Why there isn't QHttpServerRequest copy constructor?

Tarae

I'm playing with QHttpServer.

I use asynchronous processing, so my request handler returns QFuture<QHttpServerResponse>. QFuture is created by QtConcurrent::run. But QtConcurrent::run creates a copy of its arguments. And QHttpServerRequest doesn't have copy constructor so I cannot use it as an argument.

I came out with some workarounds to solve this, but they are not as nice as if QHttpServerRequest had copy constructor.

Is there any reason why QHttpServerRequest isn't copy-constructible?

There is a sample of the code to demonstrate, what I think:

// QT += httpserver concurrent

#include <QCoreApplication>
#include <QHttpServer>
#include <QtConcurrent>

QHttpServerResponse responder1()
{
  return "Hello";
}

QHttpServerResponse responder2(const QString & arg)
{
  return "Hello with arg: " + arg;
}

QHttpServerResponse responder3(const QHttpServerRequest & request)
{
  return "Hello to ip address: " + request.remoteAddress().toString();
}

int main(int argc, char *argv[])
{
  QCoreApplication a(argc, argv);

  QHttpServer server;

  // this works
  server.route("/r1", [](){return QtConcurrent::run(responder1);});
  server.route("/r2/<arg>", [](const QString & arg){return QtConcurrent::run(responder2, arg);});
  
  //this doesn't work
  server.route("/r3", [](const QHttpServerRequest & request){return QtConcurrent::run(responder3, request);});

  server.listen(QHostAddress::Any, 2222);
  return a.exec();
}

This code uses simple functions to create responses but in my real code, I use QMetaObject::invokeMethod to move computation to a different thread. So I really need to use QtConcurrent::run.

SGaist

Hi,

For the why it's not copyable, that's a question you should bring to the module developers.

That said, based on the examples of the module, it should work if you use a const reference to forward your request further.

Tarae

@SGaist Thanks for your reply!

Contacting module developers seems like a good idea. That's why I write this post. I was hoping they read it.

How exactly should I use a const reference to forward your request further?
My function responder3 has a const reference argument.

SGaist

I somehow missed that.

Taking a look at things again, my understanding is that when calling QtConcurrent::run as you do, there is a mandatory copy happening which is what triggers your issue.

However, you can "cheat" the system. QtConcurrent::run can also take a lambda so you could do something like the following:

server.route("/r3", [] (const QHttpServerRequest & request) {
    auto lambda = [&request]() -> QHttpServerResponse {
        return responder3(request);
    };
    return QtConcurrent::run(lambda);
});

Infinity

Hi

Like this you can skip the helper/workaround function:

server.route("/r2", [] (const QHttpServerRequest & request) {
   QFuture<QHttpServerResponse> future = QtConcurrent::run([&request] () {
       QString response = QString("Hello to ip address: %1\n").arg(request.remoteAddress().toString());
       return QHttpServerResponse(response, QHttpServerResponder::StatusCode::Ok);
   });
   return future;
});