Concurrent map equivalent

Defohin

Be aware that not all asynchronous computations can be canceled. For example, the QFuture returned by QtConcurrent::run() cannot be canceled; but the QFuture returned by QtConcurrent::mappedReduced() can.

:(

mrjj

@Defohin
Oh. not all can be canceled. :(
Well if u need to be able to stop at random then
threads might be the best solution.

Defohin

I don't want the pause, continue or the stop function, I just want to run a list of string over a method and pass additional arguments to it in parallel.
Using QtConcurrent::map won't work cause I can't pass additional arguments.
Using QtConcurrent::run won't work cause I'll need to call waitForFinished and it will block the GUI thread... (QFutureWatcher::cancel or QFuture::cancel won't work either cause it's not possible to cancel a QFuture returned by QtConcurrent::run)...

What can I do? I'm out of ideas.

Devopia53

@Defohin

like this:

QStringList names = { "john", "jane" }; // global or member variable

void YourClass::startJobs()
{
    QString extra = "doe";

    auto example = [extra] (QString &name) {
        qDebug() << name << extra;
    };

    QFuture<void> future = QtConcurrent::map(names, example);
}

mrjj

Hi
If cancel is a MUST have, then QThreads seems the only option :(

Defohin

@Devopia53 It's still crashing for me.
@mrjj I don't want to use cancel, as i said, I just want to run in parallel the method for each item of a string list and pass a few additional arguments. No need to cancel.

mrjj

@Defohin said in Concurrent map equivalent:

No need to cancel.

By cancel i mean to terminate before its finished. Like closing program.
In that regards you need "cancel" :)

Defohin

@mrjj Yes, if the application is closed I want to finish the threads as well. I thought it was a default behavior.

Can you give me an example of what I can do? I'm trying to use QRunnable, but I have no idea if it's going to work.

VRonin

@Defohin said in Concurrent map equivalent:

Using QtConcurrent::map won't work cause I can't pass additional arguments.

Just use a function object

struct MapHelper{
MapHelper(){}
QString m_additionalArgument1;
QString m_additionalArgument2;
 typedef QString result_type;
    QString operator()(const QString &val)
    {
        return m_additionalArgument1+val+additionalArgument2;
    }
}

MapHelper helper; // TODO: make sure to manage its lifecycle
helper.m_additionalArgument1 = "Prefix ";
helper.m_additionalArgument2 = " Suffix";
QtConcurrent::map(list_of_names,helper);

Defohin

@VRonin How to get the return from each one now?

struct NameHelper
{
    NameHelper(const QString &extra) : _extra(extra) { }

    typedef QString result_type;

    QString operator()(const QString &name)
    {
        return QString("Hello %1 %2").arg(name).arg(_extra);
    }

    QString _extra;
};

QStringList names = { "john", "jane" };
QString extra = "doe";

QFuture<QString> example = QtConcurrent::mapped(names, NameHelper(extra));

example.result() is returning only "Hello john doe".

VRonin

You can achieve the same with C++11 std::bind:

QString concatenate(const QString& prefix,const QString& val,const QString& suffix){return prefix+val+suffix;}
QtConcurrent::mapped(list_of_names,std::bind(concatenate,"Prefix ",std::placeholders::_1," Suffix"));

How to get the return from each one now?
[...]
example.result() is returning only "Hello john doe".

use results() instead of result()

Defohin

It's working nicely, but one last question; Is it GUI blocking? I tried to use a for loop inside of the function and it's now showing anything until it's finished.

VRonin

@Defohin said in Concurrent map equivalent:

Is it GUI blocking?

no, unless you force it to be

I tried to use a for loop inside of the function

could you post the code?

Defohin

struct NameHelper
{
    NameHelper(const QString &extra) : _extra(extra) { }

    typedef QString result_type;

    QString operator()(const QString &name)
    {
        for(int i = 0; i < 999999; ++i) {
            qDebug() << "block";
        }

        return QString("Hello %1 %2").arg(name).arg(_extra);
    }

    QString _extra;
};

QStringList names = { "john", "jane" };
QString extra = "doe";

QFuture<QString> example = QtConcurrent::mapped(names, NameHelper(extra));

VRonin

operator() will be executed in another thread, it will not block the GUI thread, it will not return until completed (of course)

Defohin

@VRonin But why is it not appearing the window If it's running in another thread?

VRonin

@Defohin said in Concurrent map equivalent:

But why is it not appearing the window If it's running in another thread?

WOW! we are taking this on a whole new level here! you need a QFutureWatcher on the QFuture and a slot connected to the resultReadyAt signal to display the results in the GUI

Defohin

You are a god, thank you.

VRonin

I definitely am not, trust me.

Just as a final remark, in your case, if you can use C++11, I'd use std::bind over the function object

Defohin

I'm using std::bind now... I realized something:
THe threads is not closing if the application closes, meaning that if I close the window it will still open. (if I do a heavy work inside the called function).