain items as I go — Bo Persson
– Bo Persson, Commented Mar 12, 2012 at 12:26
Not talking about just vectors or lists, also maps and other associative containers. — angryInsomniac
– angryInsomniac, Commented Mar 13, 2012 at 6:27

KillianDS · Accepted Answer · 2012-03-12 11:53:06Z

4

Use the erase-remove idiom with std::remove_if. E.g.:

std::vector<int> myvec; ... myvec.erase(std::remove_if(myvec.begin(),myvec.end(), [] (int i) -> bool { return false; }), myvec.end());

answered Mar 12, 2012 at 11:53

KillianDS

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2 Comments

angryInsomniac Over a year ago

What about associative containers like maps ? std::remove_if doesnt work for them I gather.

KillianDS Over a year ago

@angryInsomniac: no it doesn't work because it would break ordering. This question provides an answer for that problem.

Björn Pollex · Accepted Answer · 2012-03-12 11:54:18Z

0

The safest method is to use std::remove_if. This will move all items that match the given predicate to the end of the sequence. You can then delete them.

answered Mar 12, 2012 at 11:54

Björn Pollex

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2 Comments

angryInsomniac Over a year ago

What about associative containers like maps ?

Björn Pollex Over a year ago

With a map you can delete the element an iterator points to without invalidating any other iterators. That means you can do your_map.erase(it++);. See this question for more information.

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