- Qt 5.15
- Qt Core
- C++ Classes
- QThreadStorage
- Qt 5.15.19 Reference Documentation

# QThreadStorage Class

template <typename T> class QThreadStorage

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

Header: | #include <QThreadStorage>
qmake: | QT += core

- List of all members, including inherited members

Note: All functions in this class are thread-safe.

## Public Functions

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

## Detailed Description

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

The setLocalData() function stores a single thread-specific value for the calling thread. The data can be accessed later using localData().

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

If T is a pointer type, 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.

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.

     QThreadStorage<QCache<QString, SomeClass> > caches;

     void cacheObject(const QString &key, SomeClass *object)
     {
         caches.localData().insert(key, object);
     }

     void removeFromCache(const QString &key)
     {
         if (!caches.hasLocalData())
             return;

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

### Caveats

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

See also QThread.

## 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

If T is a pointer type, returns `true` if the calling thread has non-zero data available.

If T is a value type, returns whether the data has already been constructed by calling setLocalData or localData.

See also localData().

### T &QThreadStorage::localData()

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

If no data has been set, this will create a default constructed instance of type T.

See also setLocalData() and hasLocalData().

### T QThreadStorage::localData() const

This is an overloaded function.

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

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 T is a pointer type, 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.

See also localData() and hasLocalData().