Creating vector of QLineEdit and connect signal for each one

lukutis222

Hello. In my program I have a QSpinBox and depending on the value of the spinbox, I must dynamically create a number of QLabel and QLineEdit equal to the value of the QSpinBox.

I have managed to do that just fine:

In the MainWindow.h I have declared the vectors:

    QVector <QLabel*> cell_label;
    QVector <QLineEdit*> cell_edit;

In my Mainwindow.cpp:

void MainWindow::on_spinBox_valueChanged(int arg1)
{
    clearLayout(ui->test_grid); // clear the grid layout to ensure all previous widgets are deleted
    cell_label.clear(); //clear the label vector
    cell_edit.clear(); //clear the edit vector

    for(int i =0;i<arg1;i++){
        QString cell_name = "Cell " + QString::number(i);
        cell_label.append(new QLabel(cell_name,this));
        cell_edit.append(new QLineEdit("Unknown",this));
        if(i >6){
            ui->test_grid->addWidget(cell_label[i],i-7,2);
            ui->test_grid->addWidget(cell_edit[i],i-7,3);
        }
        else{
            ui->test_grid->addWidget(cell_label[i],i,0);
            ui->test_grid->addWidget(cell_edit[i],i,1);
        }

    }
}

And that seems to work fine:

My problem:
Everytime the QLineEdit text is changed, I want to change its color depending on the text. If I didint have a vector of QLineEdits, that would be very simple. I could just simple create a textChanged signal such as:

        connect(cell_edit[i], &QLineEdit::textChanged, this, &MainWindow::cell_text_changed);

But the problem I have with the above method is that there is no way of knowing which particular QLineEdit has been modified. As you can see from the image above, I have 7 different QLineEdit that bas been created and if I modify the QLineEdit of Cell 2, I need to know that Cell 2 has been modified and not something else.
textChanged signal only takes text value as a parameter but it does not take the actual QLineEdit widget as an additional parameter.

In the cell_text_changed function I need to know 2 things:

The text
Which QLineEdit widget triggered the function.

I would appreciate if someone could suggest what would be the best way to solve this.

JonB

@lukutis222
This question is asked often as it is a common requirement. The (modern) answer is to specify a C++ lambda for the slot, which allows you to pass necessary parameters from where the connect() is specified:

connect(cell_edit[i], &QLineEdit::textChanged, this, [this, i]() { cell_text_changed(i); );
// or
connect(cell_edit[i], &QLineEdit::textChanged, this, [this, i](const QString &text) { cell_text_changed(i, text); );
// or
connect(cell_edit[i], &QLineEdit::textChanged, this, [this, i](const QString &text) { cell_text_changed(cell_edit[i], text); );

lukutis222

@JonB

Thanks! That works like a charm. I used:

        connect(cell_edit[i], &QLineEdit::textChanged, this, [this, i](const QString &text) { cell_text_changed(text,cell_edit[i]);} );

To the function I pass the text and the QLineEdit widget

void MainWindow::cell_text_changed(const QString &arg1,QLineEdit* line_edit)
{
    qDebug("cell tect changed = %s",arg1.toStdString().c_str());
    if(strcmp(arg1.toStdString().c_str(),"Connected")==0){
        qDebug("connected detected");
        line_edit->setStyleSheet("QLineEdit { background-color: rgb(0, 255, 0); selection-background-color: rgb(233, 99, 0); }");
    }
}

Although I must say that I am not familiar at all with such syntax. I have never come across something like that before. Would you be able to suggest some reading material regarding this?

JonB

@lukutis222
Search for lambda in https://wiki.qt.io/New_Signal_Slot_Syntax and in https://doc.qt.io/qt-6/signalsandslots.html. For C++ lambdas themselves see https://en.cppreference.com/w/cpp/language/lambda.

Lambdas were added in C++ 11 and have since become "common", as in other languages. Once you have got over the syntax etc. they are very useful!

On a separate topic. Why in your cell_text_changed() are you using toStdString() and strcmp()? Quite unnecessary (and we rarely use C functions like strcmp() any longer). Simply:

    qDebug("cell text changed = %s",arg1);    // or qDebug() << "cell text changed =" << arg1;
    if(arg1 == "Connected"){

lukutis222

@JonB
Thanks for reading material suggestions. And thanks for noticing my string comparison. I have replaced them now with more clean looking and simple solution that you have suggested.

I used strcmp because I come from C background. Most C++ features are still new to me.

JonB

@lukutis222 said in Creating vector of QLineEdit and connect signal for each one:

I used strcmp because I come from C background. Most C++ features are still new to me.

So did I :) But these days there are better C++ (inbuilt, std:: or Qt) alternatives to nearly every function which we used to use from the C runtime library.