Question

trying to solve a mem leak I’ve changed this:

void addListener(IListener* listener, const std::string& appender )
{
     std::pair<t_ListenerVectorMap::iterator,bool> p =
    listenerVectorMap.insert(std::string(appender), new t_ListenerVector);
    p.first->second->push_back(listener);

    //add the appender
    addAppender(appender);
}

By this:

void addListener( IListener* listener,const std::string& appender )
{
    t_ListenerVector* lv = new t_ListenerVector;
    std::pair<t_ListenerVectorMap::iterator,bool> p =
    listenerVectorMap.insert(std::string(appender), lv);

    p.first->second->push_back(listener);
    if (!p.second)
    {
        delete lv;
    }

    addAppender(appender);
}

The leak I’m trying to get ride is the new t_ListenerVector (is a boost::ptr_vector). When appender already exists into listenerVectorMap, then insert operation doesn’t not modify contents of map, so new t_ListenerVector isn’t inserted and becomes a mem leak.

The change is to avoid this leak. If no new insertion as been done, lv is not inserted so I do remove it. But the problem is that delete lv throws an assertion (in vc\include\vector Line:70).
If lv hasn’t been changed, why this error?

Some ideas are welcome.
Thanks

Answer 1

For this answer I am assuming that the type of t_ListenerVectorMap is boost::ptr_map<std::string, boost::ptr_vector<IListener>>. If it isn’t then this answer is probably worthless.

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You have a misunderstanding about how boost::ptr_map works. It is true that your original code contains a memory leak, but your replacement code is even worse. listenerVectorMap.insert(std::string(appender), lv) makes listenerVectorMap take ownership of lv, regardless of whether lv is inserted or not. This means that the delete lv; line results in a double-deletion.

To fix the memory leak in the first version of your code, all you have to do is this:

void addListener(IListener* listener, const std::string& appender )
{
    std::auto_ptr<IListener> ls(listener);
    //The original code leaked here if the std::string constructor threw.
    std::pair<t_ListenerVectorMap::iterator,bool> p =
        listenerVectorMap.insert(appender, new t_ListenerVector());
    p.first->second->push_back(ls);

    //add the appender
    addAppender(appender);
}