Need more help with C++ code syntax - QTConncurent and template

Anonymous_Banned275

My QtConcurrent works in principle.
Now for details - again strictly C++ code , not a Qt problem.

This is how I define "time consuming function".
For now I pass QStringList to it, not a problem passing QVector later.

QVector<QStringList> HCI_Scan_Dialog::HCI_Scan_Scan(QStringList *string_array)

This is how I implement the function and it works as expected.

    QStringList *stringList = new  QStringList();
    futureWatcher.setFuture( QtConcurrent::run(HCI, &HCI_Scan_Dialog::HCI_Scan_Scan,stringList));

 qDebug()<< "temporary debug pause "; // wait for completion 
 qDebug()<< "test apply delay function spin to each vector

The "problem" - AFTER the time consuming function completes I need the main thread to continue . And that is where I am stuck .

QtConcurrent::run(HCI, &HCI_Scan_Dialog::HCI_Scan_Scan,stringList) returns QVector <QStringList> and I have no idea how to write the code to access this return value - where ?

    futureWatcher.setFuture(   **HERE ??**  QtConcurrent::run(HCI, &HCI_Scan_Dialog::HCI_Scan_Scan,stringList));

I am thinking to implement "emit" to "switch back" to main process.
Not really sure where it belongs .
I am already monitoring few "time marks" using "emit" SIGNAL .

Or should I just let QtConcurent finish ?
It does "emit" "finished" SIGNAL already.
(still need to access the - QtConcurrent::run(HCI, &HCI_Scan_Dialog::HCI_Scan_Scan,stringList) return value - which takes about 10 seconds to get it .

Again I am asking for C++ (template / vector) syntax help or suggestions how to continue the main thread AFTER the time consuming function is done.

JKSH

@AnneRanch said in Need more help with C++ code syntax - QTConncurent and template:

QtConcurrent::run(HCI, &HCI_Scan_Dialog::HCI_Scan_Scan,stringList) returns QVector <QStringList> and I have no idea how to write the code to access this return value

Actually, QtConcurrent::run(HCI, &HCI_Scan_Dialog::HCI_Scan_Scan, stringList) returns QFuture<QVector<QStringList>> in your case. Your code already passes this correctly to your QFutureWatcher.

The important points are:

QFuture stores the return value of your function.
QFutureWatcher stores your QFuture.

So, you can retrieve the return value of HCI_Scan_Scan() by calling auto returnValue = futureWatcher.future().result()

See

I am thinking to implement "emit" to "switch back" to main process.

You don't need to implement or emit your own signal. Just connect a slot/function to QFutureWatcher's finished() signal. That should automatically run in the main thread after your 10 seconds have elapsed.

QVector<QStringList> HCI_Scan_Dialog::HCI_Scan_Scan(QStringList *string_array)

Instead of passing a QStringList* as a function parameter, it is good practice to pass const QStringList& instead. The same idea applies when passing a QVector or any other container.

Anonymous_Banned275

@JKSH said in Need more help with C++ code syntax - QTConncurent and template:

auto returnValue = futureWatcher.future().result()

Perhaps I missed something here , but I am getting this error

/media/d/QT/QT_PROJECT_CAT/CAT_V1/configuredialog.cpp:403: error: 'returnValue' does not name a type
     auto returnValue = futureWatcher.future().result();
          ^

Is "auto " type "standard" in Qt or some option I forgot to set ?

I need to read all the links you have posted , but I noticed that " it will block until finished" . If that is the case my usage of QtConcurent is NOT what I expected , but I will know better after I get this "auto " resolved.

jsulm

@AnneRanch said in Need more help with C++ code syntax - QTConncurent and template:

Is "auto " type "standard" in Qt or some option I forgot to set ?

This has nothing to do with Qt. "auto" is a C++ keyword and was introduced in C++11 if I remember correctly. What compiler do you use (compiler version is also important).

JKSH

@AnneRanch said in Need more help with C++ code syntax - QTConncurent and template:

Is "auto " type "standard" in Qt or some option I forgot to set ?

auto is a C++ keyword that asks the compiler to automatically deduce the datatype by looking at the surrounding code. In your case, the following two lines produce exactly the same outcome on a C++11-compliant compiler:

auto returnValue = futureWatcher.future().result();
QVector<QStringList> returnValue = futureWatcher.future().result();

As @jsulm mentioned, auto has been part of the C++ standard for 10 years (but it was widely available on common compilers years before that).

