Question

I have a Qt application that was developed using Qt Creator and the GUI tool that accompanies it. I have a main thread, TheGui and a worker thread that is created by the main thread, WorkerThread (called thread).

The problem I’m having is when I create a slot in the GUI by using

public slot:
  void updateTable(string str);

within the header file of the GUI and signal void sendList(string str); within the header file of the worker thread, the slot never gets called. I connected the two using

connect(&thread, SIGNAL(sendList(string str),
        this,    SLOT(updateTable(string str)));

within the constructor in the GUI cpp file. I did something similar except with the slot in the worker thread and signal from the GUI and it worked fine. I know from using the debugger that the signal sendList is indeed getting called, it is just never going into it.

Any thoughts?

Answer 1

1. Because the signal and the slot are on distinct threads, the connection between them has the Qt::QueuedConnection type. And for queued connections, Qt has to be able to save a copy of the signal parameters, to pass them later to the slot.
   
   So, to inform Qt that the type is copyable, you have to register it with Qt’s meta-object system (see QMetaType) like this:

   // This macro call should be put in one of your .h files 
   Q_DECLARE_METATYPE(std::string)
   
   // You should call this function before any (queued) 
   // signal/slot connection involving the type
   qRegisterMetaType<std::string>();
   

2. The parameter name shouldn’t be included in the QObject::connect call, and the type names should be exactly the same as the ones you passed to Q_DECLARE_METATYPE:

   connect(&thread, SIGNAL(sendList(std::string), this, SLOT(updateTable(std::string)));
   

You can also use QString or QByteArray, which are already registered, instead of std::string, since these functions are slots and signals and as such are already Qt specific.