qserialport crashes when called from slot - c++

Question

I am writing a gui in QT for controlling usb relays. I wrote a relay class which has turn on/off member functions. also wrote a custom widget which has radio buttons for controlling, a relay pointer, as well as a switchStatus() slot in which the relay turn on/off functions were called. The mainwindow has relay members. When I call the the ui->widget->switchStatus() from the mainwindow constructor, everything works fine, the relay can be turn on and off well. However, if I connect the radio button signal to switchStatus(), the program crashes whenever I click the radio button. It crashes at the line serialPort->write. But it's not about write, whatever code related to serialPort pointer it first come to will cause crash. even I want to get the port name or port address.
myWidget::myWidget(QWidget *parent) : QWidget(parent), m_ui(new Ui::Form)
{ status = 0;
m_ui->setupUi(this);
m_ui->statusIndicator->status = &status; // status in ui pointing to null before this
m_ui->turnOffButton->setChecked(true);
connect(m_ui->turnOnButton, SIGNAL(clicked(bool)), this, SLOT(switchStatus()));
//crashes when click ratioButton,
//compare to last line in the mainwindow construtor
connect(m_ui->turnOffButton, SIGNAL(clicked(bool)), this, SLOT(switchStatus()));
}
void myWidget::switchStatus()
{
qDebug() << "swithcing";
if(status)
{
setStatus(false);
}
else
{
setStatus(true);
}
m_relay->switchStatus();
}
void relay::switchStatus()
{ if(status) turnOff();
else turnOn();
}
bool relay::turnOn(){
qDebug() << writeDataOn; // test if string is correct
qDebug() << serialPort; // crashes whenever serialPort address is called
const qint64 bytesWritten = serialPort->write(writeDataOn);
}
MainWindow::MainWindow(QWidget *parent) :
QMainWindow(parent),
ui(new Ui::MainWindow)
{
QList<QSerialPortInfo> infos = QSerialPortInfo::availablePorts();
qDebug() << infos[0].description();
QSerialPort serialPort;
serialPort.setPortName(infos[0].portName());
serialPort.setBaudRate(QSerialPort::Baud9600);
if (!serialPort.open(QIODevice::ReadWrite)) {
qDebug() << QObject::tr("Failed to open port %1, error: %2").arg(serialPort.portName()).arg(serialPort.error()) ;
}
ui->setupUi(this);
for(int i=0;i<4;i++)
{
relays[i].setRelayNumber(i);
relays[i].setPort(&serialPort);
relays[i].setStatus(relays[i].getRealStatus());
}
ui->widget->m_relay = relays;
qDebug() << ui->widget->m_relay;
qDebug() << ui->widget->m_relay->getPort();
ui->widget->switchStatus(); // this line runs well
}

Answer 1

Make your serial port a member of the MainWindow class. At the moment it is local to your constructor, so it is deallocated when the constructor finishes, hence the crashing. If you have declared a 'serialPort' member, you are hiding it by redeclaring it as a local scope variable inside the constructor.