Map Not Incrementable Error

Question

I am using a small library to have a database connection pool.

However, when I run the provided example, at the point where it calls DestroyPool I get a map is not increment-able assertion fail.

Here is the code

template <typename T>
int CommonDatabasePool<T>::DestroyPool(){
    typename map<T*, short>::iterator iter;
    for( iter = poolmap.begin(); !poolmap.empty()&& iter != poolmap.end(); iter++ ) {
        if(iter->second == DBPOOL_POOLEDCON_FREE){
            MyReleaseConcreteConnection(iter->first);
            poolmap.erase(iter);
        }
    }
    exitkeepalivepool = true;

    if(XSemPost(&semhandle)==-1){
    }
    if(XThreadJoin(&threadhandle)==-1){
    }

    return poolmap.size();
}

Is there anything here that looks suspicious that might cause such an error?

Whether or not I even use any connections it still does this...

Thanks

Generally can't delete from the container while iterating over it with an iterator. — crashmstr, Jun 10 '14 at 18:40

Bill Lynch · Accepted Answer · 2014-06-10T18:54:18.637

3

Fundamentally, you are doing this code:

for(iter = poolmap.begin(); iter != poolmap.end(); iter++) {
    poolmap.erase(iter);
}

This code attempts to call iter++ on an iterator that is no longer valid.

You could fix this with something like:

typename map<T*, short>::iterator iter = poolmap.begin();
while (iter != poolmap.end()) {
    if (iter->second == DBPOOL_POOLEDCON_FREE) {
        MyReleaseConcreteConnection(iter->first);
        iter = poolmap.erase(iter);
    } else {
        iter++;
    }
}

edited Jun 10 '14 at 18:54

answered Jun 10 '14 at 18:40

Bill Lynch

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I tried iter = poolmap.erase(iter); – jmasterx Jun 10 '14 at 18:41
If you delete the last element in the poolmap, then you would call `iter++` on `end()` which is also invalid. – Bill Lynch Jun 10 '14 at 18:42
How could I fix it? if size is one then clear map and break? – jmasterx Jun 10 '14 at 18:43
I've included an example that uses `std::remove_if`. – Bill Lynch Jun 10 '14 at 18:45
Unfortunately I am using vs 2008 and do not have c++11 support :( – jmasterx Jun 10 '14 at 18:49
@Milo: Thus, I've already added a version that has the lambda as a functor. – Bill Lynch Jun 10 '14 at 18:49
How is MyReleaseConcreteConnection() called though? This function is in the class. – jmasterx Jun 10 '14 at 18:53
`std::remove_if` shouldn't work with an `std::map`, no? – juanchopanza Jun 10 '14 at 18:53
@juanchopanza: That's... a really good point... – Bill Lynch Jun 10 '14 at 18:53
Or to have it make more sense to someone less familiar with lambdas and functors, I don't hate the idea of having a list of keys to remove from the map after you're done processing the map. The problem is you can't change the map in a loop of the map because the iterator will have an undefined behaviour. Some implementations of stl invalidate the iterator, and others do weirdness. – Kieveli Jun 10 '14 at 18:54
Just being clever about the iterator increment inside the loop should do the trick. – juanchopanza Jun 10 '14 at 18:55
@Milo: Thanks to juanchopanza's excellent point about `std::remove_if` not being applicable to a `std::map`, I've written a new way of going about this. std::map::erase returning an iterator is a C++11 feature, but it sounds like VC++2008 supports it. – Bill Lynch Jun 10 '14 at 18:57
Hmmm... I'm still getting the same error with this new solution. – jmasterx Jun 10 '14 at 18:58
@Milo: Other than what has been talked about here, I don't see any additional issues. – Bill Lynch Jun 10 '14 at 19:01

Map Not Incrementable Error

1 Answers1