CODEWITHSUNDEEP
c++dictionary
Global Warrior
Global Warrior

Reputation: 5110

Error with C++ Maps erase

I had to delete some of the entries from c++ std::map, for that purpose I iterate over the std::map and used erase. But I get an error. The error is like :

0x0017f3f6 in std::_Rb_tree_rebalance_for_erase () from /usr/local/lib/libstdc++.so.6
(gdb) bt
#0  0x0017f3f6 in std::_Rb_tree_rebalance_for_erase () from /usr/local/lib/libstdc++.so.6
#1  0x00487ef4 in getLetters () from /usr/lib/libmail.so
#2  0x0048cacc in getHeader () from /usr/lib/libmail.so

#6  0x08048802 in __gxx_personality_v0 ()
#7  0x00251e9c in __libc_start_main () from /lib/libc.so.6
#8  0x080486a1 in __gxx_personality_v0 ()
(gdb)

The code was something like this:

int nlsize=LettersMap.size();

if(nlsize > MAXNL)              
{                               
        std::map <std::string,MSG_HEADER>::iterator it;
        int i=0;                
        for( i=0, it=LettersMap.begin();  i <nlsize-MAXNL  ;i++, it++)
        {                       
                LettersMap.erase(it);                 
        }                       
}

Can someone please point out the error

Upvotes: 1

Views: 2456

Answers (2)

Sion Sheevok
Sion Sheevok

Reputation: 4217

You have an iterator to the std::map, right? In your for loop, you initialize your iterator to the beginning. You check if it is within bounds. You then erase the iterator. Your iterator is now invalid. The iterators are effectively like pointers and when you erase, you are effectively deleteing a value that was "pointed" to. As such, other iterators can't guarantee that they point to something valid.

Specifically, you erase, causing an invalidation of your iterator. A guaranteed one, because it's the iterator you erased. You need a new iterator, preferably one right after where you erased, right? Unfortunately, std::map does not offer that functionality. std::map isn't really intended to be treated as a sequential container. The idea is that you access values by their key, not their position. Seeing as you're trying to erase by their sequential order, this is problematic.

Instead, perhaps, you could identify which key you need to erase based on i? Then use find to find the key and erase to erase the key. Since you are no longer sequentially iterating through your std::map, you no longer have to worry about invalidated iterators.

Upvotes: 3

Joop Eggen
Joop Eggen

Reputation: 109547

int n = nlsize - MAXNL;
while (n-- > 0) {
    LettersMap.erase(LettersMap.begin());
}

The iterator it is becoming instable by erase.

Upvotes: 2

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