Returning QReadLocker by value from a function?

CJha

Hi, I have a class where I use a QReadWriteLock to protect access to its data while reading and writing. Since the class has to emit signals when its data changes, I have made the QReadWriteLock private and only provided read-locking (lockForRead(), tryLockForRead(int timeout), and unlock()) access in its public interface. I would also like to provide a QReadLocker access to this lock, I am wondering if this is the correct way to provide that access:

.h file

class MyData : public QObject
{
    Q_OBJECT
public:
    QReadLocker getLocker();
private:
    QReadWriteLock lock;
}

.cpp file

QReadLocker MyData::getLocker() { return QReadLocker{&lock}; }

I have tested it in a simple class and it seems to work fine. But I am not 100% sure if this would hold in real-world usage because of the confusion about copy and move constructors when returning an object by value from a function in C++. Are there any downsides to providing QReadLocker object as a return value from a function?

Kent-Dorfman

@CJha

@CJha said in Returning QReadLocker by value from a function?:

Are there any downsides to providing QReadLocker object as a return value from a function?

Yes. It won't work. semaphores are special kinds of objects. By design, they should never be copied.

Take simple counting semaphore. the count indicates whether a resource can acquire it. They are usually nothing more than an integer that is guaranteed atomic access. when you copy the integer the references are still to the old object, not the new one.

On a larger note, I think your whole model is kind of flawed. Don't "wrap" semaphores. Therein lies the introduction of race conditions. Use a reference to the sole instance of a semaphore anywhere it is needed.