Not being able to execute ftp ConnectToHost in Qt

Question

#include <qftp.h>
#include <qapplication.h>
#include "test.h"
#include "ui_test.h"
#include <qfile.h>

int main()
{
    QApplication a(argc, argv);
    Test w;
    ftp.connectToHost("192.168.26.129", 21);
    w.show();
    return a.exec();
}

I am new to Qt, ftp.connectToHost("192.168.26.129", 21) works well from within main, but whenever called from class Test, it fails (with same header inclusion ) as below.

Test::Test(QWidget *parent) :
QMainWindow(parent),
ui(new Ui::Test)
{   
    ui->setupUi(this);
    QFtp ftp;

    int x = ftp.connectToHost("192.168.26.129", 21);
    ftp.login("User1", "passwd");
}

Any suggestions would be helpful. Thanks!

Answer 1

I would suggest to use a class member since it is an async operation. You are having an object on the stack which will be unwound when the function, and for that matter the life cycle of the object, ends.

You have three alternatives to handle this:

Class member

Test::Test(QWidget *parent) :
QMainWindow(parent),
ui(new Ui::Test)
{   
    ui->setupUi(this);

    int x = m_ftp.connectToHost("192.168.26.129", 21);
    m_ftp.login("User1", "passwd");
}

This version is useful when you need the QFtp object in other method as well.

Smart pointer

Test::Test(QWidget *parent) :
QMainWindow(parent),
ui(new Ui::Test)
{   
    ui->setupUi(this);
    QPointer m_ftp = new QFtp());

    int x = ftp->connectToHost("192.168.26.129", 21);
    ftp->login("User1", "passwd");
}

This version is good when you only need the QFtp object in this method, and the Test class is not a QObject.

QObject parent-child relation

Test::Test(QWidget *parent) :
QMainWindow(parent),
ui(new Ui::Test)
{   
    ui->setupUi(this);
    QFtp ftp = new Ftp(this);    

    int x = ftp->connectToHost("192.168.26.129", 21);
    ftp->login("User1", "passwd");
}

This will not work if the "Test" class becomes a non-QObject, but it is good enough when Test inherits that, and you only need the QFtp object in this method.

Answer 2

When your Test class constructor goes out of scope, your ftp object gets destroyed. QFtp::connectToHost function does not block and returns immediately. Same goes for QFtp::login.

To solve this problem you can allocate your QFtp object using new:

QFtp *ftp = new QFtp(this);