Signal and Slot understanding in Qt - qt

Question

I am having problem with "Signal and Slot" understanding. Here below is my task description.
Select an item from Combo Box
Display that selected item, by a Label in the mainwindow.
It is very simple thing but I am not able to solve it.
Here below is my code:
#ifndef MAINWINDOW_H
#define MAINWINDOW_H
#include <QMainWindow>
namespace Ui { class MainWindow; }
class MainWindow : public QMainWindow {
Q_OBJECT
public:
explicit MainWindow(QWidget *parent = 0);
~MainWindow();
signals:
void activated(QString);
private slots:
void setLabelValue(const QString &text);
private:
Ui::MainWindow *ui;
};
#endif // MAINWINDOW_H
#include "mainwindow.h"
#include "ui_mainwindow.h"
MainWindow::MainWindow(QWidget *parent) :
QMainWindow(parent),
ui(new Ui::MainWindow)
{
ui->setupUi(this);
connect(ui->comboBox, &QComboBox::activated(QString),
this, &MainWindow::setLabelValue(QString));
}
MainWindow::~MainWindow()
{
delete ui;
}
MainWindow::setLabelValue(const QString &text)
{
//text = ui->comboBox->currentText();
text = static_cast<void (QComboBox::*)(QString)>(ui->comboBox->currentText());
ui->label->setText(text);
}
I hope you will forgive this novice's coding and understanding style.

Answer 1

The problem is you need to use the QLabel accessor function to change the text value. text() is a 'getter', not a 'setter'.
change MainWindow::setLabelValue() to
MainWindow::setLabelValue()
{
ui->label->setText(ui->comboBox->currentText());
}
Also, your connection isn't quite right. setLabelValue is a function of MainWindow, not the label, so change the connection to:
void (QComboBox::* activatedOverloadPtr)(const QString&) = &QComboBox::activated;
connect(ui->comboBox,activatedOverloadPointer, this, &MainWindow::setLabelValue);
Activated overload pointer is used so that the compiler can resolve which specific activated connection you are looking for.
The complete code would look something like:
#ifndef MAINWINDOW_H
#define MAINWINDOW_H
#include <QMainWindow>
namespace Ui { class MainWindow; }
class MainWindow : public QMainWindow {
Q_OBJECT
public:
explicit MainWindow(QWidget *parent = 0);
~MainWindow();
//signals: // this is defined by QComboBox, not you (in this case)
// void activated(QString);
private slots:
void setLabelValue(const QString &text);
private:
Ui::MainWindow *ui;
};
#endif // MAINWINDOW_H
.
#include "mainwindow.h"
#include "ui_mainwindow.h"
MainWindow::MainWindow(QWidget *parent) :
QMainWindow(parent),
ui(new Ui::MainWindow)
{
ui->setupUi(this);
void (QComboBox::* activatedOverloadPtr)(const QString&) = &QComboBox::activated;
connect(ui->comboBox,activatedOverloadPointer, this, &MainWindow::setLabelValue);
}
MainWindow::~MainWindow()
{
delete ui;
}
MainWindow::setLabelValue()
{
ui->label->setText(ui->comboBox->currentText());
}

Answer 2

Thanks all for your support. Here below the code, that has worked.
#ifndef MAINWINDOW_H
#define MAINWINDOW_H
#include <QMainWindow>
namespace Ui {
class MainWindow;
}
class MainWindow : public QMainWindow
{
Q_OBJECT
public:
explicit MainWindow(QWidget *parent = 0);
~MainWindow();
private slots:
void setLabelValue(const QString &text);
private:
Ui::MainWindow *ui;
};
#endif // MAINWINDOW_H
#include "mainwindow.h"
#include "ui_mainwindow.h"
MainWindow::MainWindow(QWidget *parent) :
QMainWindow(parent),
ui(new Ui::MainWindow)
{
ui->setupUi(this);
connect(ui->comboBox, static_cast<void (QComboBox::*)(const QString&)>(&QComboBox::activated), this,
&MainWindow::setLabelValue);
}
MainWindow::~MainWindow()
{
delete ui;
}
void MainWindow::setLabelValue(const QString &text)
{
ui->label->setText(text);
}