Qt No matching function for call to mainWindow::connect() - c++

Question

I'm trying to connect a combo box value and a label such that when the combo box changes the label reflects that. I have googled my heart out trying to find an answer but, as of yet, nothing has worked; I still get the error:no matching function for call to mainWindow::connect(QComboBox*&, const char [38], QString*, const char [26])
I have tried QObject::connect, QWidget::connect and anything else dealing with Qt, but to no avail.
Creating a label that says the combo box value is not my final intention for the program. Rather, I wish to get it working with a simple label then change it to what I want it to display (thus the tempLabel).
mainwindow.h:
class MainWindow : public QMainWindow
{
public:
MainWindow();
private slots:
QString getClass(QComboBox *box);
};
mainwindow.cpp:
MainWindow::MainWindow()
{
QString qMathClassName;
QComboBox* mathClassCombo = new QComboBox;
QLabel* label = new QLabel(qMathClassName);
// omitting layout code...
connect(mathClassCombo, SIGNAL(currentIndexChanged(const QString &)),
&qMathClassName, SLOT(getClass(mathClassCombo)));
}
QString MainWindow::getClass(QComboBox *box)
{
return box->currentText();
}
Any help would be greatly appreciated!

Answer 1

You are connecting a signal to a slot with a different signature. You have to change your slot to something like
getClass(const QString &)
to match currentIndexChanged signal.

Answer 2

I think you need to read Qt's signals and slots documentation. Again, if you've already done so. Pay special attention to their examples.
I think that you had these misconceptions about Qt in C++:
That QLabel takes a reference to a QString, and that it will update its text when that string changes. It won't. QLabel will display the value of the string when you give it the string. That is the only time it will update.
That objects constructed on the stack will not be destroyed at the end of the function. They will not. At the end of the constructor, qMathClassName will be destroyed and any reference to it will become invalid. Thus, you'd not want to make a connection to it, even if you could.
That the third argument of QObject::connect is a pointer to a place to put the return value for the slot. It's not. The third argument is a pointer to the QObject on which to call the slot. The return value of a slot is unused for any calls made to it via QObject::connect.
That you can bind values to slots in your connection. Unfortunately not. Within the SLOT macro, you must put the function signature of the slot. You may not reference any variables. The arguments section must have only class names. That is SLOT(getClass(QComboBox*)), not SLOT(getClass(mathClassCombo)).
The simplest way to ensure the contents of a combo box are displayed in a label are this:
QComboBox* mathClassCombo = new QComboBox;
QLabel* tempLabel = new QLabel;
connect(mathClassCombo, SIGNAL(currentIndexChanged(const QString&)),
tempLabel, SLOT(setText(const QString&)));
If you want to do something more complicated, I recommend just making a slot on your window that can handle those complications. For example:
mainwindow.h:
class MainWindow : public QMainWindow
{
Q_OBJECT
public:
MainWindow();
private slots:
void updateLabelText(const QString& className);
private:
QComboBox* mathClassCombo;
QLabel* tempLabel;
}
mainwindow.cpp:
MainWindow::MainWindow()
{
mathClassCombo = new QComboBox;
tempLabel = new QLabel;
// omitting layout code...
connect(mathClassCombo, SIGNAL(currentIndexChanged(const QString&)),
this, SLOT(updateLabelText(const QString&)));
}
void MainWindow::updateLabelText(const QString& className)
{
QString newLabelString = className + " is the best class ever!";
tempLabel->setCurrentText(newLabelString);
}