is it possible to use QtConcurrent::run() with a function member of a class

Question

I can't seem to be able to associate QtConcurrent::run() with a method (function member of a class) only with a simple function. How can I do this?

With a regular function I cannot emit signals and its a drag. Why would anyone find this a better alternative to QThread is beyond me and would like some input.

Answer 1

Yes, this is possible (and quite easy).

Here is an example (from the Qt documentation):

// call 'QStringList QString::split(const QString &sep, SplitBehavior behavior, Qt::CaseSensitivity cs) const' in a separate thread
QString string = ...;
QFuture<QStringList> future = QtConcurrent::run(string, &QString::split, QString(", "), QString::KeepEmptyParts, Qt::CaseSensitive);
...
QStringList result = future.result();

Basically, all you have to do is pass a pointer to the object as the first argument and the address of the method as the second argument (followed by any other arguments).

See: https://doc.qt.io/qt-5/qtconcurrentrun.html

Answer 2

The problem is that when you use a pointer to member function, you need to somehow provide the this parameter also (i.e., the object on which the member function should be called).

The syntax for this is quite difficult if you haven't used it before. It might be good to read http://www.parashift.com/c++-faq-lite/pointers-to-members.html .

Say you have a class Dog and a function Dog::walkTheDog(int howlong_minutes). Then you ought to be able to use std::bind1st and std::mem_fun to make it suitable for QtConcurrent::run:

Dog dog;
// Walk this dog for 30 minutes
QtConcurrent::run(std::bind1st(std::mem_fun(&Dog::walkTheDog), &dog), 30);

std::bind1st(std::mem_fun(&Dog::walkTheDog), &dog) returns a function-like object which has bound the member function to a particular dog. From that point you can use it much like you could use a standalone function.