signal mapping issue with QlineEdit - c++

Question

I want to pass the argument to slot. so i'm using signal mapping . I'm passing the QlinEedit variable as a argument to the slot using setmapping . but my slot function is not calling.
QSignalMapper* maper = new QSignalMapper (this) ;
connect(ui->step2,SIGNAL(returnPressed()),maper,SLOT(map()));
maper->setMapping(ui->step2,ui->step2);
connect (maper, SIGNAL(mapped(QLineEdit*)), this, SLOT(on_steps_returnPressed(QLineEdit*))) ;

Answer 1

The QSignalMapper class is deprecated and you should use the new signal/slots syntax with a lambda function:
connect(ui->step2, &QLineEdit::returnPressed, [=]() { this->on_steps_returnPressed(ui->step2); });
You can also pass only the text inside you QLineEdit instead of a pointer on it:
connect(ui->step2, &QLineEdit::returnPressed, [=]() { this->on_steps_returnPressed(ui->step2->text()); });
If you have to use a QSignalMapper anyway, you have to use the signal QSignalMapper::mapped(QWidget*). Since a slot has to have the same signature than the connected signal, on_steps_returnPressed becomes on_steps_returnPressed(QWidget*) (cast the parameter to a QLineEdit):
QSignalMapper* maper = new QSignalMapper (this) ;
connect(step2, SIGNAL(returnPressed()), maper, SLOT(map()));
maper->setMapping(step2, step2);
connect(maper, SIGNAL(mapped(QWidget*)), this, SLOT(on_steps_returnPressed(QWidget*)));
Your slot should be something like that:
void on_steps_returnPressed(QWidget* widget)
{
QLineEdit* edit = qobject_cast<QLineEdit*>(widget);
if (!edit)
return;
label->setText(edit->text());
}
A complete example to compare the two solutions:
class Widget: public QWidget
{
Q_OBJECT
public:
Widget(QWidget* parent=nullptr): QWidget(parent),
lineEdit1(new QLineEdit(this)),
lineEdit2(new QLineEdit(this)),
label(new QLabel(this))
{
QVBoxLayout* layout = new QVBoxLayout(this);
layout->addWidget(lineEdit1);
layout->addWidget(lineEdit2);
layout->addWidget(label);
connect(lineEdit1, &QLineEdit::returnPressed, [=]() { this->on_steps_returnPressed(lineEdit1); });
QSignalMapper* maper = new QSignalMapper (this) ;
connect(lineEdit2, SIGNAL(returnPressed()), maper, SLOT(map()));
maper->setMapping(lineEdit2, lineEdit2);
connect(maper, SIGNAL(mapped(QWidget*)), this, SLOT(on_steps_returnPressed(QWidget*)));
}
public slots:
void on_steps_returnPressed(QLineEdit* edit)
{
label->setText(edit->text());
}
void on_steps_returnPressed(QWidget* widget)
{
QLineEdit* edit = qobject_cast<QLineEdit*>(widget);
if (!edit)
return;
label->setText(edit->text());
}
private:
QLineEdit* lineEdit1;
QLineEdit* lineEdit2;
QLabel* label;
};