If your compiler is moderately old, you can enable C++11 by adding CONFIG += c++11 to your .pro file and re-running qmake. Nonetheless, we highly recommend that you use a recent version of your compiler + IDE, and that you take advantage of C++11 features in your code (which includes auto and powerful template features).

Anonymous_Banned275

@JKSH said in Need more help with C++ code syntax - QTConncurent and template:

QVector<QStringList> returnValue = futureWatcher.future().result();

Still no go.

/media/d/QT/QT_PROJECT_CAT/CAT_V1/configuredialog.cpp:402: error: 'class QFuture<void>' has no member named 'result'
     QVector<QStringList> returnValue = futureWatcher.future().result();
                                                               ^

More..
Adding
CONFIG += C++11

creates havoc with QT examples where "nullptr" is used . Hence I have a choice to rebuild the entire application and undefine "nullptr".
No thanks at this point - to late in the game.

PS
Thanks for the history lesson, however "auto" was introduced to C much . much earlier...

Pl45m4

@AnneRanch said in Need more help with C++ code syntax - QTConncurent and template:

class QFuture<void>' has no member named 'result'

You seem to use a QFuture<void> which has no result. In order to get any result from your QFutureWatcher's QFuture, you need to set a type.

Like QVector<T> ( -> QFuture<T>)

See here:

"Void" future, just the function is running with QtConcurrent:run, no return type.
(https://doc.qt.io/qt-5/qtconcurrentrun.html#passing-arguments-to-the-function)
QFuture is from type QFuture<QString> because the function, that runs in QtConcurrent:run is returning a QString
(https://doc.qt.io/qt-5/qtconcurrentrun.html#returning-values-from-the-function)

So check your QFutureWatcher and your QFuture setup.

Edit:

If this

QVector<QStringList> HCI_Scan_Dialog::HCI_Scan_Scan(QStringList *string_array)

is your function, your QFuture shouldn't be <void>. Does your function actually return a QVector<QStringList> / QStringList?

JonB

@AnneRanch said in Need more help with C++ code syntax - QTConncurent and template:

Thanks for the history lesson, however "auto" was introduced to C much . much earlier...

The auto keyword in C means a totally different thing from the auto we are talking about here in C++, introduced at C++11. It may be confusing that they chose to use the same keyword for a different purpose, but there you are.

The auto you are referring to in C is a storage/scope specifier. The auto we are talking about here in C++ is for type inference. Quite different.

Anonymous_Banned275

@Pl45m4 said in Need more help with C++ code syntax - QTConncurent and template:

@AnneRanch said in Need more help with C++ code syntax - QTConncurent and template:

class QFuture<void>' has no member named 'result'

You seem to use a QFuture<void> which has no result. In order to get any result from your QFutureWatcher's QFuture, you need to set a type.

Like QVector<T> ( -> QFuture<T>)

See here:

"Void" future, just the function is running with QtConcurrent:run, no return type.
(https://doc.qt.io/qt-5/qtconcurrentrun.html#passing-arguments-to-the-function)

QFuture is from type QFuture<QString> because the function, that runs in QtConcurrent:run is returning a QString
(https://doc.qt.io/qt-5/qtconcurrentrun.html#returning-values-from-the-function)

So check your QFutureWatcher and your QFuture setup.

Edit:

If this
QVector<QStringList> HCI_Scan_Dialog::HCI_Scan_Scan(QStringList *string_array)
is your function, your QFuture shouldn't be <void>. Does your function actually return a QVector<QStringList> / QStringList?

Seems that the discussion is back what I originally asked for help with

what is the correct syntax

to retrieve the data from "run " function " for further processing.
Yes, I did not clearly specify if each part is defined with correct types.

BTW at this point it is immaterial if the "run:function" actually returns anything - I need to have correct syntax and return types first.
.
I'll recheck my definitions to make sure it is correct.

