How to send data between QThreads? - c++

Question

I have an event in one thread and need to invoke handler function in another. Usually I use function connect(), but in case of two threads I had an error:
QObject::connect: Cannot queue arguments of type 'QVector<unsigned char>'
(Make sure 'QVector<unsigned char>' is registered using qRegisterMetaType().)
I tried to use qRegisterMetaType(), but don't understood clearly how and where should I declare it.
I wrote code sample, which just invoke execution in thread0 from thead1. I didn't include my attempts of using qRegisterMetaType() because they all failed =)
thread0.h:
#ifndef THREAD0_H
#define THREAD0_H
#include <QThread>
#include <QVector>
class thread0 : public QThread
{
Q_OBJECT
public:
thread0();
~thread0();
protected:
void run() Q_DECL_OVERRIDE;
public slots:
void printBuff(QVector<unsigned char> vec);
};
#endif // THREAD0_H
thread1.h:
#ifndef THREAD1_H
#define THREAD1_H
#include <QThread>
#include <QVector>
class thread1 : public QThread
{
Q_OBJECT
public:
thread1();
~thread1();
protected:
void run() Q_DECL_OVERRIDE;
signals:
void sendToPrint(QVector<unsigned char> vec);
};
#endif // THREAD1_H
main.cpp:
#include <QCoreApplication>
#include "thread0.h"
#include "thread1.h"
#include <QObject>
#include <QMetaType>
int main(int argc, char *argv[])
{
QCoreApplication a(argc, argv);
thread0 *th0 = new thread0();
thread1 *th1 = new thread1();
QObject::connect(th1, &thread1::sendToPrint, th0, &thread0::printBuff);
th0->start();
th1->start();
return a.exec();
}
and thread1.cpp:
#include "thread1.h"
thread1::thread1()
{
}
thread1::~thread1()
{
}
void thread1::run()
{
QVector<unsigned char> vec = {0, 1, 2, 3};
emit sendToPrint(vec);
}
P.S. If I use direct connection code works.
QObject::connect(th1, &thread1::sendToPrint, th0, &thread0::printBuff, Qt::DirectConnection);

Answer 1

Add this in your main()
qRegisterMetaType<QVector<unsigned char> >("QVector<unsigned char>");
Between threads, Qt uses a Queued Connection. This mechanism requires each type passed as a parameter by the signal to the slot to be known of the Qt meta-object system. Most Qt types are already registered, but not template ones as you need one qRegisterMetaType per specialization of the template
If you use direct connection, your code may seem to work, but printBuff will run in th1, not th0. And in fact, th0 thread will do nothing. This may cause the program to crash if printBuff was designed to run in thread0 only, due to thread safety issues.