QMutexLocker< Mutex > Class Template Reference

\inmodule QtCore More...

#include <qmutex.h>

Collaboration diagram for QMutexLocker< Mutex >:

Public Member Functions
Q_NODISCARD_CTOR	QMutexLocker (Mutex *) noexcept
	Constructs a QMutexLocker and locks mutex.
	~QMutexLocker () noexcept
	Destroys the QMutexLocker and unlocks the mutex that was locked in the constructor.
void	unlock () noexcept
	Unlocks this mutex locker.
void	relock () noexcept
	Relocks an unlocked mutex locker.
Mutex *	mutex () const noexcept
	Returns the mutex on which the QMutexLocker is operating.

Detailed Description

template<typename Mutex>
class QMutexLocker< Mutex >

\inmodule QtCore

The QMutexLocker class is a convenience class that simplifies locking and unlocking mutexes.

\threadsafe

Locking and unlocking a QMutex or QRecursiveMutex in complex functions and statements or in exception handling code is error-prone and difficult to debug. QMutexLocker can be used in such situations to ensure that the state of the mutex is always well-defined.

QMutexLocker should be created within a function where a QMutex needs to be locked. The mutex is locked when QMutexLocker is created. You can unlock and relock the mutex with unlock() and relock(). If locked, the mutex will be unlocked when the 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 retVal = 0;
 
    switch (flag) {
    case 0:
    case 1:
        retVal = moreComplexFunction(flag);
        break;
    case 2:
        {
            int status = anotherFunction();
            if (status < 0) {
                mutex.unlock();
                return -2;
            }
            retVal = status + flag;
        }
        break;
    default:
        if (flag > 10) {
            mutex.unlock();
            return -1;
        }
        break;
    }
 
    mutex.unlock();
    return retVal;
}

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 retVal = 0;
 
    switch (flag) {
    case 0:
    case 1:
        return moreComplexFunction(flag);
    case 2:
        {
            int status = anotherFunction();
            if (status < 0)
                return -2;
            retVal = status + flag;
        }
        break;
    default:
        if (flag > 10)
            return -1;
        break;
    }
 
    return retVal;
}

Now, the mutex will always be unlocked when the QMutexLocker object is destroyed (when the function returns since locker is an auto variable).

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<QMutex> locker;
 
public:
    SignalWaiter(QMutex *mutex)
        : locker(mutex)
    {
    }
 
    void waitForSignal()
    {
        ...
        while (!signalled)
            waitCondition.wait(locker.mutex());
        ...
    }
};

See also

QReadLocker, QWriteLocker, QMutex

Definition at line 331 of file qmutex.h.

Constructor & Destructor Documentation

QMutexLocker()

template<typename Mutex>

Q_NODISCARD_CTOR QMutexLocker< Mutex >::QMutexLocker ( Mutex * mutex )

inlineexplicitnoexcept

Constructs a QMutexLocker and locks mutex.

The mutex will be unlocked when the QMutexLocker is destroyed. If mutex is \nullptr, QMutexLocker does nothing.

See also

QMutex::lock()

Since

6.4

Move-constructs a QMutexLocker from other. The mutex and the state of other is transferred to the newly constructed instance. After the move, other will no longer be managing any mutex.

See also

QMutex::lock()

Definition at line 335 of file qmutex.h.

~QMutexLocker()

template<typename Mutex>

QMutexLocker< Mutex >::~QMutexLocker ( )

inlinenoexcept

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

See also

QMutex::unlock()

Definition at line 336 of file qmutex.h.

Member Function Documentation

mutex()

template<typename Mutex>

Mutex * QMutexLocker< Mutex >::mutex ( ) const

inlinenoexcept

Returns the mutex on which the QMutexLocker is operating.

Definition at line 340 of file qmutex.h.

relock()

template<typename Mutex>

void QMutexLocker< Mutex >::relock ( )

inlinenoexcept

Relocks an unlocked mutex locker.

See also

unlock()

Definition at line 339 of file qmutex.h.

unlock()

template<typename Mutex>

void QMutexLocker< Mutex >::unlock ( )

inlinenoexcept

Unlocks this mutex locker.

You can use relock() to lock it again. It does not need to be locked when destroyed.

See also

relock()

Definition at line 338 of file qmutex.h.

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The documentation for this class was generated from the following files:

• qtbase/src/corelib/thread/qmutex.h (1957597aa6bc6ebd8bd1f903389800897411ac5d)
• qtbase/src/corelib/thread/qmutex.cpp (ef73a5f175f8d51a0bacbdd30cacbd3b6d151d36)