JKSH

@AnneRanch said in Need more help with C++ code syntax - QTConncurent and template:

error: 'class QFuture<void>' has no member named 'result'

QFutureWatcher<void> does not expect a result (as implied by the void), hence the error.

Use QFutureWatcher<QVector<QStringList>> instead. (Also, double-check that HCI_Scan_Scan() indeed returns QVector<QStringList>)

@Pl45m4 said in Need more help with C++ code syntax - QTConncurent and template:

If this
QVector<QStringList> HCI_Scan_Dialog::HCI_Scan_Scan(QStringList *string_array)
is your function, your QFuture shouldn't be <void>. Does your function actually return a QVector<QStringList> / QStringList?

A QFutureWatcher<void> can watch any QFuture<T>. However, QFutureWatcher<void>::future() returns QFuture<void>, not the original QFuture<T>.

Pl45m4

@JKSH said in Need more help with C++ code syntax - QTConncurent and template:

However, QFutureWatcher<void>::future() returns QFuture<void>, not the original QFuture<T>.

Yes, that's what I wanted to say there :)

So future().result() on a QFuture<void> shouldn't return her QStringList or QStringList-vector, but I cant see where the <void> is coming from?! Must be a QFutureWatcher<void> somewhere in the code, where we cant see it?

JKSH

@Pl45m4 said in Need more help with C++ code syntax - QTConncurent and template:

I cant see where the <void> is coming from?! Must be a QFutureWatcher<void> somewhere in the code, where we cant see it?

@AnneRanch's original post contains a variable named futureWatcher but it didn't show the type declaration. Judging from the error message, the declaration must have been QFutureWatcher<void> futureWatcher;

That just needs to change to QFutureWatcher<QVector<QStringList>> futureWatcher;

Pl45m4

@JKSH

Found it :)

QFutureWatcher<void> is specialized to not contain any of the result fetching functions. Any QFuture<T> can be watched by a QFutureWatcher<void> as well. This is useful if only status or progress information is needed; not the actual result data.

See also QFuture and Qt Concurrent.

(https://doc.qt.io/qt-5/qfuturewatcher.html#details)

@AnneRanch

If you have a <void> type somewhere, you can't work with the result.

Anonymous_Banned275

FYI
After changing this

    // this is wrong
    //  QFutureWatcher<void> futureWatcher;
#ifdef BYPASS
    /media/d/QT/QT_PROJECT_CAT/CAT_V1/configuredialog.cpp:275: error: '>>' should be '> >' within a nested template argument list
                                                               QFutureWatcher<QVector<QStringList>> futureWatcher;
    ^
#endif
    QFutureWatcher<QVector<QStringList> > futureWatcher;

The "run" function works as expected, however , the supporting QConcurrent " 1 seconds ticks" needs work.

    qDebug()<< "test apply delay function spin to each vector                                  setup map ";
    // test map vector array with spin function
    futureWatcher.setFuture(QtConcurrent::map(vector, spin));

/media/d/QT/QT_PROJECT_CAT/CAT_V1/configuredialog.cpp:465: error: no matching function for call to 'QFutureWatcher<QVector<QStringList> >::setFuture(QFuture<void>)'
     futureWatcher.setFuture(QtConcurrent::map(vector, spin));
                                                            ^

I'll try to fix that myself...

Appreciate all the help , it has been revealing how things can get complex if one does not pay attentions to ALL PARTS of the code.

Pl45m4

@AnneRanch said in Need more help with C++ code syntax - QTConncurent and template:

The "run" function works as expected, however , the supporting QConcurrent " 1 seconds ticks" needs work.

Yes, because now you cant use your changed futureWatcher the same way as used by the Qt Example.
(spin function doesn't return a QStringList vector)

Anonymous_Banned275

@Pl45m4
Yes, I realized that. But I hacked it , at lest for now , by defining another
QFutureWatcher

   QFutureWatcher<QVector<QStringList> > futureWatcher_run;
    // 1 seconds tick 
    // keep curent SIGNAL / SLOT
    QFutureWatcher<void> futureWatcher;   // keep for  SIGNAL etc 1 second ticks

Now as I suspected - I have lost the "concurrency" - the "run" , as coded , blocks process and I have no "concurrent" 1 second ticks running in QProgressDialog.
More fun work.... perhaps just checking for "result" AFTER the "concurrent " is done will work. QProgressDialog sits on top on the area which eventually will show the results , so no hurry until it is gone from view.

