Use of QAbstractSocket stateChanged()-Signal - c++

Question

What is the correct use/implementation of the QAbstractSocket stateChanged()-Signal?
main.cpp:
#include <QCoreApplication>
#include <sslserver.h>
#include <QLoggingCategory>
int main(int argc, char *argv[]){
QCoreApplication a(argc, argv);
QLoggingCategory::setFilterRules("*.debug=true");
SslServer *myTestServer = new SslServer();
return a.exec();
}
SslServer.h:
#ifndef SSLSERVER_H
#define SSLSERVER_H
#include <QObject>
#include <QTcpServer>
#include <QTcpSocket>
#include <QTimer>
class SslServer : public QObject
{
Q_OBJECT
public:
explicit SslServer(QObject *parent = nullptr);
private slots:
void newTestConnection();
void sendTestdata();
void connectionWasClosed(QAbstractSocket::SocketState state = QAbstractSocket::UnconnectedState);
private:
QTcpServer *testServer;
QTcpSocket *testSocket;
QTimer *testTimer;
};
#endif // SSLSERVER_H
SslServer.cpp:
#include "sslserver.h"
#include <QDebug>
SslServer::SslServer(QObject *parent) : QObject(parent){
testServer = new QTcpServer(this);
testSocket = new QTcpSocket();
testTimer = new QTimer();
connect(testServer, SIGNAL(newConnection()), this, SLOT(newTestConnection())); //works
connect(testTimer, SIGNAL(timeout()), this, SLOT(sendTestdata())); //works
connect(testSocket, SIGNAL(stateChanged(QAbstractSocket::SocketState)), this, SLOT(connectionWasClosed(QAbstractSocket::SocketState)));
//Qt5 Connection was also tested, without any change to old connection-typo
//connect(testSocket, &QAbstractSocket::stateChanged, this, &SslServer::verbindungWurdeBeendet);
testTimer->start(1000);
if(testServer->listen(QHostAddress::Any, 9999)){
qDebug() << "Server running on Port 9999";
}else{
qDebug() << "Error while building server";
}
}
void SslServer::newTestConnection(){
testSocket->close();
testSocket = testServer->nextPendingConnection();
testSocket->write("Welcome on: Testserver!");
testSocket->flush();
}
void SslServer::sendTestdata(){
if(testSocket->isOpen()){
qDebug() << testSocket->state();
qDebug() << "Sending testdata to client";
testSocket->write("testdata");
}
}
void SslServer::connectionWasClosed(QAbstractSocket::SocketState){
qDebug() << "!!! SocketState changed !!!";
}
I can see from the testSocket->state() statement that the state is changing from Unconnected to Connected to Unconnected, but the SLOT-Function is never called. What am I missing here?

Answer 1

The problem is caused because you are connecting to an object:
testSocket = new QTcpSocket();
But you expect to receive the signal from another object:
testSocket = testServer->nextPendingConnection();
The solution is to make the connection when you receive the new object:
void SslServer::newTestConnection(){
if(testSocket)
testSocket->close();
testSocket = testServer->nextPendingConnection();
connect(testSocket, &QAbstractSocket::stateChanged, this, &SslServer::connectionWasClosed);
testSocket->write("Welcome on: Testserver!");
testSocket->flush();
}
The complete example can be found in the following link

Answer 2

Your SLOT signature is not matching with SIGNAL.
Change the function declaration and definition to
void verbindungWurdeBeendet(QAbstractSocket::SocketState state);
And the connect statement to
connect(testSocket, SIGNAL(stateChanged(QAbstractSocket::SocketState)), this, SLOT(verbindungWurdeBeendet(QAbstractSocket::SocketState)));
The SLOT function verbindungWurdeBeendet() (with out argument) is also acceptable, if you have default value in the function declaration.
ex:
In the below case you can have a SLOT in connect statement, without arguments.
void verbindungWurdeBeendet(QAbstractSocket::SocketState state = QAbstractSocket::UnconnectedState);