QLayout Class

The QLayout class is the base class of geometry managers. More...

Header:	#include <QLayout>
qmake:	QT += widgets
Inherits:	QObject and QLayoutItem
Inherited By:	QBoxLayout, QFormLayout, QGridLayout, and QStackedLayout

• List of all members, including inherited members
• Obsolete members

Public Types

enum	SizeConstraint { SetDefaultConstraint, SetFixedSize, SetMinimumSize, SetMaximumSize, SetMinAndMaxSize, SetNoConstraint }

Properties

• sizeConstraint : SizeConstraint
• spacing : int

Public Functions

	QLayout()
	QLayout(QWidget *parent)
bool	activate()
virtual void	addItem(QLayoutItem *item) = 0
void	addWidget(QWidget *w)
QMargins	contentsMargins() const
QRect	contentsRect() const
virtual int	count() const = 0
void	getContentsMargins(int *left, int *top, int *right, int *bottom) const
virtual int	indexOf(QWidget *widget) const
int	indexOf(QLayoutItem *layoutItem) const
bool	isEnabled() const
virtual QLayoutItem *	itemAt(int index) const = 0
QWidget *	menuBar() const
QWidget *	parentWidget() const
void	removeItem(QLayoutItem *item)
void	removeWidget(QWidget *widget)
QLayoutItem *	replaceWidget(QWidget *from, QWidget *to, Qt::FindChildOptions options = Qt::FindChildrenRecursively)
bool	setAlignment(QWidget *w, Qt::Alignment alignment)
bool	setAlignment(QLayout *l, Qt::Alignment alignment)
void	setContentsMargins(int left, int top, int right, int bottom)
void	setContentsMargins(const QMargins &margins)
void	setEnabled(bool enable)
void	setMenuBar(QWidget *widget)
void	setSizeConstraint(QLayout::SizeConstraint)
void	setSpacing(int)
QLayout::SizeConstraint	sizeConstraint() const
int	spacing() const
virtual QLayoutItem *	takeAt(int index) = 0
void	update()

Reimplemented Public Functions

virtual QSizePolicy::ControlTypes	controlTypes() const override
virtual Qt::Orientations	expandingDirections() const override
virtual QRect	geometry() const override
virtual void	invalidate() override
virtual bool	isEmpty() const override
virtual QLayout *	layout() override
virtual QSize	maximumSize() const override
virtual QSize	minimumSize() const override
virtual void	setGeometry(const QRect &r) override

Static Public Members

QSize

closestAcceptableSize(const QWidget *widget, const QSize &size)

Protected Functions

void	addChildLayout(QLayout *l)
void	addChildWidget(QWidget *w)
QRect	alignmentRect(const QRect &r) const

Reimplemented Protected Functions

virtual void

childEvent(QChildEvent *e) override

Detailed Description

This is an abstract base class inherited by the concrete classes QBoxLayout, QGridLayout, QFormLayout, and QStackedLayout.

For users of QLayout subclasses or of QMainWindow there is seldom any need to use the basic functions provided by QLayout, such as setSizeConstraint() or setMenuBar(). See Layout Management for more information.

To make your own layout manager, implement the functions addItem(), sizeHint(), setGeometry(), itemAt() and takeAt(). You should also implement minimumSize() to ensure your layout isn't resized to zero size if there is too little space. To support children whose heights depend on their widths, implement hasHeightForWidth() and heightForWidth(). See the Border Layout and Flow Layout examples for more information about implementing custom layout managers.

Geometry management stops when the layout manager is deleted.

See also QLayoutItem, Layout Management, Basic Layouts Example, Border Layout Example, and Flow Layout Example.

Member Type Documentation

enum QLayout::SizeConstraint

The possible values are:

Constant	Value	Description
QLayout::SetDefaultConstraint	0	The main widget's minimum size is set to minimumSize(), unless the widget already has a minimum size.
QLayout::SetFixedSize	3	The main widget's size is set to sizeHint(); it cannot be resized at all.
QLayout::SetMinimumSize	2	The main widget's minimum size is set to minimumSize(); it cannot be smaller.
QLayout::SetMaximumSize	4	The main widget's maximum size is set to maximumSize(); it cannot be larger.
QLayout::SetMinAndMaxSize	5	The main widget's minimum size is set to minimumSize() and its maximum size is set to maximumSize().
QLayout::SetNoConstraint	1	The widget is not constrained.

See also setSizeConstraint().

Member Function Documentation

QLayout::QLayout()

Constructs a new child QLayout.

This layout has to be inserted into another layout before geometry management will work.

QLayout::QLayout(QWidget *parent)

Constructs a new top-level QLayout, with parent parent. parent may not be nullptr.

The layout is set directly as the top-level layout for parent. There can be only one top-level layout for a widget. It is returned by QWidget::layout().

bool QLayout::activate()

Redoes the layout for parentWidget() if necessary.

You should generally not need to call this because it is automatically called at the most appropriate times. It returns true if the layout was redone.

See also update() and QWidget::updateGeometry().

