Qt offers many classes and functions for working with threads. Below are four different approaches that Qt programmers can use to implement multithreaded applications.

QThread: Low-Level API with Optional Event Loops

QThread is the foundation of all thread control in Qt. Each QThread instance represents and controls one thread.

QThread can either be instantiated directly or subclassed. Instantiating a QThread provides a parallel event loop, allowing QObject slots to be invoked in a secondary thread. Subclassing a QThread allows the application to initialize the new thread before starting its event loop, or to run parallel code without an event loop.

See the QThread class reference and the threading examples for demonstrations on how to use QThread.

QThreadPool and QRunnable: Reusing Threads

Creating and destroying threads frequently can be expensive. To reduce this overhead, existing threads can be reused for new tasks. QThreadPool is a collection of reuseable QThreads.

To run code in one of a QThreadPool's threads, reimplement QRunnable::run() and instantiate the subclassed QRunnable. Use QThreadPool::start() to put the QRunnable in the QThreadPool's run queue. When a thread becomes available, the code within QRunnable::run() will execute in that thread.

Each Qt application has a global thread pool, which is accessible through QThreadPool::globalInstance(). This global thread pool automatically maintains an optimal number of threads based on the number of cores in the CPU. However, a separate QThreadPool can be created and managed explicitly.

Qt Concurrent: Using a High-level API

The Qt Concurrent module provides high-level functions that deal with some common parallel computation patterns: map, filter, and reduce. Unlike using QThread and QRunnable, these functions never require the use of low-level threading primitives such as mutexes or semaphores. Instead, they return a QFuture object which can be used to retrieve the functions' results when they are ready. QFuture can also be used to query computation progress and to pause/resume/cancel the computation. For convenience, QFutureWatcher enables interactions with QFutures via signals and slots.

Qt Concurrent's map, filter and reduce algorithms automatically distribute computation across all available processor cores, so applications written today will continue to scale when deployed later on a system with more cores.

This module also provides the QtConcurrent::run() function, which can run any function in another thread. However, QtConcurrent::run() only supports a subset of features available to the map, filter and reduce functions. The QFuture can be used to retrieve the function's return value and to check if the thread is running. However, a call to QtConcurrent::run() uses one thread only, cannot be paused/resumed/canceled, and cannot be queried for progress.

See the Qt Concurrent module documentation for details on the individual functions.

WorkerScript: Threading in QML

The WorkerScript QML type lets JavaScript code run in parallel with the GUI thread.

Each WorkerScript instance can have one .js script attached to it. When WorkerScript.sendMessage() is called, the script will run in a separate thread (and a separate QML context). When the script finishes running, it can send a reply back to the GUI thread which will invoke the WorkerScript.onMessage() signal handler.

Using a WorkerScript is similar to using a worker QObject that has been moved to another thread. Data is transferred between threads via signals.

See the WorkerScript documentation for details on how to implement the script, and for a list of data types that can be passed between threads.

Choosing an Appropriate Approach

As demonstrated above, Qt provides different solutions for developing threaded applications. The right solution for a given application depends on the purpose of the new thread and the thread's lifetime. Below is a comparison of Qt's threading technologies, followed by recommended solutions for some example use cases.

Comparison of Solutions

Example Use Cases