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

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.

Answer 1

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.

Answer 2

Do all emits only after everything has been connected. Here the instantiation of sender happens before sender and receiver are connected, and that's where the emit was done.