I have interesting combination of warnings & errors.

Firstly, when debugging, i get warnings:

Qt Code: 
  1. can't find linker symbol for virtual table for `QFile' value
  2.   found `WebCore::JSDocument::JSDocument(JSC::Structure*, WebCore::JSDOMGlobalObject*, WTF::PassRefPtr<WebCore::Document>)' instead
  3. RTTI symbol not found for class 'WebCore::JSHTMLDocument'
  4. RTTI symbol not found for class 'WebCore::JSHTMLDocument'
  5. RTTI symbol not found for class 'WebCore::JSHTMLDocument'
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secondly, i have runtime error:

Qt Code: 
  1. QIODevice::open: File access not specified
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and momently

Qt Code: 
  1. *** glibc detected *** <path>: double free or corruption (out): 0x081f9d00 ***
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Here's the minimal code that causes that errors:

Files

In result folder created folder "resources" and in it file "vk.cookie" (everything without quotes).

Bug.pro

Qt Code: 
  1. QT       += core gui webkit network xml
  2.  
  3. TARGET = Bug
  4. TEMPLATE = app
  5.  
  6.  
  7. SOURCES += main.cpp \
  8.     api_vk.cpp \
  9.     printer.cpp
  10.  
  11. HEADERS  += \
  12.     api_vk.h \
  13.     printer.h
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main.cpp

Qt Code: 
  1. #include <QtGui/QApplication>
  2. #include "api_vk.h"
  3. #include "printer.h"
  4.  
  5. int main(int argc, char *argv[]) {
  6.     QApplication a(argc, argv);
  7.     API_VK *apivk = new API_VK;
  8.     apivk->authorise();
  9.     Printer *printer = new Printer;
  10.     QObject::connect(apivk, SIGNAL(authorisationSucceed()), printer, SLOT(printOK()));
  11.     QObject::connect(apivk, SIGNAL(authorisationFailed(QString,QString)), printer, SLOT(printFail()));
  12.  
  13.     return a.exec();
  14. }
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api_vk.h

Qt Code: 
  1. #ifndef API_VK_H
  2. #define API_VK_H
  3.  
  4. #include <QObject>
  5. #include <QTimer>
  6. #include <QUrl>
  7. #include <QtNetwork/QNetworkCookieJar>
  8.  
  9. class QWebView;
  10. class QString;
  11. class QNetworkReply;
  13. class QTimer;
  14. class QNetworkCookie;
  15.  
  16. class API_VK : public QObject
  17. {
  18.     Q_OBJECT
  19. public:
  20.     explicit API_VK(QObject *parent = 0);
  21.  
  22. signals:
  23.     void authorisationFailed(QString error, QString error_description);
  24.     void authorisationSucceed();
  25.  
  26. public slots:
  27.     void authorise();
  28.  
  29. protected:
  30.     void readCookies();
  31.     void writeCookies();
  32.  
  33. protected slots:
  34.     void newAuthoriseRequest();
  35.     void processUrl(QUrl url);
  36.  
  37. private:
  38.     static const QString app_id;
  39.     static QString access_token;
  40.     static qint32 expires_in;
  41.     QWebView *messagesPage;
  42.     QList<QNetworkCookie> cookies;
  43.     QNetworkCookieJar jar;
  44.  
  45.     static bool authorised;
  46. };
  47.  
  48. #endif
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api_vk.cpp

