//mainwindow.cpp

#include "mainwindow.h"
#include "ui_mainwindow.h"
#include <QDialog>
#include <QLineEdit>
#include <QPushButton>
#include <QVBoxLayout>
#include <QMessageBox>

MainWindow::MainWindow(QWidget *parent) :
    QMainWindow(parent),
    ui(new Ui::MainWindow)
{
    ui->setupUi(this);
    QDialog *Dialog = new QDialog;
    QVBoxLayout  *Layout = new QVBoxLayout();
    QLineEdit *line = new QLineEdit;
    QPushButton *button = new QPushButton(tr("Find"));
    Layout->addWidget(line);
    Layout->addWidget(button);
    Dialog->setLayout(Layout);

    Dialog->exec();
    searchString = line->text();

    connect(button, SIGNAL(clicked()), this, SLOT(find()));

}

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

void MainWindow::find()
{
   QMessageBox msgBox;
   msgBox.setText(tr("Find String : ") + searchString);
   msgBox.exec();
}

//mainwindow.h

#ifndef MAINWINDOW_H
#define MAINWINDOW_H

#include <QMainWindow>

namespace Ui {
class MainWindow;
}

class MainWindow : public QMainWindow
{
    Q_OBJECT

public:
    explicit MainWindow(QWidget *parent = nullptr);
    ~MainWindow();

private slots:
    void find();

private:
    Ui::MainWindow *ui;
    QString searchString;
};

#endif // MAINWINDOW_H