如何打印QLabel？

Question

I am trying to print a QLabel from a combobox in QT. Code looks like this:

QApplication a(argc, argv);
QWidget w;

QVBoxLayout *layout = new QVBoxLayout(&w);
QLabel *label = new QLabel("Here you will see the selected text from ComboBox", &w);
QComboBox *combo = new QComboBox(&w);
layout->addWidget(label);
layout->addWidget(combo);
Q_FOREACH(QSerialPortInfo port, QSerialPortInfo::availablePorts()) {
    combo->addItem(port.portName());

QObject::connect(combo, SIGNAL(currentIndexChanged(QString)), label, (SLOT(setText(QString))));

How do i print the label via cout?

Answer 1

Your code seems to be using Qt4, let's port that to Qt5 and a newer C++, shall we?

#include <QtWidgets>
#include <QtSerialPort>

int main(int argc, char ** argv) {
   QApplication app(argc, argv);
   QWidget w;

   auto layout = new QVBoxLayout(&w);
   auto label = new QLabel("Here you will see the selected text from ComboBox");
   auto combo = new QComboBox;
   layout->addWidget(label);
   layout->addWidget(combo);
   for (auto port : QSerialPortInfo::availablePorts())
       combo->addItem(port.portName());

   QObject::connect(combo, &QComboBox::currentTextChanged, [label, combo](){
       label->setText(combo->currentText());
       qDebug() << combo->currentText();
   });

   w.show();
   return app.exec();
}

• Try to not use Q_FOREACH in new code, it will probably be removed in the future ,

• Use auto when the type will be already specified by the new operator, this simplifies code,

• Use qDebug to output debug information to the terminal,

• Use lambdas in connections when the invoked code is short,

• Use the new style connections for connections, because they will guarantee that your code actually works, the old style has runtime checks, and the new has build time checks.