Unable to connect signal to slot in another class

Question

I have 2 classes. Class A and Class B. I am emitting a signal from class A which I want the B to recieve.

I am doing it following way

In Listener File

Header File:
  Class Listener:public DDSDataReaderListener
  {
     //Some code
    public:
       A m_objectSendData;
  }

Implementation File:

  void Listener::ondataavailable(DDSDataReader *reader)
 {
  m_objSendData.GetDDSData();
 }

In Class A

Header File:

Class A:public QObject
{
  Q_OBJECT
  public:
    void GetDDSData();
  signals:
    void Signal_Data();
}

.cpp File

 A::A(QWidget *parent):QObject(parent)
{
}

void A::GetDDSData()
 {
   emit Signal_Data();
 }   

In Class B

Header File:

  Class B:public QObject
  {
    Q_Object
    public:
    A objGetData;

    public slots:
    void getData();
  }

Implementation File:

 B::B(QWidget *parent):QObject(parent)
{
   //Some part of code

  connect(&objGetData,SIGNAL(Signal_Data()),this,SLOT(getData());
 }

 void B::getData()
 {
    //Watever is to be updated
  }

I tried debugging. It is going till emit part correctly. However it is not reaching the slot. Can someone please help me with this. Thank You.

Answer 1

Without full code, it's quite difficult to identify the exact issue of the problem, so I'll outline a few important points to check.

To ensure you can use the signal and slots mechanism, you should ensure that your class is derived, from QObject or a class already derived from QObject in its hierarchy and your class must contain the Q_OBJECT macro, for example: -

class A : public QObject // derived from QObject
{
    Q_OBJECT // your class must have this macro for signals and slots

    public:
    A();
};

Omitting the macro is probably the most common of mistakes.

To specify a slot, you add it to either the public or private slot section of your class: -

class B : public QObject // derived from QObject
{
    Q_OBJECT // your class must have this macro for signals and slots

    public:
    B();

    public slots:
        void SlotB(); // slot declared public

    private slots:
        void SlotBPrivate(); // slot declared private.
}; 

Once a signal is declared in a class, a slot to receive the signal should match the arguments passed in and when you connect a signal to a slot, you must not add the function argument names.

Therefore: -

connect(&objectA, SIGNAL(SignalA(int in), this, SIGNAL(SlotA(int param)); //will fail due to the argument names

It should be: -

connect(&objectA, SIGNAL(SignalA(int), this, SIGNAL(SlotA(int));

Finally, if you're using Qt 5, you can use the new connection call, which doesn't require you to specify any argument, but instead takes the addresses of slot and signal functions.

connect(&objectA, &A::SignalA, this, &B::SlotA));

Since it references the address of a function, in actuality, the functions don't need to be classed as a slot and will still be called.

Hope that helps.

Answer 2

Actually I believe an answer is given in one of the comments.

One more thing, you didn't show enough code but I suspecting that you program leaves scope of objectA variable and your emitting object is just destroyed before it can emit any signal (objectA is local variable created on stack not on heap). – Marek R 1 hour ago

you allocate your Object on stack, so it gets destroyed as soon as it gets out of scope, together with destroy it gets disconnected from all signals/slots it has connections to.

So that's why you don't see any errors/warning messages because code itself is completely legit. You should new() your object to get it allocated in heap.