QMutexLocker Class Reference

The QMutexLocker class simplifies locking and unlocking QMutexes. More...

All the functions in this class are thread-safe when Qt is built with thread support.

#include <qmutex.h>

List of all member functions.

Public Members

QMutexLocker ( QMutex * mutex )
~QMutexLocker ()
QMutex * mutex () const

Detailed Description

The QMutexLocker class simplifies locking and unlocking QMutexes.

The purpose of QMutexLocker is to simplify QMutex locking and unlocking. Locking and unlocking a QMutex in complex functions and statements or in exception handling code is error prone and difficult to debug. QMutexLocker should be used in such situations to ensure that the state of the mutex is well defined and always locked and unlocked properly.

QMutexLocker should be created within a function where a QMutex needs to be locked. The mutex is locked when QMutexLocker is created, and unlocked when QMutexLocker is destroyed.

For example, this complex function locks a QMutex upon entering the function and unlocks the mutex at all the exit points:

    int complexFunction( int flag )
    {
        mutex.lock();

        int return_value = 0;

        switch ( flag ) {
        case 0:
        case 1:
            {
                mutex.unlock();
                return moreComplexFunction( flag );
            }

        case 2:
            {
                int status = anotherFunction();
                if ( status < 0 ) {
                    mutex.unlock();
                    return -2;
                }
                return_value = status + flag;
                break;
            }

        default:
            {
                if ( flag > 10 ) {
                    mutex.unlock();
                    return -1;
                }
                break;
            }
        }

        mutex.unlock();
        return return_value;
    }

This example function will get more complicated as it is developed, which increases the likelihood that errors will occur.

Using QMutexLocker greatly simplifies the code, and makes it more readable:

    int complexFunction( int flag )
    {
        QMutexLocker locker( &mutex );

        int return_value = 0;

        switch ( flag ) {
        case 0:
        case 1:
            {
                return moreComplexFunction( flag );
            }

        case 2:
            {
                int status = anotherFunction();
                if ( status < 0 )
                    return -2;
                return_value = status + flag;
                break;
            }

        default:
            {
                if ( flag > 10 )
                    return -1;
                break;
            }
        }

        return return_value;
    }

Now, the mutex will always be unlocked when the QMutexLocker object is destroyed (when the function returns since locker is an auto variable). Note that the mutex will be unlocked after the call to moreComplexFunction() in this example, avoiding possible bugs caused by unlocking the mutex too early, as in the first example.

The same principle applies to code that throws and catches exceptions. An exception that is not caught in the function that has locked the mutex has no way of unlocking the mutex before the exception is passed up the stack to the calling function.

QMutexLocker also provides a mutex() member function that returns the mutex on which the QMutexLocker is operating. This is useful for code that needs access to the mutex, such as QWaitCondition::wait(). For example:

    class SignalWaiter
    {
    private:
        QMutexLocker locker;

    public:
        SignalWaiter( QMutex *mutex )
            : locker( mutex )
        {
        }

        void waitForSignal()
        {
            ...
            ...
            ...

            while ( ! signalled )
                waitcondition.wait( locker.mutex() );

            ...
            ...
            ...
        }
    };

Member Function Documentation

QMutexLocker::QMutexLocker ( QMutex * mutex )

Constructs a QMutexLocker and locks mutex. The mutex will be unlocked when the QMutexLocker is destroyed. If mutex is zero, QMutexLocker does nothing.

QMutexLocker::~QMutexLocker ()

Destroys the QMutexLocker and unlocks the mutex which was locked in the constructor.

See also QMutexLocker::QMutexLocker() and QMutex::unlock().

QMutex * QMutexLocker::mutex () const

Returns a pointer to the mutex which was locked in the constructor.