How to make the signals of child widgets inside a custom widget connectable?

Question

I think I have some major problem with a concept that seems pretty basic to me.

I created a custom widget, which is actually just a small collection of widgets, which, as such, would appear several times.

class CustomWidget : public QWidget {
    Q_OBJECT
public:
    explicit CustomWidget(QWidget parent=nullptr) : QWidget(parent) {
        spinboxA = new QSpinBox;
        spinboxB = new QSpinBox;
        QHBoxLayout* layout = new QHBoxLayout(this);
        layout.addWidget(spinboxA);
        layout.addWidget(spinboxB);
        this->setLayout(layout);
    }
private:
    QSpinBox* spinboxA;
    QSpinBox* spinboxB;
};

This custom widget is then used inside a gui. I want this gui to react to changes of the value of the spinboxes, of course. In my understanding I can either

1) Provide getter for the QSpinBoxes and connect their signals outside the class. 2) "Re-route" their signals like in the example below

@1) used via connect(customwidget->getSpinboxA(),SIGNAL(valueChanged(int)),this,SLOT(doSomething(int)));, I guess?

@2)

class CustomWidget : public QWidget {
    Q_OBJECT
public:
    explicit CustomWidget(QWidget parent=nullptr) : QWidget(parent) {
        spinboxA = new QSpinBox;
        spinboxB = new QSpinBox;
        QHBoxLayout* layout = new QHBoxLayout;
        layout.addWidget(spinboxA);
        layout.addWidget(spinboxB);
        this->setLayout(layout);
        connect(spinboxA,SIGNAL(valueChanged(int)),//...
            this,SLOT(onSpinboxAValueChanged(int)));
    }
private:
    QSpinBox* spinboxA;
    QSpinBox* spinboxB;
private slots:
    void onSpinboxAValueChanged(int x) {emit spinboxAValueChanged(x);}
    //...
signals:
    void spinboxAValueChanged(int x)
};

In the gui class one would connect(customwidget,SIGNAL(spinboxAValueChanged(int),this,SLOT(doSomething(int)));

Especially version 2) seems very cluttered and... I'm asking myself - how do I connect to the signals of widgets inside my custom widget?

Answer 1

A CustomWidget should be modular, that is, it should be like a black box, where inputs should be established and outputs obtained, so for me the second solution is very close to it, but I see something that can be improved: it is not necessary to create a slot only to emit a signal, signals can be connected to other signals, I also recommend using the new connection syntax.

#include <QApplication>
#include <QHBoxLayout>
#include <QSpinBox>
#include <QWidget>

#include <QDebug>

class CustomWidget : public QWidget {
    Q_OBJECT
public:
    explicit CustomWidget(QWidget *parent =nullptr):
        QWidget(parent),
        spinboxA(new QSpinBox),
        spinboxB(new QSpinBox)
    {
        QHBoxLayout* layout = new QHBoxLayout(this);
        layout->addWidget(spinboxA);
        layout->addWidget(spinboxB);
        connect(spinboxA, QOverload<int>::of(&QSpinBox::valueChanged), this, &CustomWidget::spinboxAValueChanged);
        connect(spinboxB, QOverload<int>::of(&QSpinBox::valueChanged), this, &CustomWidget::spinboxBValueChanged);
        // old syntax:
        // connect(spinboxA, SIGNAL(valueChanged(int)), this, SIGNAL(spinboxAValueChanged(int)));
        // connect(spinboxB, SIGNAL(valueChanged(int)), this, SIGNAL(spinboxBValueChanged(int)));
    }
private:
    QSpinBox *spinboxA;
    QSpinBox *spinboxB;
signals:
    void spinboxAValueChanged(int x);
    void spinboxBValueChanged(int x);
};

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    CustomWidget w;

    QObject::connect(&w, &CustomWidget::spinboxAValueChanged, [](int i){
       qDebug()<< "spinboxAValueChanged: "<< i;
    });
    QObject::connect(&w, &CustomWidget::spinboxBValueChanged, [](int i){
       qDebug()<< "spinboxBValueChanged: "<< i;
    });

    w.show();

    return a.exec();
}

#include "main.moc"