Instantiate widgets with a list of check boxes and QPair

diego-qt

Hello, I would like to create a widget with a list of QCheckBox, which the user can choose from, and a Start button that instantiates and shows other windows (widgets) according to the checked boxes, knowing that all these widgets are different classes.

Yesterday I found out about the existence of QPair and I had the idea to pair a check box and a null instantiation of a widget, e.g.:

QVector<QPair<QCheckBox*, QWidget*>> vectorOptions;
vectorOptions << qMakePair(new QCheckBox("Option 1"), (WidgetChild1*) nullptr);
vectorOptions << qMakePair(new QCheckBox("Option 2"), (WidgetChild2*) nullptr);

The idea is to connect a signal produced when the Start button is pressed, to the slot handleStartButton(). The problem is that I can't instantiate a QWidget pointer in QPair::second and I don't understand why.

void Class::handleStartButton()
{
    QVectorIterator<QPair<QCheckBox*, QWidget*>> itPairVector(vectorOptions);
    QPair<QCheckBox*, QWidget*> current;

    while(itPairVector.hasNext())
    {
        current = itPairVector.next();
        if(current.first->isChecked() && current.second == nullptr)
        {
            WidgetChild1 *tmp = new WidgetChild1;
            current = qMakePair(current.first, tmp);
            //current = qMakePair(current.first, new WidgetChild1);
            //line above would be equal to the two previous ones
            qDebug() << "in if: second initialized?" << current.second;
        }
    }

    QVectorIterator<QPair<QCheckBox*, QWidget*>> itPairVector2(vectorOptions);
    while(itPair2.hasNext())
    {
        qDebug() << "after: second initialized?" << itPair2.next().second;
    }
}

With this code, I am able to instantiate a WidgetChild1 object, show it and the debug prints something like

in if: second initialized? WidgetChild1(0x55562182c540)

However, I have the warning Potential leak of memory pointed to by field 'second' at the end of the first while loop, which means that the widgets do not stick to QPair::second.
I can verify this with the second while loop: if QPair:second was correctly initialized, it would show something similar to the first debug, but it prints

after: second initialized? QWidget(0x0)
after: second initialized? QWidget(0x0)

so it still points to nullptr.

I would appreciate some information as to how I can correctly instantiate the widget.

Christian Ehrlicher

@diego-qt said in Instantiate widgets with a list of check boxes and QPair:

current = itPairVector.next();

This creates a copy of your QPair.
Use a range-based for loop:

for (auto &current : vectorOptions)
    {
        if(current.first->isChecked() && current.second == nullptr)
        {
            current.second = new WidgetChild1;
        }
    }

diego-qt

Thank you very much! This worked instantly. I see that I was not enough documented about QPair. I will dig into the range-based loops because I didn't know about them.
However I have a warning unused variable 'current'. It doesn't make sense to me because I use it in the loop. Do you know why this happens?

Christian Ehrlicher

@diego-qt said in Instantiate widgets with a list of check boxes and QPair:

Do you know why this happens?

You did not remove your variable current which you defined (for unknown reasons) before your while loop.

diego-qt

Yep, sorry. I didn't erase because I thought it was necessary but it is already declared in the for loop. Thanks!