Thanks aamer4yu.
Perfect jpn .

It works now after using Promoting .
@ the MainWindow I've added a plain QWidget into it .
I've noticed that its header file generated by "make" creates instance of CustomSlot (which implements the slots of the form appears into the plain widget @ MainWindow form .

The Problem :now I want to replace the .UI that opened into the plain widget @ MainWindow with another .UI @ SAme plain Widget


Header File's code generated of MainWindow that Contains the plain widget :

Qt Code: 
  1. class Ui_MainForm
  2. {
  3. public:
  4.     CustomSlot *MiniWidget;  // MiniWidget = The Plain widget
  5.  
  6.     void setupUi(QWidget *MainForm)
  7.     {
  8.     if (MainForm->objectName().isEmpty())
  9.         MainForm->setObjectName(QString::fromUtf8("MainForm"));
  10.     MainForm->resize(530, 402);
  11.     MiniWidget = new CustomSlot(MainForm);
  12.     MiniWidget->setObjectName(QString::fromUtf8("MiniWidget"));
  13.     MiniWidget->setGeometry(QRect(39, 29, 451, 301));
  14.  
  15.     retranslateUi(MainForm);
  16.  
  17.     QMetaObject::connectSlotsByName(MainForm);
  18.     } // setupUi
  19.  
  20.     void retranslateUi(QWidget *MainForm)
  21.     {
  22.     MainForm->setWindowTitle(QApplication::translate("MainForm", "Main Form", 0, QApplication::UnicodeUTF8));
  23.     Q_UNUSED(MainForm);
  24.     } // retranslateUi
  25.  
  26. };
  27.  
  28. namespace Ui {
  29.     class MainForm: public Ui_MainForm {};
  30. } // namespace Ui
  31.  
  32. QT_END_NAMESPACE
  33.  
  34. #endif // UI_MAINWINDOW_H
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Main.cpp 's code :

Qt Code: 
  1. #include <QApplication>
  2. #include "ui_MainWindow.h"
  3. #include "MainApp.cpp"
  4.  
  5. int main(int argc, char *argv[])
  6. {
  7.     QApplication app(argc, argv);    
  8.  
  9.       MainApp widget;
  10.      widget.show(); 
  11.  
  12.      return app.exec();
  13.  
  14.  
  15. }
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The MainApp's code that implements slots of the MainWindow :
Qt Code: 
  1. MainApp::MainApp(QWidget *parent): QWidget(parent)
  2.  {
  3.     ui.setupUi(this);
  4.  }
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Thanks