how to use Qtconcurrent::run with either a member function or maybe a lamdba function

[JonB]

@Dean21 said in how to use Qtconcurrent::run with either a member function or maybe a lamdba function:

QMMqttClient client;

I don't know whether it's relevant to whatever your issue is, but are you aware this is a local variable in the Widget::Widget() constructor and goes out of scope at the end of that constructor?

[Christian Ehrlicher]

@JonB said in how to use Qtconcurrent::run with either a member function or maybe a lamdba function:

constructor and goes out of scope at the end of that constructor?

This is what I'm try to tell him since my first post... :(

How long do you think is the livetime of this (function local) object?

[JonB]

@Christian-Ehrlicher said in how to use Qtconcurrent::run with either a member function or maybe a lamdba function:

This is what I'm try to tell him since my first post... :(

Yep, just seen!

[Dean21]

@Christian-Ehrlicher and @JonB I have tried moving QMMqttClient client; to globally defined and globally defined as static and still have the same result

[Christian Ehrlicher]

And did you add it as member as the people @so told you? Please post your code after you did it.

[Dean21]

@Christian-Ehrlicher

This was the minimal producible example where it still crashes

#include "widget.h"
#include "./ui_widget.h"
#include <QPalette>
#include <QDebug>
#include <QTimer>
//#include <QFuture>
//#include <QThreadPool>
//#include <QtConcurrent/QtConcurrent>

#include <string>
#include <cstring>
#include <iostream>
#include <time.h>
#include </home/dave/mosquitto/include/mosquitto.h> //needed for mosquitto MQTT

//-----for qt qrapper for mosquitto-----
#include <QCoreApplication>
#include <QTime>
#include "QMMqttClient.h"
//--------------------------------------

Widget::Widget(QWidget *parent)
   : QWidget(parent)
   , ui(new Ui::Widget)

{
   ui->setupUi(this);

//----------To set background colour----------
   QPalette p(palette());
   p.setColor(QPalette::Window, Qt::darkGray);
   setPalette(p);
//--------------------------------------------


//-------------To set Font and Size-------------
   QFont f( "Ubuntu Regular", 13, QFont::Bold);
   ui->LV_Title->setFont(f);
   ui->HV_Title->setFont(f);
   ui->Currently_set_LV->setFont(f);
   ui->Currently_set_HV->setFont(f);
   ui->Light_Level_Title->setFont(f);
   ui->Number_of_photons_Title->setFont(f);
   ui->Light_Level_before_amp_Title->setFont(f);


   ui->Currently_set_LV_line->setReadOnly(true);
   ui->Currently_set_HV_line->setReadOnly(true);
   ui->Light_Level_Line_Edit->setReadOnly(true);
   ui->Number_of_Photons_Line_Edit->setReadOnly(true);
//-----------------------------------------------





       QObject::connect(&client, &QMMqttClient::onConnected, [&client](){
           qDebug() << Q_FUNC_INFO << " QMMqttClient::onConnected handler, subscribe to topic...";
           client.subscribeTopic("command/stop");
       });

       QObject::connect(&client, &QMMqttClient::onMessageReceived, [](const QString &topic, const QByteArray &msg) {
           qDebug() << Q_FUNC_INFO << " QMMqttClient::onMessageReceived: topic: " << topic << ", message: " << QString::fromStdString(msg.toStdString());
       });

       /* An example of unsecure connection */
       client.initialize("12", "130.246.58.64", 1883);




       client.connect();

  /

   //timer->start();

}

Widget::~Widget()
{
   delete ui;
}

and here is the widget.h file where I tried to add as a member

#ifndef WIDGET_H
#define WIDGET_H

#include <QWidget>
#include <string>
#include "QMMqttClient.h"

QT_BEGIN_NAMESPACE
namespace Ui { class Widget; }
QT_END_NAMESPACE

class Widget : public QWidget
{
    Q_OBJECT

public:
    Widget(QWidget *parent = nullptr);
    ~Widget();
    QMMqttClient client;

private slots:

private:
    Ui::Widget *ui;


};
#endif // WIDGET_H

But adding the QMMqttClient client; line here produces errors:

capture of non variable Widget::client,
This was not captured by this lamdba function
Invalid use of non-static data member Widget::client

[JonB]

@Dean21 said in how to use Qtconcurrent::run with either a member function or maybe a lamdba function:

   QObject::connect(&client, &QMMqttClient::onConnected, [&client](){
       qDebug() << Q_FUNC_INFO << " QMMqttClient::onConnected handler, subscribe to topic...";
       client.subscribeTopic("command/stop");
   });

I'm not sure whether that [&client] doesn't pass as pointer rather than reference? Try [=] or [client] or even [&], does that make it compile??

Alternatively try passing this for slot object:

QObject::connect(&client, &QMMqttClient::onConnected, this, [&client](){

does that make it compile?

[Dean21]

@JonB using [=] and [client] both introduce errors, using [&] didnt add errors but doesnt fix it crashing
and using this for slot object also still causes crash :(

[jsulm]

@Dean21 If client is member of the class then simply capture this:

connect(&client, &QMMqttClient::onConnected, this, [this](){ // Use client here

If you get error then please post those....

[Dean21]

@jsulm said in how to use Qtconcurrent::run with either a member function or maybe a lamdba function:

connect(&client, &QMMqttClient::onConnected, this, this{ // Use client here

Hi jsulm, thanks for your help, but that didnt work, the error message was

client' is not captured 

and it the error says the error is on the client.subscribeTopic("command/stop") ; line

[jsulm]

@Dean21 Please show your current code.
This code works for me just fine (ui is member of the class calling connect):

connect(ui->pushButton, &QPushButton::pressed, [this]() { ui->pushButton->setText("DONE"); });

[JonB]

@Dean21
Let's start with: did you actually delete the QMMqttClient client; statement in the constructor?

[Dean21]

@JonB yes sorry I forgot to uncomment this i think this is working now

[Dean21]

@jsulm sorry i forgot to comment a line of code, this is working now thank you so much for your help I was really stuck and dont think I would have fixed without help from everyone here thank you.
But would this prevent any functionality of any other code from working, for example I have a lineEdit box and once I enter a value its suppose to show in another lineEdit box but that functionality seems to have stopped working, since this new part started working

[jsulm]

@Dean21 Please show your current code if something does not work...