Qt: Using signals and slots between two child threads

Question

I'm new to Qt, and C++ in general, and I'm trying to make a program that runs two child threads, threadA and threadB. Both threads are created in main.cpp like so:

ThreadA threadA;
threadA.start();
ThreadB threadB;
threadB.start();

The two threads then run independently. I need to make threadA able to call a method in threadB, passing data as an argument in the process. I have tried using signals and slots, by adding this to my main.cpp:

QThread::connect(&threadA, SIGNAL(mySignal(uint)),
    &threadB, SLOT(mySlot(uint)));

Where threadA::mySignal(uint) is used in a:

//a.h
signals:
  void mySignal(unsigned int blah);
//a.cpp
  emit mySignal(42);

And threadB::mySlot(uint) is in b:

//b.h
public slots:
    void mySlot(unsigned int fluff);
//b.cpp
void threadB::mySlot(unsigned int fluff)
{ doStuff(fluff); ... }

The program compiles and runs sucessfully, but I get a message from QObject in my debug log:

QObject::connect: No such slot QThread::mySlot(uint) in ../Project/main.cpp:42

Meaning that the compiler is looking in QThread instead of threadB for mySlot. Can anyone tell me where I'm going wrong here? Any help would be gratly appreciated, and I can provide more details if nessesary.

Answer 1

First of all for the error you mentioned

The program compiles and runs sucessfully, but I get a message from QObject in my debug log:

QObject::connect: No such slot QThread::mySlot(uint) in ../Project/main.cpp:42

You are using mySlot and have defined MySlot please refer back your code

You have written

QThread::connect(&threadA, SIGNAL(mySignal(uint)), &threadB, SLOT( mySlot (uint)));

and defined

//bh

public slots:
    void MySlot(unsigned int fluff);

Answer 2

You are probably missing the Q_OBJECT macro in your threadb.h (and possibly threada.h ).

class ThreadB : public QObject 
{
   Q_OBJECT 
...

From the docs: All classes that contain signals or slots must mention Q_OBJECT at the top of their declaration. They must also derive (directly or indirectly) from QObject. All classes that contain signals or slots must mention Q_OBJECT at the top of their declaration. They must also derive (directly or indirectly) from QObject.

I'm also assuming that you have subclassed QThread . You should know that QThread is used for managing threads, not processing data. You should subclass QObject instead. Read this for more info about that.