Question

I’m checking for memory leaks in my Qt program using QtCreator and Valgrind. I am deleting a few entries in a QHash in my destructor like this:

QHash<QString, QVariant*> m_Hash;

/**
 * @brief
 * Destruct a Foo Class instance
 */
Foo ::~Foo ()
{

    // Do Cleanup here

    // Delete hash leftovers
    foreach( QString key, m_Hash.keys() )
    {

        qDebug() << "Deleting an entry..";

        // Delete the hash item
        delete m_Hash.take(key);

    }

}

If I debug with Valgrind this code is fine and deletes the contents when the destructor is called:

>> Deleting an entry.. 
>> Deleting an entry.. 

If I launch with GDB within QtCreator, launch without GDB from QtCreator, or just run my Qt App from the command line I get Segmentation Faults!

Signal name : 
SIGSEGV
Signal meaning : 
Segmentation fault

If I commend out the ‘delete’ line then I can run my app just fine using any method but I do leak memory.

What gives? Does valgrind introduce some sort of delay that allows my destructor to work? How can I solve this?

Answer 1

You can not modify the container you iterate over with foreach. Use iterators instead. Correct code using method iterator QHash::erase ( iterator pos ):

 QMap<QString, QVariant* >::iterator it = m_Hash.begin();
 // auto it = m_Hash.begin(); // in C++11
 while (it != m_Hash.end()) {
     delete it.value();
     it = m_Hash.erase(it);
 }

Also, any particular reason why you are storing QVariant pointers, instead of values? QVariant is usually suitable for keeping as value, since most data you’d store in QVariant is either implicitly shared, or small.