Global variable contains garbage when push button is clicked - c++

Question

#include "mainwindow.h"
#include "ui_mainwindow.h"
#include <QtDebug>
#include <iostream>
char * ip_ch;
void test(const char* a)
{
qDebug()<<"test is "<<a;
}
MainWindow::MainWindow(QWidget *parent) :
QMainWindow(parent),
ui(new Ui::MainWindow)
{
ui->setupUi(this);
}
MainWindow::~MainWindow()
{
delete ui;
}
void MainWindow::on_lineEdit_editingFinished()
{
QString st_ip_ch=ui->lineEdit->text();
QByteArray ip_ch_temp = st_ip_ch.toLatin1();
ip_ch = ip_ch_temp.data();
qDebug()<<"ip_ch is "<<ip_ch;
test(ip_ch);
}
void MainWindow::on_pushButton_clicked()
{
qDebug()<<"when push button ";
test(ip_ch);
}
The code uses the following UI. I want to type some string in the line input widget like this, and then push the button.
It prints out this:
ip_ch is 123
test is 123
When button is clicked, it prints:
test is `??`
Why does ip_ch point to garbage?

Answer 1

ip_ch becomes a dangling pointer as soon as on_lineEdit_editingFinished() returns. That's because you're making it point to a temporary buffer ip_ch_temp that goes out of scope:
ip_ch = ip_ch_temp.data(); // Don't do that!
Since you're coding in C++, you shouldn't be writing C. Store your Latin1 representation by value:
class MainWindow : public QMainWindow {
QByteArray ip_ch;
Ui::MainWindow ui;
public:
MainWindow(QWidget * parent = 0) {
ui.setupUi(this);
}
// no explicit destructor needed!
void test(const QByteArray & data) { qDebug() << "test:" << data; }
void on_lineEdit_editingFinished()
{
ip_ch = ui.lineEdit->test().toLatin1();
qDebug() << "ip_ch is" << ip_ch;
test(ip_ch);
}
void on_pushButton_clicked()
{
qDebug() << "button was pushed";
test(ip_ch);
}
};
Other problems with your code:
#include <QDebug>, not <QtDebug>
#include <QMainWindow>, not <qmainwindow.h>
Most likely, you don't need to derive your dialog from QMainWindow; derive from a QDialog instead.
Don't use a pointer to ui; store it by value instead. That way you have less code that can go wrong: let the compiler manage memory for you. That's what it's for!