How to do it...

To learn how to achieve asynchronous operations using the signals and slots mechanism, let's follow this example:

1. Create a Qt Console Application project:

1. This type of project will only provide a main.cpp file for you, instead of mainwindow.h and mainwindow.cpp, like our previous example projects. Let's open up main.cpp and add the following headers to it:

#include <QNetworkAccessManager>
#include <QNetworkReply>
#include <QDebug>

1. Then, add the following code to our main() function. We will be using the QNetworkAccessManager class to initiate a GET request to the following web URL:

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

    QString *html = new QString;
    qDebug() << "Start";

    QNetworkAccessManager manager;
    QNetworkRequest req(QUrl("http://www.dustyfeet.com"));
    QNetworkReply* reply = manager.get(req);

1. Then, we use C++11's lambda expression to connect QNetworkReply signals to inline functions:

    QObject::connect(reply, &QNetworkReply::readyRead,
    [reply, html]() {
        html->append(QString(reply->readAll()));
    });

    QObject::connect(reply, &QNetworkReply::downloadProgress,
    [reply](qint64 bytesReceived, qint64 bytesTotal) {
        qDebug() << "Progress: " << bytesReceived << "bytes /" << bytesTotal << "bytes";
    });

1. We can also use a lambda expression with connect() to call a function that is not under a QObject class:

    QObject::connect(reply, &QNetworkReply::finished,
    [=]() {
        printHTML(*html);
    });

    return a.exec();
}

1. Lastly, we define the printHTML() function, as shown in the following code:

void printHTML(QString html) {
    qDebug() << "Done";
    qDebug() << html;
}    

1. If you build and run the program now, you will see that it's functional even without declaring any slot function. Lambda expressions make declaring a slot function optional, but this is only recommended if your code is really short: