Qt C++ GUI QSpinBox Storing Input? - c++

Question

How would I take the user input from the a spinbox and use that as a value? In other words if I wanted to store the input from the QSpinBox into a variable how would I go about doing this. Im really new at Qt GUI so any input would be greatly appreciated.

Answer 1

To react to GUI elements in Qt, you connect to the signals that those elements give off. Also if you have a pointer to the instance of it, you can query and change its states and properties.
Here is a quick example of what you are looking for
#include <QApplication>
#include <QVBoxLayout>
#include <QLabel>
#include <QSpinBox>
int main(int argc, char *argv[])
{
QApplication a(argc, argv);
// The widget, contains a layout
QWidget * w;
w = new QWidget;
// The layout arranges and holds
// all the children of the widget
QVBoxLayout * vbox;
vbox = new QVBoxLayout;
// The user input element, the spinbox!
QSpinBox * spinbox;
spinbox = new QSpinBox();
spinbox->setValue(5);// example of using a pointer to edit its states
// now add it to the layout
vbox->addWidget(spinbox);
// add in an element to connect to,
// the infamous QLabel
QLabel * label;
label = new QLabel("spinbox output");
// add it also to the layout
vbox->addWidget(label);
// connection can happen anytime as long as the two elements
// are not null!
// This connection takes advantage of signals and slots
// and making their connection at runtime.
// if a connect call fails you should be able to see why in the
// application output.
QObject::connect(spinbox, SIGNAL(valueChanged(QString)),
label, SLOT(setText(QString)));
// associate the layout to the widget
w->setLayout(vbox);
// make the widget appear!
w->show();
return a.exec();
}
I usually put most of the initializing and connecting of GUI elements into the constructor or a method of the main QWidget or the QMainWindow. I often take the signal from a GUI input element, like a spinbox and I connect it to a custom slot defined on my subclassed QWidget. Then if I want to display it with the input value differently or increase the output by 2, I can do so easily.
// in the constructor of my Widget class
// after spinbox has been initialized
QObject(m_spinbox, SIGNAL(valueChanged(int)),
this, SLOT(on_spinboxValueChanged(int)));
void Widget::on_spinboxValueChanged(int i)
{
// here m_label is a member variable of the Widget class
m_label->setText(QString::number(i + 2));
// if accessing the value in this function is inconvenient, you can always
// use a member variable pointer to it to get its stored value.
// for example:
int j = m_spinbox->value();
qDebug() << "Spinbox value" << j;
}
Identical things can be done in QML and Qt Quick, and for many people it is easier and more intuitive because of how close it is to javascript and css.
Also Qt Creator has a tool for generating forms, and it provides another way to create your widgets with layouts and then when you access your elements you do it through a ui variable.
Also the Qt docs and examples are awesome. Taking time to learn them and read up on them is well worth the effort.
Hope that helps.