Qt Code: 
  1. #include "api_vk.h"
  2. #include <QtGui>
  3. #include <QWebView>
  4. #include <QtNetwork/QNetworkRequest>
  5. #include <QtNetwork/QNetworkAccessManager>
  6. #include <QtNetwork/QNetworkReply>
  7. #include <QtNetwork/QNetworkCookie>
  8. #include <QtNetwork/QNetworkCookieJar>
  9. #include <QString>
  10. #include <QStringList>
  11. #include <QFile>
  12. #include <QTextStream>
  13. #include <QTimer>
  14. #include <QUrl>
  15. #include <QtXml>
  16. #include <QVariant>
  17. #include <QDateTime>
  18. #include <QDebug>
  19.  
  20.  
  21. bool API_VK::authorised = false;
  22. const QString API_VK::app_id = "2783286";
  23. QString API_VK::access_token = "";
  24. int API_VK::expires_in = 0;
  25.  
  26.  
  27.     // defining class methods
  28.  
  29. API_VK::API_VK(QObject *parent) :
  30.     QObject(parent)
  31. {
  32. }
  33.  
  34. void API_VK::authorise() {
  35.     newAuthoriseRequest();    // 1. going here
  36. }
  37.  
  38. void API_VK::newAuthoriseRequest() {
  39.     // gets new access_token
  40.     // 2. going here
  41.  
  42.     messagesPage = new QWebView;
  43.     readCookies();
  44.     jar.setCookiesFromUrl(cookies, QUrl("http://vk.com"));
  45.     messagesPage->page()->networkAccessManager()->setCookieJar(&jar);
  46.  
  47.     QUrl url("http://oauth.vk.com/authorize");
  48.     url.addQueryItem("client_id", app_id);
  49.     url.addQueryItem("scope", "messages");
  50.     url.addQueryItem("redirect_uri","http://api.vk.com/blank.html");
  51.     url.addQueryItem("display","page");
  52.     url.addQueryItem("response_type","token");
  53.  
  54.     messagesPage->load(QNetworkRequest(url));
  55.     connect(messagesPage, SIGNAL(urlChanged(QUrl)), this, SLOT(processUrl(QUrl)));
  56.     messagesPage->show();
  57.  
  58. }
  59.  
  60. void API_VK::processUrl(QUrl url) {  // 3. going here
  61. /* firstly we're here when oath.vk.com redirects us to api.vk.com/login...
  62.  * in this case we're exiting at 4.
  63.  * secondly, user logs in, and api.vk.com/login redirects us back to oath.vk.com,
  64.  * where we get access_token, etc
  65.  * and when debugging, we start receiving warnings about "can't find linker symbol" secondly, not firstly
  66.  */
  67.  
  68. //    if (!url.hasQueryItem("access_token"))
  69. //        return;
  70.  
  71.     /* I commented previous part because most of you doesn't have VK accounts so you can't go
  72.      * to the next part of code */
  73.  
  74.     access_token = url.queryItemValue("access_token");
  75.     expires_in = url.queryItemValue("expires_in").toInt();
  76.  
  77.     emit authorisationSucceed();
  78.  
  79.     authorised = true;
  80.     cookies = messagesPage->page()->networkAccessManager()->cookieJar()->cookiesForUrl(QUrl("http://vk.com"));
  81.     messagesPage->deleteLater();
  82.     writeCookies();
  83. }
  84.  
  85. void API_VK::readCookies() {
  86.     QFile file("./resouces/vk.cookie");
  87.     if (!file.open(QIODevice::ReadOnly | QIODevice::Text)) {
  88.         return;
  89.     }
  90.  
  91.     QTextStream in(&file);
  92.     QByteArray name, value;
  93.     while (!in.atEnd()) {
  94.         in >> name >> value;
  95.         cookies.append(QNetworkCookie(name, value));
  96.     }
  97. }
  98.  
  99. void API_VK::writeCookies() {
  100.     QFile file("./resouces/vk.cookie");   // note: this file exists
  101.     if (!file.open(QIODevice::Truncate | QIODevice::Text)) {  // 5. at this line i receive runtime errors
  102.         return;
  103.     }
  104.  
  105.     QTextStream out(&file);
  106.     for (QList<QNetworkCookie>::const_iterator i = cookies.begin(); i != cookies.end(); ++i) {
  107.         out << (*i).name() << ' ' << (*i).value() << '\n';
  108.     }
  109. }
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printer.h

Qt Code: 
  1. #ifndef PRINTER_H
  2. #define PRINTER_H
  3.  
  4. #include <QObject>
  5. #include <QDebug>
  6.  
  7. struct Printer: public QObject {
  8.     Q_OBJECT
  9.  
  10. public slots:
  11.     void printOK() { qDebug() << "OK"; }
  12.     void printFail() { qDebug() << "Fail"; }
  13. };
  14.  
  15. #endif // PRINTER_H
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printer.cpp

Qt Code: 
  1. #include "printer.h"
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Here's the full output with memory dump: http://pastebin.com/btVNe4nd

At 5 QtCreator says that app received signal from OS (signal: SIGABRT) and decompiles

Qt Code: 
  1. Disassembler (__kernel_vsyscall)
  2.  
  3. 0x132414  <+0x0000>         int    $0x80
  4. 0x132416  <+0x0002>         ret
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And valgrind says:

Qt Code: 
  1. Analysis of memory <path>
  2. QMetaObject::connectSlotsByName: No matching signal for on_cancelButton_clicked()
  3. "sni-qt/23102" WARN  20:28:53.697 void StatusNotifierItemFactory::connectToSnw() Invalid interface to SNW_SERVICE 
  4. ** Analysis finished **
  5. ** Unknown error **
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Question on StackOwerflow: http://stackoverflow.com/questions/1...corruption-out

So, why I receive they? I don't use malloc()/free(), you can see it.