Signal-Slot makes a mess

I am trying to create a image-saving application using Qt. Now the stub

class ImageSaver:public QObject
{
    int index;
    QWebPage * main_Page;
    QNetworkAccessManager * manager;
    QNetworkReply * reply;
    QString file_Name;
    QSet<QString> image_Addresses;
    QString web_Address;
    Q_OBJECT
signals:
    void image_Saved();
public slots:
    void request_Image();
    void on_Finished(bool status);
    void got_Reply(QNetworkReply * reply);
public:
    ImageSaver();
    void start();
};

ImageSaver::ImageSaver()
{
    index = 0;

    manager = new QNetworkAccessManager;

    reply = NULL;

    connect(main_Page,SIGNAL(loadFinished(bool)),this,SLOT(on_Finished(bool)));

    connect(manager,SIGNAL(finished(QNetworkReply*)),this,SLOT(got_Reply(QNetworkReply*)));

    connect(this,SIGNAL(image_Saved()),this,SLOT(request_Image()));
}

void ImageSaver::start()
{
    //loads the url 
    // In the end of the loading it will emit load_Finished(bool)
    // So that signal will execute on_Finished(bool)
}

void ImageSaver::request_Image()
{
    QString temp_Address = *(image_Addresses.begin()+index);

   //makes a request to the server to give the image "temp_Address"
   //When the server gives the reply signal finished(QNetworkReply*) will be emitted
   // this in turn will call the got_Reply(QNetworkReply*)
}

void ImageSaver::on_Finished(bool status)
{
       //collects all the images's url addresses, and pushes them in the list 
        //"image_Addresses"
        //Then emits image_Saved();
        //This signal will wake up the function request_Image()
}

void ImageSaver::got_Reply(QNetworkReply * reply)
{
    //Image is extracted from the reply and got saved in the same name as in the page
    //index got increased;
    //emits the signal image_Saved();
    //This signal will activate the function request_Image()
}

int main(int argc,char * argv[])
{
    QApplication app(argc,argv);
    ImageSaver a;
    a.start();
    return app.exec();
}

#include "main.moc"

In short First call is to "start".That calls "on_Finished" and there is no problem untill this. So all the image files's addresses got pushed in the list. Next is one by one request for image[i] made, and the reply image got saved. This thing is happening repeatedly. Here only I am getting problem. Crashes are appearing in this operation especially in saving the image.

My assumption is "signal-slot" is not like function call, thy are more or less like thread but operates on the same function( pointer). So when one signal requests for painter, which is already rendering something then the crash will appear.

Can anybody say the fact behind the crash and how to save all the images without crash?

EDIT: Hi, This is the full code. Run this one, and click the message boxes contineously

    #include <QApplication>
#include <QDir>
#include <QImage>
#include <QObject>
#include <QMessageBox>
#include <QPainter>
#include <QPixmap>
#include <QSet>
#include <QTimer>
#include <QtNetwork/QNetworkAccessManager>
#include <QtNetwork/QNetworkReply>
#include <QtNetwork/QNetworkRequest>
#include <QtWebKit/QWebElement>
#include <QtWebKit/QWebFrame>
#include <QtWebKit/QWebPage>
#include <QUrl>

class ImageSaver:public QObject
{
    int index;
    QWebPage * main_Page;
    QNetworkAccessManager * manager;
    QNetworkReply * reply;
    QString file_Name;
    QSet<QString> image_Addresses;
    QString web_Address;
    Q_OBJECT
signals:
    void image_Saved();
public slots:
    void request_Image();
    void on_Finished(bool status);
    void got_Reply(QNetworkReply * reply);
public:
    ImageSaver();
    void start();
protected:
    //void show_Frame(QWebFrame * frame);
};

ImageSaver::ImageSaver()
{
    index = 0;

    this->main_Page = new QWebPage;

    manager = new QNetworkAccessManager;

    reply = NULL;

    connect(main_Page,SIGNAL(loadFinished(bool)),this,SLOT(on_Finished(bool)));

    connect(manager,SIGNAL(finished(QNetworkReply*)),this,SLOT(got_Reply(QNetworkReply*)));

    connect(this,SIGNAL(image_Saved()),this,SLOT(request_Image()));
}

void ImageSaver::start()
{
    web_Address = "yahoo.com";

