Написать программу с окном типа QMainWindow

//mainwindow.h
#ifndef MINWINDOW_H
#define MINWINDOW_H
 
#include <QtWidgets/QMainWindow>
#include <QPushButton>
#include <QLabel>
 
class MainWindow : public QMainWindow
{
    Q_OBJECT
 
public:
    MainWindow(QWidget *parent = 0);
 
private slots:
    void openFileDialog();
    void openGetSymbolDialog();
 
private:
    QPushButton* m_openSymbolDialogPButton;
    QPushButton* m_openFileDialogPButton;
    QLabel* m_textLabel;
};
 
#endif // MINWINDOW_H
 
//mainwindow.cpp
#include "mainwindow.h"
#include <QDockWidget>
#include <QVBoxLayout>
#include <QFileDialog>
#include <QFile>
#include <QDialog>
#include <QLineEdit>
 
 
MainWindow::MainWindow(QWidget *parent)
    : QMainWindow(parent)
{
    m_openSymbolDialogPButton = new QPushButton("Get symbol");
    m_openFileDialogPButton = new QPushButton("Select file");
    QVBoxLayout* vLayout = new QVBoxLayout;
    vLayout->addWidget(m_openSymbolDialogPButton);
    vLayout->addWidget(m_openFileDialogPButton);
    QWidget* widget = new QWidget;
    widget->setLayout(vLayout);
 
    QDockWidget* dock = new QDockWidget(this);
    dock->setWidget ( widget );
    addDockWidget(Qt::LeftDockWidgetArea, dock);
 
    m_textLabel = new QLabel;
    m_textLabel->setStyleSheet("background-color: magenta");
    setCentralWidget(m_textLabel);
 
    connect(m_openFileDialogPButton,   SIGNAL(clicked()), SLOT(openFileDialog()));
    connect(m_openSymbolDialogPButton, SIGNAL(clicked()), SLOT(openGetSymbolDialog()));
 
    m_openSymbolDialogPButton->setEnabled(false);
}
 
 
void MainWindow::openFileDialog()
{
    QString fileName = QFileDialog::getOpenFileName(this, tr("Open File"), ".",  "Text (*.txt)");
    
    QFile file(fileName);
    if(file.open(QIODevice::ReadOnly))
    {
        setWindowTitle(fileName);
        m_textLabel->setText(QString::fromLocal8Bit(file.readAll()));
        m_openSymbolDialogPButton->setEnabled(true);
    }
}
 
void MainWindow::openGetSymbolDialog()
{
    QDialog dialog(this);
 
    QPushButton* okButton = new QPushButton("Ok");
    QLineEdit* lineEdit = new QLineEdit;
    lineEdit->setMaxLength(1);
    QVBoxLayout* vBox = new QVBoxLayout;
    vBox->addWidget(lineEdit);
    vBox->addWidget(okButton);
    dialog.setLayout(vBox);
 
    connect(okButton, SIGNAL(clicked()), &dialog, SLOT(accept()));
    if(dialog.exec() == QDialog::Accepted)
    {
        QString symbol= lineEdit->text();
        if(!text.isEmpty())
        {
            lineEdit->setDisabled(true);
            lineEdit->setMaxLength(32767);
            lineEdit->setText(QString("Count of symbol \"%1\" is: %2").arg(symbol).arg(m_textLabel->text().count(symbol, Qt::CaseInsensitive)));
            dialog.exec();
        }
    }
}
 
//main.cpp
#include "mainwindow.h"
#include <QtWidgets/QApplication>
 
int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    MinWindow w;
    w.show();
    w.resize(300, 300);
    return a.exec();
}