i've centos 6.6 as server and client in vm ware. one has mysql server and other has client mysql. i've two dialogs in client. one is "MyAppMainWindow" and other one is "MyDialog". problenm is that when i want to create MyDialog's object in MyAppMainWindow implementation file "myappmainwindow.cpp" programm crashes.

here is myappmainwindow.cpp
messageboxes as only to track error.

Qt Code: 
  1. #include "myappmainwindow.h"
  2. #include "ui_myappmainwindow.h"
  3.  
  4. MyAppMainWindow::MyAppMainWindow(QWidget *parent) :
  5.     QMainWindow(parent),
  6.     ui(new Ui::MyAppMainWindow)
  7. {
  8.     ui->setupUi(this);
  9.  
  10.     db = new QSqlDatabase(QSqlDatabase::addDatabase("QMYSQL"));
  11.     db->setHostName("oracle");
  12.     db->setDatabaseName("test");
  13.     //db->setDatabaseName("music");
  14.     db->setUserName("rahul");
  15.     db->setPassword("rahul");
  16.     db->setPort(3306);
  17.  
  18.     query = new QSqlQuery(*db);
  19.     model = new QSqlQueryModel(this);
  20.     //mydlg = new MyDialog(this);
  21. }
  22.  
  23. MyAppMainWindow::~MyAppMainWindow()
  24. {
  25.     db->close();
  26.     delete ui;
  27. }
  28.  
  29. void MyAppMainWindow::on_pushButtonLogin_clicked()
  30. {
  31.  
  32.     QString str;
  33.  
  34.     QString su = ui->lineEditUser->text().trimmed();
  35.     QString sp = ui->lineEditPassword->text().trimmed();
  36.  
  37.     str = "select *from tablelogin where user = \"" + su + "\" and password = \"" + sp +"\";";
  38.  
  39.     if(db->open() )
  40.     {
  41.         query = new QSqlQuery(str, *db);
  42.  
  43.         if(query->exec())
  44.         {
  45.             int count = 0;
  46.             while(query->next())
  47.                 count++;
  48.  
  49.             if(count == 1)
  50.             {
  51.  
  52.                 QMessageBox::information(this, "sqlqueymodel", "user name and password is correct");
  53.                 model->setQuery(*query);
  54.                 ui->tableViewLogin->setModel(model);
  55.                 QMessageBox::information(this, "sqlqueymodel", "ui->tableViewLogin->setModel(model);");
  56.                 mydlg = new MyDialog(this);//here program crashes
  57.                 QMessageBox::information(this, "sqlqueymodel", " mydlg = new MyDialog(this);");
  58.                 mydlg->setModal(true);
  59.                 mydlg->exec();
  60.  
  61.             }
  62.             else if( count > 1 )
  63.                 QMessageBox::information(this, "sqlqueymodel", "duplicate username ... access denied ");
  64.             else
  65.                 QMessageBox::critical(this, "sqlqueymodel", "user name or password is not correct");
  66.         }
  67.     }
  68.     else
  69.     {
  70.         QMessageBox::critical(this, "db->open", db->lastError().text().trimmed());
  71.     }
  72.  
  73.     db->close();
  74. }
  75.  
  76. void MyAppMainWindow::on_pushButtonSecondForm_clicked()
  77. {
  78.     this->hide();
  79.  
  80.     mydlg = new MyDialog(this);
  81.  
  82.     mydlg->setModal(true);
  83.  
  84.     mydlg->exec();
  85. }
  86.  
  87. QSqlDatabase * MyAppMainWindow::open_db(QSqlDatabase *db)
  88. {
  89.     return (db = this->db);
  90. }
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myappaminwindow.h
-------------------
Qt Code: 
  1. #include <QtDebug>
  2. #include <QSqlDriver>
  3. #include <QSqlError>
  4. #include <QSqlQueryModel>
  5.  
  6. #include <QMessageBox>
  7.  
  8. #include "mydialog.h"
  9.  
  10. namespace Ui {
  11. class MyAppMainWindow;
  12. }
  13.  
  14. //class MyDialog;
  15.  
  16. class MyAppMainWindow : public QMainWindow
  17. {
  18.     Q_OBJECT
  19.  
  20. public:
  21.     explicit MyAppMainWindow(QWidget *parent = 0);
  22.     ~MyAppMainWindow();
  23.  
  24. private slots:
  25.     void on_pushButtonLogin_clicked();
  26.  
  27.     void on_pushButtonSecondForm_clicked();
  28.  
  29. public slots:
  30.  
  31.     QSqlDatabase *open_db(QSqlDatabase *db);
  32.  
  33. private:
  34.     Ui::MyAppMainWindow *ui;
  36.     QSqlQuery *query;
  37.  
  39.  
  40.     MyDialog *mydlg;
  41.  
  42.  
  43. };
  44.  
  45. #endif // MYAPPMAINWINDOW_H
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mydialog.h
Qt Code: 
  1. #ifndef MYDIALOG_H
  2. #define MYDIALOG_H
  3.  
  4. #include <QDialog>
  5. //#include "myappmainwindow.h"
  6. #include <QSqlQuery>
  7. #include <QSqlDatabase>
  8. #include <QtSql>
  9.  
  10. namespace Ui {
  11. class MyDialog;
  12. }
  13.  
  14. class MyAppMainWindow;
  15.  
  16. class MyDialog : public QDialog
  17. {
  18.     Q_OBJECT
  19.  
  20. public:
  21.     explicit MyDialog(QWidget *parent = 0);
  22.     ~MyDialog();
  23.  
  24.     void fillcombo();
  25.  
  26. signals:
  27.  
  28.     void open_db(QSqlDatabase *dbdlg);
  29.  
  30. private slots:
  31.     void on_pushButton_clicked();
  32.  
  33.     void on_comboBoxEmployee_currentTextChanged(const QString &arg1);
  34.  
  35. private:
  36.     Ui::MyDialog *ui;
  37.     MyAppMainWindow *mywindow;
  38.     QSqlQuery *query;
  39.     QSqlDatabase *dbdlg;
  40.  
  41. };
  42.  
  43. #endif // MYDIALOG_H
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mydialog.cpp