[protected] void QLayout::addChildLayout(QLayout *l)

This function is called from addLayout() or insertLayout() functions in subclasses to add layout l as a sub-layout.

The only scenario in which you need to call it directly is if you implement a custom layout that supports nested layouts.

See also QBoxLayout::addLayout(), QBoxLayout::insertLayout(), and QGridLayout::addLayout().

[protected] void QLayout::addChildWidget(QWidget *w)

This function is called from addWidget() functions in subclasses to add w as a managed widget of a layout.

If w is already managed by a layout, this function will give a warning and remove w from that layout. This function must therefore be called before adding w to the layout's data structure.

[pure virtual] void QLayout::addItem(QLayoutItem *item)

Implemented in subclasses to add an item. How it is added is specific to each subclass.

This function is not usually called in application code. To add a widget to a layout, use the addWidget() function; to add a child layout, use the addLayout() function provided by the relevant QLayout subclass.

Note: The ownership of item is transferred to the layout, and it's the layout's responsibility to delete it.

See also addWidget(), QBoxLayout::addLayout(), and QGridLayout::addLayout().

void QLayout::addWidget(QWidget *w)

Adds widget w to this layout in a manner specific to the layout. This function uses addItem().

[protected] QRect QLayout::alignmentRect(const QRect &r) const

Returns the rectangle that should be covered when the geometry of this layout is set to r, provided that this layout supports setAlignment().

The result is derived from sizeHint() and expanding(). It is never larger than r.

[override virtual protected] void QLayout::childEvent(QChildEvent *e)

Reimplements: QObject::childEvent(QChildEvent *event).

[static] QSize QLayout::closestAcceptableSize(const QWidget *widget, const QSize &size)

Returns a size that satisfies all size constraints on widget, including heightForWidth() and that is as close as possible to size.

QMargins QLayout::contentsMargins() const

Returns the margins used around the layout.

By default, QLayout uses the values provided by the style. On most platforms, the margin is 11 pixels in all directions.

This function was introduced in Qt 4.6.

See also setContentsMargins().

QRect QLayout::contentsRect() const

Returns the layout's geometry() rectangle, but taking into account the contents margins.

This function was introduced in Qt 4.3.

See also setContentsMargins() and getContentsMargins().

[override virtual] QSizePolicy::ControlTypes QLayout::controlTypes() const

Reimplements: QLayoutItem::controlTypes() const.

[pure virtual] int QLayout::count() const

Must be implemented in subclasses to return the number of items in the layout.

See also itemAt().

[override virtual] Qt::Orientations QLayout::expandingDirections() const

Reimplements: QLayoutItem::expandingDirections() const.

Returns whether this layout can make use of more space than sizeHint(). A value of Qt::Vertical or Qt::Horizontal means that it wants to grow in only one dimension, whereas Qt::Vertical | Qt::Horizontal means that it wants to grow in both dimensions.

The default implementation returns Qt::Horizontal | Qt::Vertical. Subclasses reimplement it to return a meaningful value based on their child widgets's size policies.

See also sizeHint().

[override virtual] QRect QLayout::geometry() const

Reimplements: QLayoutItem::geometry() const.

See also setGeometry().

void QLayout::getContentsMargins(int *left, int *top, int *right, int *bottom) const

For each of left, top, right and bottom that is not nullptr, stores the size of the margin named in the location the pointer refers to.

By default, QLayout uses the values provided by the style. On most platforms, the margin is 11 pixels in all directions.

This function was introduced in Qt 4.3.

See also setContentsMargins(), QStyle::pixelMetric(), PM_LayoutLeftMargin, PM_LayoutTopMargin, PM_LayoutRightMargin, and PM_LayoutBottomMargin.

[virtual] int QLayout::indexOf(QWidget *widget) const

Searches for widget widget in this layout (not including child layouts).

Returns the index of widget, or -1 if widget is not found.

The default implementation iterates over all items using itemAt()

int QLayout::indexOf(QLayoutItem *layoutItem) const

Searches for layout item layoutItem in this layout (not including child layouts).

Returns the index of layoutItem, or -1 if layoutItem is not found.

This function was introduced in Qt 5.12.

[override virtual] void QLayout::invalidate()

Reimplements: QLayoutItem::invalidate().

[override virtual] bool QLayout::isEmpty() const

Reimplements: QLayoutItem::isEmpty() const.

bool QLayout::isEnabled() const

Returns true if the layout is enabled; otherwise returns false.

See also setEnabled().

[pure virtual] QLayoutItem *QLayout::itemAt(int index) const

Must be implemented in subclasses to return the layout item at index. If there is no such item, the function must return 0. Items are numbered consecutively from 0. If an item is deleted, other items will be renumbered.

This function can be used to iterate over a layout. The following code will draw a rectangle for each layout item in the layout structure of the widget.

static void paintLayout(QPainter *painter, QLayoutItem *item)
{
    QLayout *layout = item->layout();
    if (layout) {
        for (int i = 0; i < layout->count(); ++i)
            paintLayout(painter, layout->itemAt(i));
    }
    painter->drawRect(item->geometry());
}

void MyWidget::paintEvent(QPaintEvent *)
{
    QPainter painter(this);
    if (layout())
        paintLayout(&painter, layout());
}

See also count() and takeAt().

