Solved QtConcurrent: function object with qhash as member

[JanW]

Hi,

I'm having some trouble with the QtConcurrent api. When I try to use a function object in the QtConcurrent::blockingMap function which contains a QHash as a member variable, my application crashes when i try to look up something in that qhash in the parallell call. My (little bit simplified) code looks like this:

struct ParentChildFunctionObject
{
	ParentChildFunctionObject(QHash<QUuid, QString> lookupHash)
		: _lookupHash(lookupHash)
	{}
	void operator()(const QUuid& guid)
	{
          //...
          QHash<QUuid, QString>::iterator it = _lookupHash.find(guid)  //->  crash here
          //...
	}
	QHash<QUuid, QString> _lookupHash
};

void ImportOperation::fillParentChildStructure()
{
	QtConcurrent::blockingMap(_myListOfQUuids, ParentChildFunctionObject(_myLookupHash));
        //_myListOfQUuids is a QVector<QUuid> and _myLookupHash is a QHash<QUuid, QString>
}

The crash occurs in the find method of the qhash (when I just assign a fixed value to the string i need, everything works fine).

What am I missing?
Thanks,

Jan

Ps: An alternative would be to give the string I need as an extra parameter in the blockingMap parallel called function. Is that possible?

Pps: Qt 5.12, msvc2017 compiler, Win10, but i don't think it matters :)

[Christian Ehrlicher]

Since find() is done on a non-const QHash the first thing which is done is detach(). Since detach() itself is not thread-safe you will maybe get a crash here due to concurrency.
When you don't modify the QHash I would pass it as const reference (this should be done in any case to avoid a copy) and store it as 'const QHash<QUuid, QString> _lookupHash' or even as 'QHash<QUuid, QString> &_lookupHash' so find() will work on a const container so it doesn't get detached.

[JanW]

That indeed seems to be the problem. After changing to const ref, it doesn't crash anymore.
Thanks!

Jan