mainwindow.cpp

#include "mainwindow.h"
#include "ui_mainwindow.h"
#include<iostream>
#include<fstream>
#include <QFileDialog>
#include <QFile>
#include <QMessageBox>
#include <QTextStream>

MainWindow::MainWindow(QWidget *parent) :
    QMainWindow(parent),
    ui(new Ui::MainWindow)
{
    ui->setupUi(this);
}

MainWindow::~MainWindow()
{
    delete ui;
}

void MainWindow::on_QuitButton_clicked()
{
    qApp->quit();
}

void MainWindow::on_actionOpen_triggered()
{
    QString fileName = QFileDialog::getOpenFileName(this, tr("Open File"), QString(),
               tr("Text Files (*.txt);;C++ Files (*.cpp *.h)"));

       if (!fileName.isEmpty()) {
           QFile file(fileName);
           if (!file.open(QIODevice::ReadOnly)) {
               QMessageBox::critical(this, tr("Error"), tr("Could not open file"));
               return;
           }
           QTextStream in(&file);
           ui->textEdit->setText(in.readAll());
           file.close();
       }
}