Having trouble using an STL map's iterator while removing elements

Question

I'm looping through an STL map to delete elements and do some clean up. Unfortunately for me my program crashes on the ++iter line. Am I doing something wrong in how I'm looping over the map?

std::map<string,BoneHierarchy* >::iterator iter = boneMap.begin();
while(iter != boneMap.end()) {
    string playerName = iter->first;
    boneMap[playerName]->clear();
    boneQueue->push(boneMap[playerName]);
    boneMap.erase(iter);
    ++iter;
}

I think iterators should only read/modify, when you delete it the iteration in progress is broken. — kenny, Apr 21 '12 at 15:53

score 6 · Accepted Answer · edited May 23 '17 at 11:49

6

Don't increment the iterator after deleting it (it's been erased, so how can you expect to increment it?). Erase a copy of the iterator with:

boneMap.erase(iter++);

or do

iter = boneMap.erase(iter);

if using C++11.

(See also Removing elements from a C++ map through a for-loop)

edited May 23 '17 at 11:49

Community

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answered Apr 21 '12 at 15:55

Cornstalks

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Nawaz · Answer 2 · 2012-04-21T16:00:25.037

3

boneMap.erase(iter);
++iter;                     //incorrect

should be written as:

iter = boneMap.erase(iter); //correct (in C++11)
//no need to increment!

Because map::erase() returns iterator following the erased item.

edited Apr 21 '12 at 16:00

answered Apr 21 '12 at 15:55

Nawaz

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`map::erase(iterator position)` returns `void`. – jpm Apr 21 '12 at 15:57
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@jpm: It doesn't exist anymore (in C++11). In C++11, it returns iterator. – Nawaz Apr 21 '12 at 15:59
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Neat, I wasn't familiar with that update. Thanks for the info. Also, +1. – jpm Apr 21 '12 at 16:03

Having trouble using an STL map's iterator while removing elements

2 Answers2