    QDir dir;

    dir.mkdir(web_Address);

    QUrl url = QUrl::fromUserInput(web_Address);

    main_Page->mainFrame()->load(url);
}

void ImageSaver::request_Image()
{
    QString temp_Address = *(image_Addresses.begin()+index);

        int a = temp_Address.lastIndexOf("/");
        file_Name = temp_Address.mid(a+1);

        //Without the below message box, 开发者_JS百科the program closes shortly
        //This message box is slowing down that effect
        QMessageBox hh;
        hh.setText(file_Name);
        hh.exec();
        QNetworkRequest request= QNetworkRequest(QUrl(temp_Address));
        request.setRawHeader("img","src");
        manager->get(request);
}

void ImageSaver::on_Finished(bool status)
{
    if(status)
    {
        QMessageBox mm;
        mm.setText("Finished");
        mm.exec();

        QWebElementCollection temp_Collection= main_Page->mainFrame()->findAllElements("*");

        for(int i=0;i<temp_Collection.count();++i)
        {
            QWebElement temp_Element = temp_Collection[i];
            if(temp_Element.tagName().contains("img",Qt::CaseInsensitive) && temp_Element.attributeNames().contains("src",Qt::CaseInsensitive))
            {
                QString image_Web_Address = temp_Element.attribute("src");
                if(!image_Addresses.contains(image_Web_Address))
                    image_Addresses.insert(image_Web_Address);
            }
        }
        emit image_Saved();
        QMessageBox kk;
        kk.setText("Image is going to be saved");
        kk.exec();
    }

    else
    {
        QMessageBox mm;
        mm.setText("Not ready");
        mm.exec();
    }

    QMessageBox mm;
    mm.setText("Getting out of finished");
    mm.exec();
}

void ImageSaver::got_Reply(QNetworkReply * reply)
{
    QImage image;
    if(image.load(static_cast<QIODevice *>(reply),0))
        image.save(web_Address+QDir::separator()+file_Name,0);

    ++index;
    emit image_Saved();
}

/*
void ImageSaver::show_Frame(QWebFrame * temp_Frame)
{
    QImage image(temp_Frame->contentsSize(),QImage::Format_ARGB32_Premultiplied);
    image.fill(Qt::transparent);

    QPainter painter(&image);
    painter.setRenderHint(QPainter::Antialiasing,true);
    painter.setRenderHint(QPainter::TextAntialiasing,true);
    painter.setRenderHint(QPainter::SmoothPixmapTransform,true);

    temp_Frame->documentElement().render(&painter);

    painter.end();

    foreach(QWebFrame * temp_Frame0,temp_Frame->childFrames())
        show_Frame(temp_Frame0);
}
*/

int main(int argc,char * argv[])
{
    QApplication app(argc,argv);

    ImageSaver a;
    a.start();

    return app.exec();
}

#include "main.moc"

This is the pro file

QT += webkit network

SOURCES += \
    main.cpp

Your code crashes because you read beyond the boundaries of the image_Addresses set.

void ImageSaver::request_Image()
{
    QString temp_Address = *(image_Addresses.begin()+index);
    ...

You increment index after every image received, but there isn't any check anywhere in the code whether index is still less than image_Addresses.size(), so it crashes once dereferencing image_Addresses.begin()+index for index == image_Addresses.size().

There may be many solutions to this problem. I think that you should have a look at The State Machine Framework. In easy situations you can just use boolean variable to check if you can go on. You should also think what to do when you're busy processing the image. You can queue request or just reject them. Also you can implement threading, so that new requests are served by new threads.

P.S. Signals are more like events than threads to me.

What's the error and why do you have a #include at the end?

FYI there is a QImage class you can use instead which includes saving and loading from a QIODevice* such as QNetworkReply for example. It's extremely rare to need to reinvent the wheel in the massive framework that is Qt.