Anonymous_Banned275

I need to discuss / clarify ONE more "problem" .

I hope I can explain "the problem" using my interpretations of Qt documentation. It is somewhat dupe of what I have said / posted before.

My "primary objective" was to utilize / write multi threaded code to account for a function which takes abut 10 seconds to execute .
I am using "QtConcurrent "framework " which does support multi threading .
I do "QtConcurrent::run" to run time consuming function in ONE thread AND futureWatcher.setFuture(QtConcurrent::map(vector, spin)); in additional FOUR threads.
As is - It all works as expected.

The "time consuming function" retrieves data for further processing.
I have a lengthy discussion elsewhere about how to pass / retrieve such data for such processing and it DOES work using following code:

QVector<QStringList> returnValue = futureWatcher_run.future().result();

The problem is , as far as I can identify it - the futureWatcher_run.future().result() no longer "runs " as multi threaded / QtConcurrent framework - it appears to run in main thread and is blocking.

Am I wrong ?

I do not have code implemented to actually see / debug the thread execution of the above code.

I do realize my problem is harder to visualize without full code, but how can I verify / change the "returnValue" code to make sure it runs in QtConcurrent framework and in its OWN thread , and not to be blocking?

    QStringList *stringList = new  QStringList(); // temporary pass QStringList
    QElapsedTimer timerHCI;
    timerHCI.start();

    futureWatcher_run.setFuture(QtConcurrent::run(HCI, &HCI_Scan_Dialog::HCI_Scan_Scan,stringList));
    
    
    // test returnValue
    int returnValueIndex = 0;
    int returnValueSize = 0;
    
    qDebug()<< "futureWatcher.setFuture(QtConcurrent::run(HCI, "
               "&HCI_Scan_Dialog::HCI_Scan_Scan,stringList)) elapsed time " <<  timerHCI.elapsed() ;
    
#ifdef BYPASS
    // takes real time
    QVector<QStringList> returnValue = futureWatcher_run.future().result();

J.Hilk

@AnneRanch I' don't quite understand your situation, why do you think its not running in its own thread?

Any way, if you want to, take a look at my git repo
https://github.com/DeiVadder/QtThreadExample

In that I have a project that covers all Qt ways of parallel processing, Related functions are number form 1 to 5.

Qt Concurrent is the number 4

Anonymous_Banned275

@J-Hilk said in Need more help with C++ code syntax - QTConncurent and template:

@AnneRanch I' don't quite understand your situation, why do you think its not running in its own thread?

Actually - that is silly for me to say.
Of course it is running in its own thread, problem is it is NOT QtConcurrent - one of he expected multi threads .
But I cannot simply prove it - the "debug" is getting too complicated.
I know for sure it is blocking - it is causing the main window to "go gray".
That is from my initial experience - when I was not running QtConcurrent correctly - the main window would turn gray at completion of the function.

I'll take a look at your code.
thanks.

Any way, if you want to, take a look at my git repo
https://github.com/DeiVadder/QtThreadExample

In that I have a project that covers all Qt ways of parallel processing, Related functions are number form 1 to 5.

Qt Concurrent is the number 4

JKSH

@AnneRanch said in Need more help with C++ code syntax - QTConncurent and template:

Appreciate all the help , it has been revealing how things can get complex if one does not pay attentions to ALL PARTS of the code.

You're welcome. Indeed, complexity tends to grow exponentially.

futureWatcher.setFuture(QtConcurrent::map(vector, spin));

You started this thread by asking about QtConcurrent::run(), and now you're adding QtConcurrent::map() to the mix. These two functions are very different.

Let's step back a bit. First, please describe in your own words:

What does QtConcurrent::run() do? What does it return?
What does QtConcurrent::map() do? What does it return?

perhaps just checking for "result" AFTER the "concurrent " is done will work.

Yes, you should do this.

The reason for the "block" is documented at https://doc.qt.io/qt-5/qfuture.html#result