Qt Code: 
  1. #include "mydialog.h"
  2. #include "ui_mydialog.h"
  3.  
  4. #include "myappmainwindow.h"
  5.  
  6. MyDialog::MyDialog(QWidget *parent) :
  7.     QDialog(parent),
  8.     ui(new Ui::MyDialog)
  9. {
  10.     ui->setupUi(this);
  11.     mywindow = new MyAppMainWindow(this);
  12.  
  13.     //mywindow->db->open();
  14.     connect(this, SIGNAL(open_db(QSqlDatabase *)), mywindow, SLOT(open_db(QSqlDatabase *)) );
  15.  
  16.     emit open_db(dbdlg);
  17.  
  18.     this->query = new QSqlQuery(*dbdlg);
  19.     fillcombo();
  20. }
  21.  
  22. MyDialog::~MyDialog()
  23. {
  24.     delete ui;
  25. }
  26.  
  27. void MyDialog::on_pushButton_clicked()
  28. {
  29.  
  30.     QString strname = ui->lineEditUserName->text().trimmed();
  31.  
  32.     ui->comboBoxEmployee->addItem(strname);
  33.  
  34. }
  35.  
  36. void MyDialog::on_comboBoxEmployee_currentTextChanged(const QString &arg1)
  37. {
  38.     //ui->labelUserNameCombo->setText(ui->comboBoxEmployee->currentText().trimmed());
  39.     ui->lineEditUserName->setText(arg1);
  40. }
  41.  
  42. void MyDialog::fillcombo()
  43. {
  44.     //if(mywindow->db->open())
  45.     if(dbdlg->open())
  46.     {
  47.         QString str = "select user\ name from tableEmployeeData;";
  48.         this->query = new QSqlQuery(str, *dbdlg);
  49.         if(this->query->exec())
  50.             while(this->query->next())
  51.                 ui->comboBoxEmployee->addItem(this->query->value(1).toString());
  52.  
  53.     }
  54. }
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Qt Code: 
  1. #include "myappmainwindow.h"
  2. #include <QApplication>
  3.  
  4. int main(int argc, char *argv[])
  5. {
  6.     QApplication a(argc, argv);
  7.     MyAppMainWindow w;
  8.     w.show();
  9.  
  10.     return a.exec();
  11. }
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how to solve this problem.