Thanks fnmblot as well as Thanks again jpn .
It seems that I'm that kind of annoying students who won't stop questioning

I've made what you 've told me & separated the declaration from the implementation .
However , my slot didn't work yet ?!!
+ How does the header file which generated ,ui_Test.h , can call my CustomSlot ?!!

I've changed a little in my App :
Here are my Codes :

=> Main.cpp 's code is :

Qt Code: 
  1. #include <QApplication>
  2. #include <QDialog>
  3.  
  4. #include "ui_Test.h"
  5.  
  6. int main(int argc, char *argv[])
  7. {
  8.     QApplication app(argc, argv);
  9.  
  10.  
  11.  
  12. QWidget *widget = new QWidget;
  13.      Ui_MyForm ui;
  14.      ui.setupUi(widget);
  15.  
  16.      widget->show();
  17.      return app.exec();
  18. }
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---------------------------------------------------------------------------------------------------------------


ui_Test.h 's code is :

Qt Code: 
  1. /********************************************************************************
  2. ** Form generated from reading ui file 'Test.ui'
  3. **
  4. ** Created: Thu Jun 5 19:58:05 2008
  5. **      by: Qt User Interface Compiler version 4.4.0
  6. **
  7. ** WARNING! All changes made in this file will be lost when recompiling ui file!
  8. ********************************************************************************/
  9.  
  10. #ifndef UI_TEST_H
  11. #define UI_TEST_H
  12.  
  13. #include <QtCore/QVariant>
  14. #include <QtGui/QAction>
  15. #include <QtGui/QApplication>
  16. #include <QtGui/QButtonGroup>
  17. #include <QtGui/QLineEdit>
  18. #include <QtGui/QPushButton>
  19. #include <QtGui/QWidget>
  20.  
  21. QT_BEGIN_NAMESPACE
  22.  
  23. class Ui_MyForm
  24. {
  25. public:
  26.     QPushButton *btnCreateFile;
  27.     QLineEdit *txtData;
  28.  
  29.     void setupUi(QWidget *MyForm)
  30.     {
  31.     if (MyForm->objectName().isEmpty())
  32.         MyForm->setObjectName(QString::fromUtf8("MyForm"));
  33.     MyForm->resize(533, 393);
  34.     btnCreateFile = new QPushButton(MyForm);
  35.     btnCreateFile->setObjectName(QString::fromUtf8("btnCreateFile"));
  36.     btnCreateFile->setGeometry(QRect(170, 190, 75, 24));
  37.     txtData = new QLineEdit(MyForm);
  38.     txtData->setObjectName(QString::fromUtf8("txtData"));
  39.     txtData->setGeometry(QRect(160, 90, 113, 20));
  40.  
  41.     retranslateUi(MyForm);
  42.     QObject::connect(btnCreateFile, SIGNAL(pressed()), MyForm, SLOT(CreateFile()));
  43.  
  44.     QMetaObject::connectSlotsByName(MyForm);
  45.     } // setupUi
  46.  
  47.     void retranslateUi(QWidget *MyForm)
  48.     {
  49.     MyForm->setWindowTitle(QApplication::translate("MyForm", "Form", 0, QApplication::UnicodeUTF8));
  50.     btnCreateFile->setText(QApplication::translate("MyForm", "PushButton", 0, QApplication::UnicodeUTF8));
  51.     txtData->setText(QString());
  52.     Q_UNUSED(MyForm);
  53.     } // retranslateUi
  54.  
  55. };
  56.  
  57. namespace Ui {
  58.     class MyForm: public Ui_MyForm {};
  59. } // namespace Ui
  60.  
  61. QT_END_NAMESPACE
  62.  
  63. #endif // UI_TEST_H
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---------------------------------------------------------------------------------------------------------------

=> CustomSlot.h 's code is :

Qt Code: 
  1. #ifndef CustmSlot_Declaration_H
  2. #define CustmSlot_Declaration_H
  3. #include "ui_Test.h"
  4.  
  5. class CustomSlot: public QWidget
  6.  {
  7.      Q_OBJECT
  8.  
  9.  public:
  10.      CustomSlot(QWidget *parent = 0);
  11.  
  12.  private slots:
  13.      void CreateFile();
  14.  
  15.  
  16.  private:
  17.      Ui_MyForm ui;
  18.  };
  19. #endif
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---------------------------------------------------------------------------------------------------------------

=> CustomSlot.cpp 's code is :

Qt Code: 
  1. #include"CustomSlot.h"
  2. #include "ui_Test.h"
  3. #include <stdio.h>
  4.  
  5.  
  6.  
  7.  
  8. CustomSlot::CustomSlot(QWidget *parent)
  9.      : QWidget(parent)
  10.  {
  11.      ui.setupUi(this);
  12.  }
  13.  
  14. void CustomSlot:: CreateFile()
  15.  {
  16.     FILE * pFile;
  17.     pFile = fopen ("C:/myfile.txt","w");
  18.  }
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Thanks