Concurrent Task

The QtConcurrent::task returns an instance of an auxiliary class called QtConcurrent::QTaskBuilder. Normally, you don't need to create an instance of this class manually. The QtConcurrent::QTaskBuilder provides an interface to adjust different task parameters in a chain-like manner. This approach is known as a fluent interface.

You can just set the parameters you need and then kick a task off. In order to finalize the configuration of a task you must invoke QtConcurrent::QTaskBuilder::spawn. This function is non-blocking (i.e. returns a future object immediately), but it's not guaranteed that the task starts immediately. You can use the QFuture and QFutureWatcher classes to monitor the status of the task.

See more examples and explanations below.

To run a function in another thread, use QtConcurrent::QTaskBuilder::spawn:

QtConcurrent::task([]{ qDebug("Hello, world!"); }).spawn();

This will run a lambda function in a separate thread obtained from the default QThreadPool.

Invoking a function with arguments is done by passing them to QtConcurrent::QTaskBuilder::withArguments:

auto task = [](const QString &s){ qDebug() << ("Hello, " + s); };
QtConcurrent::task(std::move(task))
    .withArguments("world!")
    .spawn();

A copy of each argument is made at the point where QtConcurrent::QTaskBuilder::withArguments is called, and these values are passed to the thread when it begins executing the task. Changes made to the arguments after calling QtConcurrent::QTaskBuilder::withArguments are not visible to the thread.

If you want to run a function that accepts arguments by reference, you should use std::ref/cref auxiliary functions. These functions create thin wrappers around passed arguments:

QString s("Hello, ");
QtConcurrent::task([](QString &s){ s.append("world!"); })
    .withArguments(std::ref(s))
    .spawn();

Make sure that all wrapped objects live long enough. It is possible to get undefined behavior if a task outlives the object wrapped by std::ref/cref.

You can obtain the result of a task with the QFuture API:

auto future = QtConcurrent::task([]{ return 42; }).spawn();
auto result = future.result(); // result == 42

Note that QFuture::result() is a blocking call, it waits for the result to become available. Use QFutureWatcher to get a notification when the task has finished execution and the result is available.

In case you want to pass a result to another asynchronous task, you can use QFuture::then() to create a chain of dependent tasks. See the QFuture documentation for more details.

Using different types of callable objects▲

std::is_invocable_v<std::decay_t<Task>, std::decay_t<Args>...>

QVariant value(42);
auto result = QtConcurrent::task(&qvariant_cast<int>)
                  .withArguments(value)
                  .spawn()
                  .result(); // result == 42

QString result("Hello, world!");

QtConcurrent::task(&QString::chop)
    .withArguments(&result, 8)
    .spawn()
    .waitForFinished(); // result == "Hello"

auto result = QtConcurrent::task(std::plus<int>())
                  .withArguments(40, 2)
                  .spawn()
                  .result() // result == 42

struct CallableWithState
{
    void operator()(int newState) { state = newState; }

    // ...
};

// ...

CallableWithState object;

QtConcurrent::task(std::ref(object))
   .withArguments(42)
   .spawn()
   .waitForFinished(); // The object's state is set to 42

Using custom thread pool▲

QThreadPool pool;
QtConcurrent::task([]{ return 42; }).onThreadPool(pool).spawn();

Setting priority for a task▲

QtConcurrent::task([]{ return 42; }).withPriority(10).spawn();

QtConcurrent::task([]{ qDebug("Hello, world!"); }).spawn(FutureResult::Ignore);

void increment(QPromise<int> &promise, int i)
{
    promise.addResult(i + 1);
}

int result = QtConcurrent::task(&increment).withArguments(10).spawn().result(); // result == 11