Layout managres in Qt provide mechanisms to control how widgets resize and reposition when their container changes size. A key concept in this behavior is the stretch factor, which defines the relative proportion by which widgets grow or shrink.

Understanding Stretch Factors

• By default, widgets managed by a layout expand equally when space becomes available.
• Custom stretch factors can be assigned to individual widgets or layout rows/columns to control resizing behavior.
• In QBoxLayout, stretch factors are set using addStretch() or via widget-specific policies.
• Note: Initial widget sizes are determined before layout application, so visual proportions may only become apparent after resizing the window.

Using QGridLayout

QGridLayout arranges child widgets in a two-dimensional grid. Each widget occupies one or more cells defined by row and column indices.

Stretch factors can also be applied to entire rows or columns in a grid layout:

• setRowStretch(row, stretch) controls vertical expansion of a row.
• setColumnStretch(column, stretch) controls horizontal expansion of a column.

Additionally, layouts can be nested—QGridLayout can contain other layout managers as items, enabling complex UI structures.

Example Implementation

The following code demonstrates a 2×2 grid of buttons with custom minimum sizes and a vertical stretch factor applied to the first row.

#ifndef WIDGET_H
#define WIDGET_H

#include <QWidget>
#include <QPushButton>

class GridWidget : public QWidget
{
    Q_OBJECT

private:
    QPushButton buttonA;
    QPushButton buttonB;
    QPushButton buttonC;
    QPushButton buttonD;

    void setupGridLayout();

public:
    explicit GridWidget(QWidget *parent = nullptr);
    ~GridWidget() override = default;
};

#endif // WIDGET_H

#include "Widget.h"
#include <QGridLayout>
#include <QSizePolicy>

GridWidget::GridWidget(QWidget *parent)
    : QWidget(parent)
    , buttonA(this)
    , buttonB(this)
    , buttonC(this)
    , buttonD(this)
{
    setupGridLayout();
}

void GridWidget::setupGridLayout()
{
    buttonA.setText("Button A");
    buttonB.setText("Button B");
    buttonC.setText("Button C");
    buttonD.setText("Button D");

    auto policy = QSizePolicy(QSizePolicy::Expanding, QSizePolicy::Expanding);
    policy.setHorizontalStretch(1);
    policy.setVerticalStretch(1);

    buttonA.setSizePolicy(policy);
    buttonB.setSizePolicy(policy);
    buttonC.setSizePolicy(policy);
    buttonD.setSizePolicy(policy);

    buttonA.setMinimumSize(160, 50);
    buttonB.setMinimumSize(160, 50);
    buttonC.setMinimumSize(160, 50);
    buttonD.setMinimumSize(160, 50);

    auto layout = new QGridLayout(this);
    layout->addWidget(&buttonA, 0, 0);
    layout->addWidget(&buttonB, 0, 1);
    layout->addWidget(&buttonC, 1, 0);
    layout->addWidget(&buttonD, 1, 1);

    layout->setSpacing(10);
    layout->setRowStretch(0, 2); // First row takes twice the vertical space

    setLayout(layout);
}

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

int main(int argc, char *argv[])
{
    QApplication app(argc, argv);
    GridWidget window;
    window.show();
    return app.exec();
}

Key Takeaways

• QGridLayout organizes widgets in a flexible grid structure.
• Widgets can span multiple rows or columns using overloaded addWidget() variants.
• Both QBoxLayout and QGridLayout support stretch factors to define relative resizing behavior.
• Stretch factors influence how extra space is distributed but do not override minimum or maximum size constraints.