QT: How to download from url while pressing a button - c++

Question

I have this code:
QNetworkAccessManager man;
QNetworkRequest req(QUrl("URL"));
QString ua("HttpRequestDemo/0.1 (Win64) Qt/5.14.0");
req.setHeader(QNetworkRequest::UserAgentHeader, QVariant(ua));
QNetworkReply* reply = man.get(req);
QObject::connect(reply){
QByteArray read = reply->readLine();
QFile out("file.txt");
out.open(QIODevice::WriteOnly|QIODevice::Text);
out.write(read);
out.close();
})
This works on the main.cpp file, using the QCoreApplication, but I want to use the QApplication and download a specific data while pressing a button.
I put the same code on the on_pushButton_clicked() in the mainwindow.cpp file and it didn't even generate the file from the url.

Answer 1

The problem is that man and req go out of scope and are destroyed as soon as your on_pushButton_clicked() function returns, at which point the request probably hasn't even been sent yet.
You need to make sure that these objects outlive the current scope, either by making them members of the window class, or by allocating them on the heap and setting some QObject (maybe also the window class) as the parent.

Answer 2

The problem is that if you put the same code in a method like X you make QNetworkAccessManager a local variable that will be removed instantly that the connection is asynchronous. The solution is to make QNetworkAccessManager an attribute of the class.
*.h
private:
QNetworkAccessManager man;
*.cpp
void Klass::on_pushButton_clicked(){
QNetworkRequest req(QUrl("URL"));
QString ua("HttpRequestDemo/0.1 (Win64) Qt/5.14.2");
req.setHeader(QNetworkRequest::UserAgentHeader, QVariant(ua));
QNetworkReply* reply = man.get(req);
connect(reply, &QNetworkReply::finished, [&]() {
QByteArray read = reply->readAll();
QFile out("file.txt");
out.open(QIODevice::WriteOnly|QIODevice::Text);
out.write(read);
out.close();
reply->close();
reply->deleteLater();
})
}

Answer 3

If you are planning on potentially queuing very many downloads, I strongly recommend using libcurl in your Qt app. I was using QNetworkAccessManager to down 100+ financial quote files, and it would fail downloading ~ 1/3 of the time, and take a while to download. I switched to libcurl, and after figuring out how to get my crypto root certificates setup for https, it runs much faster, and almost never fails. I run it as a dll.
And yes, you will need to make sure the network manager, whether QNetworkManager or curl, doesn't go out of scope upon exiting the button handler. A more conventional pattern, although not necessarily better, is to either have a pointer to e.g. QNetworkManager in your parent class, and new it, or use a std::unique_ptr and std::make_unique (purportedly safer). Creating large objects on the stack can cause problems (in the old days, dare I say, stack overflows), and so is usually done on the heap. In this case, it's not very big, so it doesn't really matter. Alternatively, a form creating big objects might itself be created on the heap.