Bad Acces Memory with map iterator

Question

Possible Duplicate:
Problem with std::map::iterator after calling erase()

May be you could enlighten me with an issue I'm currently having. So problem is that I get a bad access memory when I'm trying to erase some elements in the map. We suppose the following typedef :

typedef std::map < std::string *, Document *, pStringCompare > Map;

And we suppose we already have instantiated a map which contains two elements (for example) before the execution of all the following code. This code with working great :

Map::iterator it = documents.begin();

std::string *s = it->first;
Document *d = it->second;

documents.erase(it);

delete d;
delete s;

But when I'm trying to loop with the iterator, i get the error.

for (Map::iterator it = documents.begin() ; it != documents.end() ; it++)
{
    std::string s = * ( it->first);
    Document dd = * (it->second);
    std::cout << s << " || " << dd;
    documents.erase(it);  // This line causes the bad access memory error.
}

Thanks for your help ! It's really appreciated !

Aleks

Are you sure it's that line on which the error occurs? I'd expect it to occur on the increment (`it++`), because `it` becomes *invalid* after a call to `erase()`. — Angew is no longer proud of SO, Nov 09 '12 at 19:10

score 6 · Accepted Answer · edited May 23 '17 at 10:24

You are accessing invalidated iterator. Once you pass it to erase() it is no longer valid, so you cannot increment it in your for loop. See Iterator Invalidation Rules for more details on that.

Your loop should be structured like this to avoid accessing invalidated iterators:

for (Map::iterator it = documents.begin() ; it != documents.end() ; )
{
    std::string s = * ( it->first);
    Document dd = * (it->second);
    std::cout << s << " || " << dd;
    documents.erase(it++);
}

Though I really recommend you read more documentation and understand what is going on here. For example, see this.

Good Luck!

Thanks for the help and the doc, it's been very helpful ! – Aleks Nov 09 '12 at 21:34 — Aleks, Nov 09 '12 at 21:34

Ryan Guthrie · Answer 2 · 2012-11-09T19:36:21.140

1

Alternatively:

while ((Map::iterator iter = documents.begin()) != documents.end())
{
   std::string s = * ( iter->first);
   Document dd = * (iter->second);
   std::cout << s << " || " << dd;
   documents.erase(iter);
}

edited Nov 09 '12 at 19:36

answered Nov 09 '12 at 19:27

Ryan Guthrie

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Bad Acces Memory with map iterator

2 Answers2