Qt SIGNAL and SLOT in hierarcy, not threads (yet)

Question

OK, I'm a total beginner but I'm missing something here. Been all over the Qt documentation/examples and everything I can dig up through Google. All similar information is slightly different in context...

I'm just starting out with Qt SIGNALS and SLOTS, I'm successful with gui examples and within a class. Now I want to connect a SIGNAL in a child class with a SLOT in a sibling class with the Connect defined in the parent main. Ultimately my aim is to receive iamges in a class handling a QTcpSocket and emit the data as char* to be handled (saved or displayed) by another class.

For now I've just created the most basic version of the arrangement in a Console app as a learning exercise. I've got a sender class...

sender.h

#ifndef SENDER_H
#define SENDER_H

#include <QObject>

class sender : public QObject
{
    Q_OBJECT

public:
    sender(QObject *parent = 0);
    ~sender();
signals:
    void output(int data);
public slots:
    void test(int data);
private:    
};

#endif // SENDER_H

sender.cpp

#include <iostream>
#include "sender.h"

sender::sender(QObject *parent)
    : QObject(parent)
{
    std::cout << "Created sender" << std::endl;
    int stuff = 47;
    std::cout << "stuff = " << stuff << std::endl;

    connect(this, SIGNAL(output(int)), this, SLOT(test(int)));

    emit output(stuff);
}

void sender::test(int data)
{
    std::cout << "Got to test, data = " << data << std::endl;
}

sender::~sender()
{
    std::cout << "Destroying sender" << std::endl;
}

...and a receiver class...

receiver.h

#ifndef RECEIVER_H
#define RECEIVER_H

#include <QObject>

class receiver : public QObject
{
    Q_OBJECT

public:
    receiver(QObject *parent = 0);
    ~receiver();
public slots:
    void input (int data);
private:
};

#endif // RECEIVER_H

receiver.cpp

#include <iostream>
#include "receiver.h"

receiver::receiver(QObject *parent)
    : QObject(parent)
{
    std::cout << "Created receiver" << std::endl;
}

void receiver::input(int data)
{
    std::cout << "Got data as = " << data << std::endl;
}

receiver::~receiver()
{
    std::cout << "Destroying receiver" << std::endl;
}

My main looks like this...

main.cpp

#include <QtCore/QCoreApplication>
#include <iostream>
#include "sender.h"
#include "receiver.h"

int main(int argc, char *argv[])
{
    QCoreApplication a(argc, argv);

    receiver myReceiver;
    sender mySender;

    if (QObject::connect(&mySender, SIGNAL(output(int)), 
        &myReceiver, SLOT(input(int))))
    {
        std::cout << "Got here so connect returned true" << std::endl;
    }

    return a.exec();
}

I've added the cout outputs and the sender::test function to try and figure out what's happening.

For me, this compiles cleanly and runs without any warnings or errors but while the sender::test SLOT gets called the receiver::input SLOT doesn't. The test on the connect in main returns true and neither sender or receiver are destroyed prematurely. Console output is...

Created receiver
Created sender
stuff = 47
Got to test, data = 47
Got here so connect returned true

So the SIGNAL is emitted, the SIGNAL and SLOT parameters match, I've got the Q_OBJECT macros in both sender.h and receiver.h, and both inherit from and #include QObject.

What's wrong???

P.S. I'm running 4.8.3 and IDE is VS2010 with Qt plugin.

ixSci ixSci · Accepted Answer · 2013-02-19T17:23:37

The answer is quite simple: you are sending signal before you've connected it to the receiver and after you've connected it to itself. So your output is absolutely correct.

Qt SIGNAL and SLOT in hierarcy, not threads (yet)

2 Answers