QThreadStorage Class

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# QThreadStorage Class Reference

The QThreadStorage class provides per-thread data storage. More...

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

`#include <qthreadstorage.h>`

List of all member functions.

## Public Members

- QThreadStorage ()
- ~QThreadStorage ()
- bool hasLocalData () const
- T & localData ()
- T localData () const
- void setLocalData ( T data )

## Detailed Description

The QThreadStorage class provides per-thread data storage.

QThreadStorage is a template class that provides per-thread data storage.

Note that due to compiler limitations, QThreadStorage can only store pointers.

The setLocalData() function stores a single thread-specific value for the calling thread. The data can be accessed later using the localData() functions. QThreadStorage takes ownership of the data (which must be created on the heap with new) and deletes it when the thread exits (either normally or via termination).

The hasLocalData() function allows the programmer to determine if data has previously been set using the setLocalData() function. This is useful for lazy initializiation.

For example, the following code uses QThreadStorage to store a single cache for each thread that calls the cacheObject() and removeFromCache() functions. The cache is automatically deleted when the calling thread exits (either normally or via termination).

        QThreadStorage<QCache<SomeClass> *> caches;

        void cacheObject( const QString &key, SomeClass *object )
        {
            if ( ! caches.hasLocalData() )
                caches.setLocalData( new QCache<SomeClass> );

            caches.localData()->insert( key, object );
        }

        void removeFromCache( const QString &key )
        {
            if ( ! caches.hasLocalData() )
                return; // nothing to do

            caches.localData()->remove( key );
        }

### Caveats

- As noted above, QThreadStorage can only store pointers due to compiler limitations. Support for value-based objects will be added when the majority of compilers are able to support partial template specialization.

- The destructor does not delete per-thread data. QThreadStorage only deletes per-thread data when the thread exits or when setLocalData() is called multiple times.

- QThreadStorage can only be used with threads started with QThread. It cannot be used with threads started with platform-specific APIs.

- As a corollary to the above, platform-specific APIs cannot be used to exit or terminate a QThread using QThreadStorage. Doing so will cause all per-thread data to be leaked. See QThread::exit() and QThread::terminate().

- QThreadStorage can be used to store data for the main() thread after QApplication has been constructed. QThreadStorage deletes all data set for the main() thread when QApplication is destroyed, regardless of whether or not the main() thread has actually finished.

- The implementation of QThreadStorage limits the total number of QThreadStorage objects to 256. An unlimited number of threads can store per-thread data in each QThreadStorage object.

See also Environment Classes and Threading.

## Member Function Documentation

### QThreadStorage::QThreadStorage ()

Constructs a new per-thread data storage object.

### QThreadStorage::~QThreadStorage ()

Destroys the per-thread data storage object.

Note: The per-thread data stored is not deleted. Any data left in QThreadStorage is leaked. Make sure that all threads using QThreadStorage have exited before deleting the QThreadStorage.

See also hasLocalData().

### bool QThreadStorage::hasLocalData () const

Returns TRUE if the calling thread has non-zero data available; otherwise returns FALSE.

See also localData().

### T & QThreadStorage::localData ()

Returns a reference to the data that was set by the calling thread.

Note: QThreadStorage can only store pointers. This function returns a reference to the pointer that was set by the calling thread. The value of this reference is 0 if no data was set by the calling thread,

See also hasLocalData().

### T QThreadStorage::localData () const

This is an overloaded member function, provided for convenience. It behaves essentially like the above function.

Returns a copy of the data that was set by the calling thread.

Note: QThreadStorage can only store pointers. This function returns a pointer to the data that was set by the calling thread. If no data was set by the calling thread, this function returns 0.

See also hasLocalData().

### void QThreadStorage::setLocalData ( T data )

Sets the local data for the calling thread to data. It can be accessed later using the localData() functions.

If data is 0, this function deletes the previous data (if any) and returns immediately.

If data is non-zero, QThreadStorage takes ownership of the data and deletes it automatically either when the thread exits (either normally or via termination) or when setLocalData() is called again.

Note: QThreadStorage can only store pointers. The data argument must be either a pointer to an object created on the heap (i.e. using new) or 0. You should not delete data yourself; QThreadStorage takes ownership and will delete the data itself.

See also localData() and hasLocalData().

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