How to send data through signal and slot?

Question

Could someone please help how to connect signal and slot?

I've got function1 which receive in realtime data ( one value)

void function1(int,double)
{
   if(condition)
     {
     //some code
     numb3 = 100;// double numb3 received new data
     emit mySignal(numb3);
     }
}

then in other function I've got variable which should receive captured value

   void function2(int,double)
    {

     double parameter2 = numb3;
    }

I tried combinations like

Q_SIGNAL double mySignal(double newValue=0){return newValue;};
Q_SLOT double slot1(double param=0) {emit mySignal(param); };

and then in function2{
connect(customPlot,SIGNAL(mySignal()), qApp, SLOT(slot1()));
double parameter2 = slot1();}

but they are not working as I would like to.

Thanks in advance!

Take a look here: http://stackoverflow.com/questions/5842124/can-qt-signals-return-a-value — Kirill Gamazkov, Mar 14 '14 at 20:52

score 5 · Answer 1 · answered Feb 20 '14 at 17:03

Define your custom signal in the header of the class that emits the signal:

signals:
       void   signalName(paramType1,paramType2...);

Define your slot to receive the signal in the class that will receive it:

public slots:
      void   slotName(paramType1,paramType2...) ; // Should be THE SAME parameter types

Now connect inside the cpp where you want to start connecting both:

connect(classObjectWhereTheSignalIs, SIGNAL(signalName(paramType1,paramType2)),classObjectWhereTheSlotIs,SLOT(slotName(paramType1,paramType2)));

And now you emit a signal like this, whenever you want:

emit (signalName(paramOfTypeParamType1, paramOfTypeParamType2...));

Cheers.

Kunal · Answer 2 · 2015-10-07T22:34:57.383

1

you should define your signal like this in header.

Q_SIGNAL:

void mySignal(int);

your slot will look like below in header file.

Q_SLOT:

void mySlot(int val);

Now in cpp file you can connect signal to slot like below.

connect(signalObject, SIGNAL(mySignal(int)),slotObject,SLOT(mySlot(int)));

edited Oct 07 '15 at 22:34

answered Feb 20 '14 at 04:30

Kunal

3,475
21
14

ok, but how to proceed with one parameter which should be send between signal ans slot ? – gen_next Feb 20 '14 at 11:10
@gen_next I wrote a more complete answer. – Darkgaze Feb 20 '14 at 17:04

score 0 · Answer 3 · answered Feb 20 '14 at 14:51

0

You're mixing C++ 'return' and Qt signal/slot. Typically, signals and slots have return type void. Also, signal's body is generated by MOC, so you usually just declare it.

Like this:

#include <QObject>
#include <QDebug>

class CustomPlot: QObject {
    Q_OBJECT

public:
    void bla() {
        emit doSomething(3.1415926535);
    }

signals:
    void doSomething(double value);
};

class EventHandler: QObject {
    Q_OBJECT

public slots:
    void onDoSomething(double value) {
        qDebug() << "we are called with value =" << value;
    }

};

//in main:

CustomPlot plot;
EventHandler handler;

QObject::connect(&plot, &CustomPlot::doSomething, &handler, &EventHandler::onDoSomething);

plot.bla();  // in QtCreator's 'Application Output' panel you'll see:
             //we are called with value = 3.1415926535

answered Feb 20 '14 at 14:51

Kirill Gamazkov

3,277
1
18
22

Hi, thanks but in this example first function bla() is started which triggers emit signal. I need as result slot which was triggered by emit function + slot result must be simultaneously assigned to double object.... – gen_next Feb 20 '14 at 21:33
Sorry, I didn't understand you. Looks like you've got just 5 seconds to express everything :). I don't understand why you trying to read the slot's return. Slot is usually return void. And if it does not, how will you handle the case when several slots are connected to signal? Whose return value will you use? – Kirill Gamazkov Mar 07 '14 at 10:12
Once again: when it comes to signal/slots, the return values are almost always void. When you want to 'return' some value, you pass that value to signal, i. e. call 'emit signal_name(value_to_return)'. So you can treat signals as 'inverted' methods whose params are actually return values that are passed to connected slots – Kirill Gamazkov Mar 07 '14 at 10:22

How to send data through signal and slot?

3 Answers3