qt todoapp

widget.h

#ifndef WIDGET_H
#define WIDGET_H
#include 
#include 
#include 
#include 
#include 
#include 
QT_BEGIN_NAMESPACE
namespace Ui {
class Widget;
}
QT_END_NAMESPACE
class Widget : public QWidget
{
    Q_OBJECT
public:
    Widget(QWidget *parent = nullptr);
    ~Widget();
private slots:
    void on_btnAdd_clicked();
    void on_btnRemove_clicked();
    void on_btnRemoveAll_clicked();
private:
    Ui::Widget *ui;
    QString path = QStandardPaths::writableLocation(QStandardPaths::DocumentsLocation) + "\\todoFile.txt";
};
#endif // WIDGET_H

widget.cpp

#include "widget.h"
#include "ui_widget.h"
Widget::Widget(QWidget *parent)
    : QWidget(parent)
    , ui(new Ui::Widget)
{
    ui->setupUi(this);
    this->setWindowTitle("Todo List App");
    QFile file(path);
    if(!file.open(QIODevice::ReadWrite)) {
        QMessageBox::information(0, "error", file.errorString());
    }
    QTextStream in(&file);
    while(!in.atEnd()) {
        QListWidgetItem* item = new QListWidgetItem(in.readLine(), ui->listWidget);
        ui->listWidget->addItem(item);
        item->setFlags(item->flags() | Qt::ItemIsEditable);
    }
}
Widget::~Widget()
{
    // write file
    QFile file(path);
    if(!file.open(QIODevice::WriteOnly)) {
        QMessageBox::information(0, "error", file.errorString());
    }
    QTextStream out(&file);
    for (int i=0; ilistWidget->count(); i++) {
        out listWidget->item(i)->text() 
    if(ui-lineEdit->text().isEmpty()) {
        return;
    }
    QListWidgetItem * item = new QListWidgetItem(ui->lineEdit->text(), ui-> listWidget);
    ui->listWidget->addItem(item);
    item->setFlags(item->flags() | Qt::ItemIsEditable);
    ui->lineEdit->clear();
    ui->lineEdit->setFocus();
}
void Widget::on_btnRemove_clicked()
{
    QListWidgetItem* item = ui->listWidget->takeItem(ui->listWidget->currentRow());
    delete item;
}
void Widget::on_btnRemoveAll_clicked()
{
    ui->listWidget->clear();
}