Emitting signals from unrelated file causes break

Contempt

I am attempting to emit multiple of the same signal in one function. Is this not possible, or am I doing it wrong?
Function with Emitting:

void mydialog::testfunction(int t, int b)
{
	emit testsignal("Hello, World!");
	Sleep(2000);
	qDebug("Number: %d/%d", t, b);
	Sleep(10000);
	emit testsignal("Hello, World2!");
}

Calling it

	QtConcurrent::run(this, &mydialog::testfunction, 9, 8);

Error thrown after the second emit (12 Seconds Later)

Exception thrown at 0x000000005391BFEE (Qt5Core.dll) in legit.exe: 0xC0000005: Access violation reading location 0x0000000000000023. occurred

If anymore info is needed then please ask.

bludger

My first bet would be that the instance of mydialog gets deleted/out-of-scope while the signal still must be delivered. But I'm not sure, I need to see some more code then.

Contempt

This post is deleted!

Contempt

This post is deleted!

Contempt

@bludger With more testing and moving my tests to a new project I think i've come up with a more narrow scenario so you can help me easier. I've now switch from emitting from a closing dialog to emitting from an unrelated file, yet I still hit breakpoints during the emit, but this time even if I emit once.
emittertest.h

#include <QObject>

class emittertest : public QObject
{
	Q_OBJECT
public:
	void fnc(QString r);
signals:
	void fnctest(QString);
private:
	int value;
};

emittertest.cpp

#include "emittertest.h"
#include "qdebug.h"
#include <windows.h>
void emittertest::fnc(QString t)
{
	qDebug() << t << "Thanks";
	//emit fnctest("Word");
	emit fnctest(QString("Rio"));
}

My app's .cpp file

void gq(QString Rex)
{
	while (true) {
		qDebug() << "Name:" << Rex << "Thread:" << QThread::currentThread();
		Sleep(2000);
	}
}
void QtTestApp::myslot(QString t)
{
	qDebug() << "Signal Emitted: " << t;
}
QtTestApp::QtTestApp(QWidget *parent)
	: QMainWindow(parent)
{
	ui.setupUi(this);
	emittertest b;
	connect(&b, SIGNAL(fnctest(QString)), this, SLOT(myslot(QString)));
	QFuture<void> f2 = QtConcurrent::run(&b, &emittertest::fnc, QString("NAME"));

}

Error

Exception thrown at 0x0000000063DFCE09 (Qt5Cored.dll) in QtTestApp.exe: 0xC0000005: Access violation reading location 0x000001FA8800F021. occurred

bludger

The emittertest variable gets out of scope before the signal is emitted. Upon connect you use the address of b so Qt's metasystem uses that address until the signal and slot are being disconnected. When leaving the constructor all local variables are going out-of-scope and so does b, meaning that the address of b became invalid.
A quick fix would be moving the local variable b to be a class member variable. Or making it a pointer instead so the object lives on the heap (make sure you delete it properly to avoid any memory leaks).

class QtTestApp : public QMainWindow
{
   Q_OBJECT
....
private:
   emittertest m_b;
}

Or as a pointer: (also a class member)

class QtTestApp : public QMainWindow
{
   Q_OBJECT
...
private:
   emittertest* m_b;
}

Contempt

@bludger Trying it your way and I get these errors

Severity	Code	Description	Project	File	Line	Suppression State
Error	C2065	'm_b': undeclared identifier	QtTestApp	C:\Users\Contempt\source\repos\QtTestApp\QtTestApp\QtTestApp.cpp	21	

Severity	Code	Description	Project	File	Line	Suppression State
Error	C4430	missing type specifier - int assumed. Note: C++ does not support default-int	QtTestApp	c:\users\contempt\source\repos\qttestapp\qttestapp\QtTestApp.h	15	

Severity	Code	Description	Project	File	Line	Suppression State
Error	C3646	'm_b': unknown override specifier	QtTestApp	c:\users\contempt\source\repos\qttestapp\qttestapp\QtTestApp.h	15

My QtTestApp.h

class QtTestApp : public QMainWindow
{
	Q_OBJECT

public:
	QtTestApp(QWidget *parent = Q_NULLPTR);
	private slots:
	void myslot(QString);
private:
	emittertest m_b;
	Ui::QtTestAppClass ui;
};

And I can not run emittertest::fnc() in a QtConcurrent thread anymore.

VRonin

It's still a race condition as there is nothing stopping your QMainWindow destroying before QtConcurrent::run finished. There is a way of making it work even with QtConcurrent but it's harder than just using QThread: see https://mayaposch.wordpress.com/2011/11/01/how-to-really-truly-use-qthreads-the-full-explanation/

bludger

@Contempt I guess you forgot to include the emittertest header file in QtTestApp.h. As long as it is not known within the QtTestApp header you will get the errors you're reporting.