Have a look at this, this is one way to do it.

Qt Code: 
  1. #include <QApplication>
  2. #include <QtGui>
  3. #include <QtCore>
  4.  
  5. class QTextEditLoader : public QObject // Added <<<<<<<<<<<<<<<<<<<<
  6. {
  7.     Q_OBJECT
  8.  
  9. public:
  10.     explicit QTextEditLoader(QTextEdit *textEdit, QLineEdit * filename)
  11.         : QObject(textEdit)
  12.         , parent(textEdit)
  13.         , fileName(filename)
  14.     {
  15.         ;
  16.     }
  17.  
  18. public slots:
  19.     void loadTextFromFile(void)
  20.     {
  21.         QString text("<Error, Unable to open file>");
  22.         QFile file(fileName->text());
  23.  
  24.         if(file.open(file.ReadOnly | file.Text))
  25.         {
  26.             text = file.readAll();
  27.             if(text.isEmpty())
  28.                 text = "<Error, Empty/Non-Text File>";
  29.             file.close();
  30.         }
  31.  
  32.         parent->setText(text);
  33.     }
  34.  
  35. private:
  36.     QTextEdit *parent;
  37.     QLineEdit *fileName;
  38. };
  39.  
  40. int main(int argc, char *argv[])
  41. {
  42.     QApplication app(argc, argv);
  43.     QWidget *window = new QWidget;
  44.     window->setWindowTitle("Text Reader");
  45.  
  46.     QGridLayout *layout = new QGridLayout;
  47.  
  48.     QTextEdit *textbox = new QTextEdit();
  49.     QLineEdit *enterFile = new QLineEdit();
  50.     QTextEditLoader *loader = new QTextEditLoader(textbox, enterFile); // Added <<<<<<<<<<<<<<<<<<<<
  51.     QPushButton *goButton = new QPushButton("Ok");
  52.     QPushButton *quitButton = new QPushButton("Quit");
  53.  
  54.     layout->addWidget(textbox, 0,0,1,1);
  55.     layout->addWidget(enterFile, 1,0,1,1);
  56.     layout->addWidget(goButton, 2,0,1,1);
  57.     layout->addWidget(quitButton, 3,0,1,1);
  58.  
  59.     window->setLayout(layout);
  60.  
  61.     QObject::connect(quitButton, SIGNAL(clicked()), qApp, SLOT(quit()));
  62.     QObject::connect(goButton, SIGNAL(clicked()), loader, SLOT(loadTextFromFile())); // Added <<<<<<<<<<<<<<<<<<<<
  63.  
  64.     //
  65.     //STUFF TO BE ADDED
  66.     //
  67.  
  68.     window->show();
  69.     return app.exec(); //Starts main event loop
  70. }
  71.  
  72. #include "Main.moc"
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