Application crashing after calling a method - c++

Question

I want to return a QString value from a function that exists in class called DB in my MainWindow class, but it always crashes the application.
db.h
#ifndef DB_H
#define DB_H
#include <QMainWindow>
#include <QtSQl>
#include <QSqlQuery>
#include <QSqlError>
#include <QObject>
#include <QDialog>
class DB : public QMainWindow
{
Q_OBJECT
public:
explicit DB(QWidget *parent = 0);
QString getDriver() const;
void setDriver(const QString &value);
private:
QString Driver="test";
signals:
public slots:
};
#endif // DB_H
and this is db.cpp
#include "db.h"
DB::DB(QWidget *parent) : QMainWindow(parent)
{
}
QString DB::getDriver() const
{
return Driver;
}
void DB::setDriver(const QString &value)
{
Driver = value;
}
mainwindow.h
#ifndef MAINWINDOW_H
#define MAINWINDOW_H
#include <QMainWindow>
#include <QDebug>
#include <QFileInfo>
#include <QDialog>
#include <QObject>
#include "Db.h"
namespace Ui {
class MainWindow;
}
class MainWindow : public QMainWindow
{
Q_OBJECT
public:
explicit MainWindow(QWidget *parent = 0);
~MainWindow();
DB *conn;
private slots:
void on_pushButton_clicked();
private:
Ui::MainWindow *ui;
};
#endif // MAINWINDOW_H
mainwindow.cpp
#include "mainwindow.h"
#include "ui_mainwindow.h"
#include <QDebug>
MainWindow::MainWindow(QWidget *parent) :
QMainWindow(parent),
ui(new Ui::MainWindow)
{
ui->setupUi(this);
}
MainWindow::~MainWindow()
{
delete ui;
}
void MainWindow::on_pushButton_clicked()
{
QString driverfromdb = conn->getDriver();
qDebug() << driverfromdb;
}
And this what happen when I click the pushbutton: image

Answer 1

You are declaring a pointer to DB class inside your MainWindow class
DB *conn;
but you never initialize it. At the moment it does not point to anything, so if you try to access it:
QString driverfromdb = conn->getDriver(); // boom, conn is somewhere in the void
The solution is simple, you need to create a DB object before using it, e.g.:
void MainWindow::on_pushButton_clicked()
{
conn = new DB();
QString driverfromdb = conn->getDriver();
qDebug() << driverfromdb;
}
or just let conn be a typical variable, not an pointer:
class MainWindow : public QMainWindow
{
Q_OBJECT
public:
explicit MainWindow(QWidget *parent = 0);
~MainWindow();
DB conn; // conn is created after creating MainWindow object
This way you have not to initialize anything and you will avoid crashes.

Answer 2

or:
MainWindow::MainWindow(QWidget *parent) :
QMainWindow(parent), conn(new DB),
ui(new Ui::MainWindow)
{
ui->setupUi(this);
}
MainWindow::~MainWindow()
{
delete conn;
delete ui;
}