Concurrent Run

The QtConcurrent::run() function runs a function in a separate thread. The return value of the function is made available through the QFuture API.

This function is a part of the Qt Concurrent framework.

Running a Function in a Separate Thread

To run a function in another thread, use QtConcurrent::run():

extern void aFunction();
QFuture<void> future = QtConcurrent::run(aFunction);

This will run aFunction in a separate thread obtained from the default QThreadPool. You can use the QFuture and QFutureWatcher classes to monitor the status of the function.

To use a dedicated thread pool, you can pass the QThreadPool as the first argument:

extern void aFunction();
QThreadPool pool;
QFuture<void> future = QtConcurrent::run(&pool, aFunction);

Passing Arguments to the Function

Passing arguments to the function is done by adding them to the QtConcurrent::run() call immediately after the function name. For example:

extern void aFunctionWithArguments(int arg1, double arg2, const QString &string);

int integer = ...;
double floatingPoint = ...;
QString string = ...;

QFuture<void> future = QtConcurrent::run(aFunctionWithArguments, integer, floatingPoint, string);

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

Returning Values from the Function

Any return value from the function is available via QFuture:

extern QString functionReturningAString();
QFuture<QString> future = QtConcurrent::run(functionReturningAString);
QString result = future.result();

As documented above, passing arguments is done like this:

extern QString someFunction(const QByteArray &input);

QByteArray bytearray = ...;

QFuture<QString> future = QtConcurrent::run(someFunction, bytearray);
QString result = future.result();

Note that the QFuture::result() function blocks and waits for the result to become available. Use QFutureWatcher to get notification when the function has finished execution and the result is available.

Additional API Features

Using Member Functions

QtConcurrent::run() also accepts pointers to member functions. The first argument must be either a const reference or a pointer to an instance of the class. Passing by const reference is useful when calling const member functions; passing by pointer is useful for calling non-const member functions that modify the instance.

For example, calling QByteArray::split() (a const member function) in a separate thread is done like this:

// call 'QList<QByteArray>  QByteArray::split(char sep) const' in a separate thread
QByteArray bytearray = "hello world";
QFuture<QList<QByteArray> > future = QtConcurrent::run(bytearray, &QByteArray::split, ',');
QList<QByteArray> result = future.result();

Calling a non-const member function is done like this:

// call 'void QImage::invertPixels(InvertMode mode)' in a separate thread
QImage image = ...;
QFuture<void> future = QtConcurrent::run(&image, &QImage::invertPixels, QImage::InvertRgba);
// At this point, the pixels in 'image' have been inverted

Using Bound Function Arguments

You can use std::bind() to bind a number of arguments to a function when called. If C++11 support is not available, boost::bind() or std::tr1::bind() are suitable replacements.

There are number of reasons for binding:

  • To call a function that takes more than 5 arguments.
  • To simplify calling a function with constant arguments.
  • Changing the order of arguments.

See the documentation for the relevant functions for details on how to use the bind API.

Calling a bound function is done like this:

void someFunction(int arg1, double arg2);
QFuture<void> future = QtConcurrent::run(std::bind(someFunction, 1, 2.0));

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