About using layouts, widget parenting

Introduction#

The layouts are a necessary in every Qt application. They manage the object, their position, their size, how they are resized.

Remarks#

From Qt layout documentation:

When you use a layout, you do not need to pass a parent when constructing the child widgets. The layout will automatically reparent the widgets (using QWidget::setParent()) so that they are children of the widget on which the layout is installed.

So do :

QGroupBox *box = new QGroupBox("Information:", widget);
layout->addWidget(box);

or do :

QGroupBox *box = new QGroupBox("Information:", nullptr);
layout->addWidget(box);

is exactly the same.

Basic Horizontal Layout

The horizontal layout set up the object inside it horizontally.

basic code:

#include <QApplication>

#include <QMainWindow>
#include <QWidget>
#include <QHBoxLayout>
#include <QPushButton>

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

    QMainWindow window;
    QWidget *widget = new QWidget(&window);
    QHBoxLayout *layout = new QHBoxLayout(widget);

    window.setCentralWidget(widget);
    widget->setLayout(layout);

    
    
    layout->addWidget(new QPushButton("hello world", widget));
    layout->addWidget(new QPushButton("I would like to have a layout !", widget));
    layout->addWidget(new QPushButton("layouts are so great !", widget));

    window.show();

    return a.exec();
}

this will output:

Basic Vertical Layout

The vertical layout set up the object inside it vertically.

#include "mainwindow.h"
#include <QApplication>

#include <QMainWindow>
#include <QWidget>
#include <QVBoxLayout>
#include <QPushButton>

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

    QMainWindow window;
    QWidget *widget = new QWidget(&window);
    QVBoxLayout *layout = new QVBoxLayout(widget);

    window.setCentralWidget(widget);
    widget->setLayout(layout);



    layout->addWidget(new QPushButton("hello world", widget));
    layout->addWidget(new QPushButton("I would like to have a layout !", widget));
    layout->addWidget(new QPushButton("layouts are so great !", widget));

    window.show();

    return a.exec();
}

output:

Combining Layouts

You can combine mulple layout thanks to other QWidgets in your main layout to do more specifics effects like an information field: for example:

#include <QApplication>

#include <QMainWindow>
#include <QWidget>
#include <QVBoxLayout>
#include <QPushButton>
#include <QLabel>
#include <QLineEdit>
#include <QGroupBox>

#include <QTextEdit>

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

    QMainWindow window;
    QWidget *widget = new QWidget(&window);
    QVBoxLayout *layout = new QVBoxLayout(widget);

    window.setCentralWidget(widget);
    widget->setLayout(layout);

    QGroupBox *box = new QGroupBox("Information:", widget);
    QVBoxLayout *boxLayout = new QVBoxLayout(box);

    layout->addWidget(box);


    QWidget* nameWidget = new QWidget(box);
    QWidget* ageWidget = new QWidget(box);
    QWidget* addressWidget = new QWidget(box);

    boxLayout->addWidget(nameWidget);
    boxLayout->addWidget(ageWidget);
    boxLayout->addWidget(addressWidget);

    QHBoxLayout *nameLayout = new QHBoxLayout(nameWidget);
    nameLayout->addWidget(new QLabel("Name:"));
    nameLayout->addWidget(new QLineEdit(nameWidget));


    QHBoxLayout *ageLayout = new QHBoxLayout(ageWidget);
    ageLayout->addWidget(new QLabel("Age:"));
    ageLayout->addWidget(new QLineEdit(ageWidget));

    QHBoxLayout *addressLayout = new QHBoxLayout(addressWidget);
    addressLayout->addWidget(new QLabel("Address:"));
    addressLayout->addWidget(new QLineEdit(addressWidget));



    QWidget* validateWidget = new QWidget(widget);
    QHBoxLayout *validateLayout = new QHBoxLayout(validateWidget);
    validateLayout->addWidget(new QPushButton("Validate", validateWidget));
    validateLayout->addWidget(new QPushButton("Reset", validateWidget));
    validateLayout->addWidget(new QPushButton("Cancel", validateWidget));

    layout->addWidget(validateWidget);

    window.show();

    return a.exec();
}

will output :

Grid layout example

The grid layout is a powerful layout with which you can do an horizontal and vertical layout a once.

example:

#include "mainwindow.h"
#include <QApplication>

#include <QMainWindow>
#include <QWidget>
#include <QVBoxLayout>
#include <QPushButton>
#include <QLabel>
#include <QLineEdit>
#include <QGroupBox>

#include <QTextEdit>

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

    QMainWindow window;
    QWidget *widget = new QWidget(&window);
    QGridLayout *layout = new QGridLayout(widget);

    window.setCentralWidget(widget);
    widget->setLayout(layout);

    QGroupBox *box = new QGroupBox("Information:", widget);
    layout->addWidget(box, 0, 0);

    QVBoxLayout *boxLayout = new QVBoxLayout(box);

    QWidget* nameWidget = new QWidget(box);
    QWidget* ageWidget = new QWidget(box);
    QWidget* addressWidget = new QWidget(box);

    boxLayout->addWidget(nameWidget);
    boxLayout->addWidget(ageWidget);
    boxLayout->addWidget(addressWidget);

    QHBoxLayout *nameLayout = new QHBoxLayout(nameWidget);
    nameLayout->addWidget(new QLabel("Name:"));
    nameLayout->addWidget(new QLineEdit(nameWidget));


    QHBoxLayout *ageLayout = new QHBoxLayout(ageWidget);
    ageLayout->addWidget(new QLabel("Age:"));
    ageLayout->addWidget(new QLineEdit(ageWidget));

    QHBoxLayout *addressLayout = new QHBoxLayout(addressWidget);
    addressLayout->addWidget(new QLabel("Address:"));
    addressLayout->addWidget(new QLineEdit(addressWidget));



    layout->addWidget(new QPushButton("Validate", widget), 1, 0);
    layout->addWidget(new QPushButton("Reset", widget), 1, 1);
    layout->addWidget(new QPushButton("Cancel", widget), 1, 2);


    window.show();

    return a.exec();
}

give :

so you can see that the group box is only in the first column and first row as the addWidget was layout->addWidget(box, 0, 0);

However, if you change it to layout->addWidget(box, 0, 0, 1, 3);, the new 0 and 3 represent how many line and column you want for your widget so it give :

exactly the same as you created a horizontal and then a vertical layout in a subwidget.