Simple version checker with Qt - c++

Question

I’m trying to implement a simple version checker below.
I’m getting a zero error code reading the file, but the file
contents show up blank. You can access the file via browser,
permissions are ok.
void check_version()
{
QNetworkAccessManager *nam = new QNetworkAccessManager();
QUrl data_url("http://www.example.com/version.txt");
QNetworkRequest req(data_url);
QNetworkReply *reply = nam->get(req);
QByteArray data = reply->readAll() ;
QString s1(data);
int err = reply->error();
QString s2 = QString::number(err);
delete reply;
delete nam;
QMessageBox::critical(0, "",s1+" "+s2,QMessageBox::Cancel);
}
I gather the problem is that I need to wait to read until the get is finished, so I need a signal and a slot: the signal tells the slot to read the data.
pseudocode:
QObject::connect(&rep, SIGNAL( rep is finished ),
QByteArray newver , SLOT( reply->readAll() ));
How do I set up a signal/slot for my task?

Answer 1

You're right, you have to wait until the get() is "finished" in order to obtain the whole response with a call to readAll().
Following a working example to start with:
// ...
QNetworkAccessManager *nam = new QNetworkAccessManager();
QUrl data_url("http://www.example.com/version.txt");
QNetworkReply* reply = nam->get(QNetworkRequest(data_url));
QEventLoop eventLoop;
QObject::connect(reply, SIGNAL(finished()), &eventLoop, SLOT(quit()));
eventLoop.exec();
if (reply->error() != QNetworkReply::NoError)
{
// Something went wrong. Error can be retrieved with: reply->error()
}
else
{
// Call reply->readAll() and do what you need with the data
}
// ...
This example will block until the reply is ready. If you need an asynchronous behavior you can just connect the signal finished() to a custom slot and check for error and/or read there. I do not recommend to connect the signal finished() directly to readAll() because "sometimes errors happen".