undefined reference to vtable in class

[stampede]

I have separated the code into three files: header and .cpp for the class, and third file for main() function.
As high_flyer said, the class meta code is not generated, so you are getting undefined references.
I haven't changed anything in the code itself, just when you add the class header to the list of headers in your .pro file, it gets "moc-ed", the Meta-Object code is created in moc_headerName.cpp. You don't have the header file, so this code is not generated.
Split this into three files and add header to project .pro file.
I think you can use the "moc" on your file manually, but splitting it makes more sense to me.
----
Edit:

Split this into three files and add header to project .pro file.

Add all of them to .pro file, .cpp to SOURCES and header to HEADERS

[JeanC]

Thank you, I understand.
Personally I think it's a bit overkill to use separate files for such a small program, against my taste, but if that's what Qt wants, ok.
Now I tried to add that class in Qt Creator but I can't find how. Off course I could manually edit the .pro file but it seems a bit odd that I cannot do that in Creator. If I try to edit the .pro file in Creator it just ignores my 'File Open'.

I changed things by hand, now I get:
QObject::startTimer: QTimer can only be used with threads started with QThread
and
QObject::connect: Cannot connect (null)::destroyed() to QHostInfoLookupManager::waitForThreadPoolDone()

My guess is that the class is destroyed before it can do anything. How do I avoid that? I'd like to keep this a small console program.

[stampede]

If I try to edit the .pro file in Creator it just ignores my 'File Open'.

Really ? It opens the .pro for editing when I double click it.
If you really want, you can keep it as one file:

#include <QSettings>
#include <QDebug>
#include <QNetworkAccessManager>
#include <QUrl>
#include <QNetworkRequest>
#include <QObject>
 
class ClientHandler : public QObject
{
Q_OBJECT
  QNetworkAccessManager *manager;
private slots:
  void replyFinished(QNetworkReply *);
public:
  void CheckSite(void);
  ~ClientHandler();
};
 
void ClientHandler::replyFinished(QNetworkReply *reply) { qDebug() << "DONE"; }
ClientHandler::~ClientHandler() {}
 
void ClientHandler::CheckSite(void)
{
QUrl qrl("http://checkip.dyndns.org");
manager = new QNetworkAccessManager(this);
connect(manager, SIGNAL(finished(QNetworkReply*)), this, SLOT(replyFinished(QNetworkReply*)));
manager->get(QNetworkRequest(qrl));
}
 
int main(int argc, char *argv[])
{
QSettings ini("Jean", "ddns");
if (!ini.contains("LastIp"))
  {
  qDebug() << "no ip";
  ClientHandler c;
  c.CheckSite();
  }
return 0;
}
 
#include "moc_main.cpp"

#include <QSettings>
#include <QDebug>
#include <QNetworkAccessManager>
#include <QUrl>
#include <QNetworkRequest>
#include <QObject>
 
class ClientHandler : public QObject
{
Q_OBJECT
  QNetworkAccessManager *manager;
private slots:
  void replyFinished(QNetworkReply *);
public:
  void CheckSite(void);
  ~ClientHandler();
};
 
void ClientHandler::replyFinished(QNetworkReply *reply) { qDebug() << "DONE"; }
ClientHandler::~ClientHandler() {}
 
void ClientHandler::CheckSite(void)
{
QUrl qrl("http://checkip.dyndns.org");
manager = new QNetworkAccessManager(this);
connect(manager, SIGNAL(finished(QNetworkReply*)), this, SLOT(replyFinished(QNetworkReply*)));
manager->get(QNetworkRequest(qrl));
}
 
int main(int argc, char *argv[])
{
QSettings ini("Jean", "ddns");
if (!ini.contains("LastIp"))
  {
  qDebug() << "no ip";
  ClientHandler c;
  c.CheckSite();
  }
return 0;
}

#include "moc_main.cpp"

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in order to get "moc_main.cpp" file you need to run "moc":

moc main.cpp -o moc_main.cpp

moc main.cpp -o moc_main.cpp

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You don't need to add anything in the pro file this way.
And yes, your ClientHandler goes out of scope and is deleted before finishing the network request:

...
c.CheckSite();
  } // c goes out of scope here

...
c.CheckSite();
  } // c goes out of scope here

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How do I avoid that?

One of possible solutions is to use QCoreApplication:

int main(int argc,char**argv){
    QCoreApplication app(argc,argv);
    QSettings ini("Jean", "ddns");
    if (!ini.contains("LastIp"))
   {
   qDebug() << "no ip";
   ClientHandler c;
   c.CheckSite();
   }
   return app.exec();
}
 
// 
void ClientHandler::replyFinished(QNetworkReply *reply) { 
    qDebug() << "DONE"; 
    qApp->quit();  
}

int main(int argc,char**argv){
    QCoreApplication app(argc,argv);
    QSettings ini("Jean", "ddns");
    if (!ini.contains("LastIp"))
   {
   qDebug() << "no ip";
   ClientHandler c;
   c.CheckSite();
   }
   return app.exec();
}

// 
void ClientHandler::replyFinished(QNetworkReply *reply) { 
    qDebug() << "DONE"; 
    qApp->quit();  
}

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I suppose it would be better to connect to QNetworkReply signals as well, as in example from QNetworkAccessManager docs ,in order to handle errors.
-----
sorry, now i see the wrong code ( c was deleted anyway ), should be something like:

if (!ini.contains("LastIp"))
   {
   qDebug() << "no ip";
   ClientHandler c;
   c.CheckSite();
   app.exec();
   }

if (!ini.contains("LastIp"))
   {
   qDebug() << "no ip";
   ClientHandler c;
   c.CheckSite();
   app.exec();
   }

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