TQMutexLocker Class Reference

The TQMutexLocker class simplifies locking and unlocking TQMutexes. More...

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

#include <qmutex.h>

List of all member functions.

Public Members

TQMutexLocker ( TQMutex * mutex )
~TQMutexLocker ()
TQMutex * mutex () const

Detailed Description

The TQMutexLocker class simplifies locking and unlocking TQMutexes.

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

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

For example, this complex function locks a TQMutex 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 TQMutexLocker greatly simplifies the code, and makes it more readable:

    int complexFunction( int flag )
    {
        TQMutexLocker 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 TQMutexLocker 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.

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

    class SignalWaiter
    {
    private:
        TQMutexLocker locker;

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

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

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

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

Member Function Documentation

TQMutexLocker::TQMutexLocker ( TQMutex * mutex )

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

TQMutexLocker::~TQMutexLocker ()

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

See also TQMutexLocker::TQMutexLocker() and TQMutex::unlock().

TQMutex * TQMutexLocker::mutex () const

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

Trademarks

TQt 3.3.8