QTConcurent and lambda in QT6

JulsPower

Now for my other question which is more to educate me than fix something

when I tried in my main thread to use db.open().then [] () function to get the return of the open function at a later point.
I get the correct return value, but I tried to show a QMessageBox::critical in case of error
it shows but the program crash in sigsev shortly after.

why is that? the then function is executed in the main thread right? not by the database qthreadpool one?

JulsPower

@Christian-Ehrlicher said in QTConcurent and lambda in QT6:

This usecase is weird - use a dedicated QThread then. As soon as someone else increases the thread count you will get into big trouble.

ill take that into consideration for sure I find how with he used qthreadpool instead of qthread

I don't see how someone could increase the threadcount those its limited to 1 by: m_pool.setMaxThreadCount(1);

JonB

@JulsPower said in QTConcurent and lambda in QT6:

it shows but the program crash in sigsev shortly after.

Run under a debugger or show your code. And make sure you call no UI code from your secondary thread.

Christian Ehrlicher

@JulsPower said in QTConcurent and lambda in QT6:

I don't see how someone could increase the threadcount those its limited to 1 by: m_pool.setMaxThreadCount(1);

Another developer who is modifying your code and don't know about such strange constraints for instance.

I get the correct return value, but I tried to show a QMessageBox::critical in case of error
it shows but the program crash in sigsev shortly after.

You must not show an ui element from outside the main (gui) thread.

JulsPower

@Christian-Ehrlicher said in QTConcurent and lambda in QT6:

You must not show an ui element from outside the main (gui) thread.

But the then function is run in the main thread that own the gui that shouldn't be the problem.

Christian Ehrlicher

@JulsPower said in QTConcurent and lambda in QT6:

But the then function is run in the main thread that own the gui that shouldn't be the problem.

You a) did not show us code and b) did not do what @JonB suggested so how should we help?

JulsPower

@JonB said in QTConcurent and lambda in QT6:

@JulsPower said in QTConcurent and lambda in QT6:

it shows but the program crash in sigsev shortly after.

Run under a debugger or show your code. And make sure you call no UI code from your secondary thread.

here is the code that open the db in the main thread (which call the thread to open the db in fact)

    m_jdb->open().then([&](const bool &b){
        if(!b)
            QMessageBox::critical(this, "Error", "Couldn't connect to DB");
    });

the debugger doesn't help me much here it look like a gui bug but if I breakpoint into the then function "this" is the mainwindow pointer

JulsPower

Here most of the related code
jdatabase.h

#ifndef JDATABASE_H
#define JDATABASE_H
#include <QThreadPool>
#include <QFuture>
#include <QSqlDatabase>
#include <QtConcurrent>


class JDatabase : public QObject
{
    Q_OBJECT

signals:
    void Connected();
    void FailToConnect();
public:
    JDatabase(QWidget *parent = nullptr);
    ~JDatabase();
    QFuture<bool> open();

private:
    QThreadPool m_pool;
};

#endif // JDATABASE_H

jdatabase.cpp

#include "jdatabase.h"

JDatabase::JDatabase(QWidget *parent)
{
    // limit to one thread
    m_pool.setMaxThreadCount(1);
    // prevent automatic deletion and recreation
    m_pool.setExpiryTimeout(-1);
}

JDatabase::~JDatabase()
{
//    m_pool.deleteLater();
}

QFuture<bool> JDatabase::open()
{

    //m_pool,
    return QtConcurrent::run(&m_pool, [&]()
    {
        QString connectString = "Driver={SQL Server};Server=192.168.21.174;Database=DBMachines;";

        auto db = QSqlDatabase::addDatabase("QODBC");
        db.setUserName("Julien");
        db.setPassword("MB6262");
        db.setDatabaseName(connectString);
        auto s= db.connectOptions();
        db.open();
        if(db.isOpen())
            emit Connected();
        else
            emit FailToConnect();
        return db.isOpen();
    });
}

mainwindow.h declaration

    JDatabase *m_jdb;

and in mainwindow.cpp

    //Connection base de donnée
    m_jdb = new JDatabase(this);
    connect(m_jdb, &JDatabase::Connected, this, [&](){QMessageBox::critical(this, "YAY", "Connecté, refresh"); RefreshDB();});
//    connect(m_jdb, &JDatabase::FailToConnect, this, [&](){QMessageBox::critical(this, "Erreur", "Impossible de ce connecter à la base de donnée");});
    m_jdb->open().then([&](const bool &b){
        if(!b)
            QMessageBox::critical(this, "Error", "Couldn't connect to DB");
    });

the signal work as intended but I also tought that the then function would work

thanks guys

Christian Ehrlicher

Take a look at the complete backtrace - I'm pretty sure it's not executed in the main thread as I don't see how this should be achieved.

JulsPower

@Christian-Ehrlicher said in QTConcurent and lambda in QT6:

Take a look at the complete backtrace - I'm pretty sure it's not executed in the main thread as I don't see how this should be achieved.

the breakpoint in the then lambda give me this

then I run the app the msgbox shows
and as soon my pointer the into the box I get sigsev

how could I find if its the right thread showing the msgbox?, did I understand the then function correctly?

Christian Ehrlicher

Take a breakpoint on 'b' and look at the backtrace to see if it is called from main() and/or check the currentThreadId() with the main .thread id (qApp->threadId())

JulsPower

@Christian-Ehrlicher said in QTConcurent and lambda in QT6:

Take a breakpoint on 'b' and look at the backtrace to see if it is called from main() and/or check the currentThreadId() with the main .thread id (qApp->threadId())

HI
what is the "backtrace"?
here is a breakpoint to b

should the then part be executed in the main loop?

Christian Ehrlicher

So as I already said in all of my previous post the message box is not drawn in the main thread. Fix it by e.g. using signals and slots or don't use threads at all.

JulsPower

@Christian-Ehrlicher said in QTConcurent and lambda in QT6:

So as I already said in all of my previous post the message box is not drawn in the main thread. Fix it by e.g. using signals and slots or don't use threads at all.

Hi
I understand the signal and slot and know how to use them for what I want to do
but I wanted to understand better the then function. from what I understood the qfuture would allow to execute code (lambda or a function call) at a later time when the result would have been received. but I don't get why it would be executed by the other thread.

thanks again im just trying to understand how thing work (or should work) so I can use those function correctly.

Christian Ehrlicher

@JulsPower said in QTConcurent and lambda in QT6:

but I don't get why it would be executed by the other thread.

Because there is no other way - the future is ready and can't simply switch the thread to the main thread (and there is even no reason to do so) so it's executed in the future's thread.

JulsPower

@Christian-Ehrlicher said in QTConcurent and lambda in QT6:

@JulsPower said in QTConcurent and lambda in QT6:

but I don't get why it would be executed by the other thread.

Because there is no other way - the future is ready and can't simply switch the thread to the main thread (and there is even no reason to do so) so it's executed in the future's thread.

then I cannot see a usage for future then function ...

Christian Ehrlicher

@JulsPower said in QTConcurent and lambda in QT6:

then I cannot see a usage for future then function

The documentation has a nice common use case for it.