Calling slots from a Class in Qt

Question

I am trying to program an App that fetches files from a server.

I have a 'Window' class(mainwindow.cpp, which is a widget class that would be the UI) and then I have a 'Backend' class(Backend.cpp).

The GUI has a push button and two radio buttons. If the radio button "remote" is seleted, then upon clicking the push button will lead to fetching files from server.

However, there is some problem in the 'connect' call in Backend.cpp which I can't figure out. The error I get is: no matching function call to 'QObject::connect(QNetworkReply*&), const char[13], Backend* const, const char[20])'

Here are the codes:

ANSWER: Avoid circular inclusions!!!! Here are the updated codes:

mainwindow.h

#ifndef WINDOW_H
#define WINDOW_H

#include <QWidget>
#include <QRadioButton>
#include <QtNetwork/QTcpSocket>
#include <QtNetwork/QHostAddress>
#include <QFile>
#include <QUrl>

#include "Backend.h"
class QGroupBox;

class Window : public QWidget
{
    Q_OBJECT

public:
    Window(QWidget *parent = 0);
    QTcpSocket *conn;
    QFile *file;

    QUrl url;
    Backend backend_inst;

private:

    QRadioButton *button_local;
    QRadioButton *button_remote;
    QGroupBox *createPushButtonGroup();


private slots:
    void onClick_button1();
    void onCheck_local();
    void onCheck_remote();
};

#endif

mainwindow.c

#include <QtGui>

#include "mainwindow.h"

Window::Window(QWidget *parent)
    : QWidget(parent)
{
    QGridLayout *grid = new QGridLayout;
    grid->addWidget(createPushButtonGroup(), 1, 1);
    setLayout(grid);

    setWindowTitle(tr("File-Fetch App"));
    resize(480, 420);
}

QGroupBox *Window::createPushButtonGroup()
{

    QGroupBox *groupBox = new QGroupBox();

    QPushButton *pushButton1 = new QPushButton(tr("Fetch Files!!"));

    button_local = new QRadioButton(tr("&Download Files from Local Storage"));
    button_remote = new QRadioButton(tr("&Download Files from a Web-Server"));
    button_local->setChecked(1);


    connect(pushButton1,SIGNAL(clicked()), this, SLOT(onClick_button1()));

    QVBoxLayout *vbox = new QVBoxLayout;
    vbox->addWidget(pushButton1);
    vbox->addSpacing(50);
    vbox->addWidget(button_local);
    vbox->addWidget(button_remote);
    vbox->addStretch(1);
    groupBox->setLayout(vbox);

    return groupBox;
}

void Window::onClick_button1()
{
    QTextStream out(stdout);
    out << "fetch button clicked\n";
    if (button_local->isChecked()){
        onCheck_local();
    }
    else if (button_remote->isChecked()){
         onCheck_remote();
    }

}
void Window::onCheck_local()
{

    QTextStream out(stdout);
    out << "local update button checked\n";

}
void Window::onCheck_remote()
{
    QTextStream out(stdout);
    out << "remote update button checked\n";
    QString pathname= "http://192.168.1.1:8000/example.txt";
    QUrl webaddr = pathname;
   backend_inst.FetchFile(webaddr);
}

Backend.h

#ifndef BACKEND_H
#define BACKEND_H

#include <QObject>
#include <QNetworkReply>
#include <QNetworkAccessManager>
#include <QUrl>
#include <QTextStream>

class Backend : public QObject
{
    Q_OBJECT

public:

    Backend(QObject* parent=0);
    void FetchFile(QUrl fpath);

public slots:
    void getBytesFromFile();

private:
    QNetworkReply *reply;
    QNetworkAccessManager qnam;

};

#endif // BACKEND_H

Backend.cpp

#include "Backend.h"


Backend::Backend(QObject* parent)
    : QObject(parent)
{
}

void Backend::FetchFile(QUrl fpath)
{
    reply = qnam.get(QNetworkRequest(fpath));
    QObject::connect(reply, SIGNAL(readyRead()),this, SLOT(getBytesFromFile()));

    //qnam = new QNetworkAccessManager;
    //QObject::connect(&qnam, SIGNAL(finished(QNetworkReply*)), this, SLOT(getBytesFromFile()));
}

void Backend::getBytesFromFile(){

    QByteArray downloadedData;
    QTextStream out(stdout);
    out << "we are loading data from URL\n";
    downloadedData =reply->readAll();
    out << downloadedData;
    delete reply;

}

main.cpp

#include <QApplication>

#include "mainwindow.h"

int main(int argc, char *argv[])
{
    QApplication app(argc, argv);
    Window window;
    window.show();
    return app.exec();
}

Answer 1

To use signals and slots, your classes (both signaling and slotting) must derive from QObject, i.e.

#include "mainwindow.h"

#include <QObject>

class Backend : public QObject
{
    Q_OBJECT
public:
    Backend(QObject* parent=0);

[...]

Backend::Backend(QObject* parent)
    : QObject(parent)
{
}