it's code for example:

ui.h:
Qt Code: 
  1. #ifndef UI_GUI_H
  2. #define UI_GUI_H
  3.  
  4. #include <QtCore/QVariant>
  5. #include <QtGui/QAction>
  6. #include <QtGui/QApplication>
  7. #include <QtGui/QButtonGroup>
  8. #include <QtGui/QLabel>
  9. #include <QtGui/QMainWindow>
  10. #include <QtGui/QMenuBar>
  11. #include <QtGui/QPushButton>
  12. #include <QtGui/QWidget>
  13.  
  14. class Ui_MainWindow
  15. {
  16. public:
  17.     QWidget *centralwidget;
  18.     QPushButton *pushButtonOpen;
  19.     QPushButton *pushButtonClose;
  20.     QLabel *label;
  21.     QMenuBar *menubar;
  22.  
  23.     void setupUi(QMainWindow *MainWindow)
  24.     {
  25. 	.....
  26.     label = new QLabel(centralwidget);
  27.     label->setObjectName(QString::fromUtf8("label"));
  28.     label->setGeometry(QRect(60, 100, 46, 14));
  29. 	.......
  30.  
  31.     retranslateUi(MainWindow);
  32.     //QObject::connect(pushButtonOpen, SIGNAL(clicked()), label, SLOT(clear()));
  33.  
  34.     QMetaObject::connectSlotsByName(MainWindow);
  35.     } // setupUi
  36.  
  37.     void retranslateUi(QMainWindow *MainWindow)
  38.     {
  39. 	........
  40.     label->setText(QApplication::translate("MainWindow", "TextLabel", 0, QApplication::UnicodeUTF8));
  41.     Q_UNUSED(MainWindow);
  42.     } // retranslateUi
  43.  
  44. };
  45.  
  46. namespace Ui {
  47.     class MainWindow: public Ui_MainWindow {};
  48. } // namespace Ui
  49.  
  50. #endif // UI_GUI_H
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main.cpp:
Qt Code: 
  1. #include <QtGui>
  2. #include <QApplication>
  3.  
  4. #include "ventana.h"
  5.  
  6. int main(int argc, char *argv[])
  7. {
  8.     QApplication app(argc, argv);
  9.  
  10.     intervalo = 0;
  11.  
  12.     Ventana widget;
  13.     widget.show();
  14.  
  15.     return app.exec();
  16. }
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ventana.h:
Qt Code: 
  1. #include "ui.h"
  2.  
  3. class Ventana : public QMainWindow, private Ui::MainWindow
  4. {
  5. 	Q_OBJECT
  6.  
  7. 	public:
  8. 	    Ventana();
  9.  
  10. 	public slots:
  11. 		void Bopen();
  12. 		void Bclose();
  13. };
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ventana.cpp:
Qt Code: 
  1. #include <QtGui>
  2. #include "ventana.h"
  3. #include "function.h"
  4.  
  5. Ventana::Ventana()
  6. {
  7.     setupUi(this);
  8.  
  9.     connect(pushButtonOpen, SIGNAL(clicked()), this, SLOT(Bopen()));
  10.     connect(pushButtonClose, SIGNAL(clicked()), this, SLOT(Bclose()));    
  11. }
  12.  
  13. void Ventana::Bclose(){
  14. 	label->setText(QString("CloseOk"));
  15.         functionDoSomething();
  16. }
  17.  
  18. void Ventana::Bopen(){
  19.     label->setText(QString("OpenOK"));      
  20. }
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From HERE im call label->setText();
But say is undeclared.

function.cpp:
Qt Code: 
  1. #include <QtGui>
  2. #include "function.h"
  3.  
  4. void functionDoSomething(){
  5.        //here call to Qlabel
  6.        label->setText(QString("HELLO WORLD"));
  7. }
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function.h:
Qt Code: 
  1. void functionDoSomething();
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