[override virtual] QLayout *QLayout::layout()

Reimplements: QLayoutItem::layout().

[override virtual] QSize QLayout::maximumSize() const

Reimplements: QLayoutItem::maximumSize() const.

Returns the maximum size of this layout. This is the largest size that the layout can have while still respecting the specifications.

The returned value doesn't include the space required by QWidget::setContentsMargins() or menuBar().

The default implementation allows unlimited resizing.

QWidget *QLayout::menuBar() const

Returns the menu bar set for this layout, or nullptr if no menu bar is set.

See also setMenuBar().

[override virtual] QSize QLayout::minimumSize() const

Reimplements: QLayoutItem::minimumSize() const.

Returns the minimum size of this layout. This is the smallest size that the layout can have while still respecting the specifications.

The returned value doesn't include the space required by QWidget::setContentsMargins() or menuBar().

The default implementation allows unlimited resizing.

QWidget *QLayout::parentWidget() const

Returns the parent widget of this layout, or nullptr if this layout is not installed on any widget.

If the layout is a sub-layout, this function returns the parent widget of the parent layout.

See also parent().

void QLayout::removeItem(QLayoutItem *item)

Removes the layout item item from the layout. It is the caller's responsibility to delete the item.

Notice that item can be a layout (since QLayout inherits QLayoutItem).

See also removeWidget() and addItem().

void QLayout::removeWidget(QWidget *widget)

Removes the widget widget from the layout. After this call, it is the caller's responsibility to give the widget a reasonable geometry or to put the widget back into a layout or to explicitly hide it if necessary.

Note: The ownership of widget remains the same as when it was added.

See also removeItem(), QWidget::setGeometry(), and addWidget().

QLayoutItem *QLayout::replaceWidget(QWidget *from, QWidget *to, Qt::FindChildOptions options = Qt::FindChildrenRecursively)

Searches for widget from and replaces it with widget to if found. Returns the layout item that contains the widget from on success. Otherwise nullptr is returned. If options contains Qt::FindChildrenRecursively (the default), sub-layouts are searched for doing the replacement. Any other flag in options is ignored.

Notice that the returned item therefore might not belong to this layout, but to a sub-layout.

The returned layout item is no longer owned by the layout and should be either deleted or inserted to another layout. The widget from is no longer managed by the layout and may need to be deleted or hidden. The parent of widget from is left unchanged.

This function works for the built-in Qt layouts, but might not work for custom layouts.

This function was introduced in Qt 5.2.

See also indexOf().

bool QLayout::setAlignment(QWidget *w, Qt::Alignment alignment)

Sets the alignment for widget w to alignment and returns true if w is found in this layout (not including child layouts); otherwise returns false.

bool QLayout::setAlignment(QLayout *l, Qt::Alignment alignment)

This is an overloaded function.

Sets the alignment for the layout l to alignment and returns true if l is found in this layout (not including child layouts); otherwise returns false.

void QLayout::setContentsMargins(int left, int top, int right, int bottom)

Sets the left, top, right, and bottom margins to use around the layout.

By default, QLayout uses the values provided by the style. On most platforms, the margin is 11 pixels in all directions.

This function was introduced in Qt 4.3.

See also contentsMargins(), getContentsMargins(), QStyle::pixelMetric(), PM_LayoutLeftMargin, PM_LayoutTopMargin, PM_LayoutRightMargin, and PM_LayoutBottomMargin.

void QLayout::setContentsMargins(const QMargins &margins)

Sets the margins to use around the layout.

By default, QLayout uses the values provided by the style. On most platforms, the margin is 11 pixels in all directions.

This function was introduced in Qt 4.6.

See also contentsMargins().

void QLayout::setEnabled(bool enable)

Enables this layout if enable is true, otherwise disables it.

An enabled layout adjusts dynamically to changes; a disabled layout acts as if it did not exist.

By default all layouts are enabled.

See also isEnabled().

[override virtual] void QLayout::setGeometry(const QRect &r)

Reimplements: QLayoutItem::setGeometry(const QRect &r).

See also geometry().

void QLayout::setMenuBar(QWidget *widget)

Tells the geometry manager to place the menu bar widget at the top of parentWidget(), outside QWidget::contentsMargins(). All child widgets are placed below the bottom edge of the menu bar.

See also menuBar().

[pure virtual] QLayoutItem *QLayout::takeAt(int index)

Must be implemented in subclasses to remove the layout item at index from the layout, and return the item. If there is no such item, the function must do nothing and return 0. Items are numbered consecutively from 0. If an item is removed, other items will be renumbered.

The following code fragment shows a safe way to remove all items from a layout:

QLayoutItem *child;
while ((child = layout->takeAt(0)) != nullptr) {
    ...
    delete child->widget(); // delete the widget
    delete child;   // delete the layout item
}

See also itemAt() and count().

void QLayout::update()

Updates the layout for parentWidget().

You should generally not need to call this because it is automatically called at the most appropriate times.

See also activate() and invalidate().