Qt 4.3: QLayoutItem Class Reference

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# QLayoutItem Class Reference
[QtGui module]

The QLayoutItem class provides an abstract item that a QLayout manipulates. More...

     #include <QLayoutItem>

Inherited by QLayout, QSpacerItem, and QWidgetItem.

- List of all members, including inherited members

### Public Functions

- QLayoutItem ( Qt::Alignment alignment = 0 )
- virtual ~QLayoutItem ()
- Qt::Alignment alignment () const
- QSizePolicy::ControlTypes controlTypes () const
- virtual Qt::Orientations expandingDirections () const = 0
- virtual QRect geometry () const = 0
- virtual bool hasHeightForWidth () const
- virtual int heightForWidth ( int w ) const
- virtual void invalidate ()
- virtual bool isEmpty () const = 0
- virtual QLayout * layout ()
- virtual QSize maximumSize () const = 0
- virtual int minimumHeightForWidth ( int w ) const
- virtual QSize minimumSize () const = 0
- void setAlignment ( Qt::Alignment alignment )
- virtual void setGeometry ( const QRect & r ) = 0
- virtual QSize sizeHint () const = 0
- virtual QSpacerItem * spacerItem ()
- virtual QWidget * widget ()

## Detailed Description

The QLayoutItem class provides an abstract item that a QLayout manipulates.

This is used by custom layouts.

Pure virtual functions are provided to return information about the layout, including, sizeHint(), minimumSize(), maximumSize() and expanding().

The layout's geometry can be set and retrieved with setGeometry() and geometry(), and its alignment with setAlignment() and alignment().

isEmpty() returns whether the layout item is empty. If the concrete item is a QWidget, it can be retrieved using widget(). Similarly for layout() and spacerItem().

Some layouts have width and height interdependencies. These can be expressed using hasHeightForWidth(), heightForWidth(), and minimumHeightForWidth(). For more explanation see the Qt Quarterly article Trading Height for Width.

See also QLayout.

## Member Function Documentation

### QLayoutItem::QLayoutItem ( Qt::Alignment alignment = 0 )

Constructs a layout item with an alignment. Not all subclasses support alignment.

### QLayoutItem::~QLayoutItem () `[virtual]`

Destroys the QLayoutItem.

### Qt::Alignment QLayoutItem::alignment () const

Returns the alignment of this item.

See also setAlignment().

### QSizePolicy::ControlTypes QLayoutItem::controlTypes () const

Returns the control type(s) for the layout item. For a QWidgetItem, the control type comes from the widget's size policy; for a QLayoutItem, the control types is derived from the layout's contents.

See also QSizePolicy::controlType().

### Qt::Orientations QLayoutItem::expandingDirections () const `[pure virtual]`

Returns whether this layout item 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.

### QRect QLayoutItem::geometry () const `[pure virtual]`

Returns the rectangle covered by this layout item.

See also setGeometry().

### bool QLayoutItem::hasHeightForWidth () const `[virtual]`

Returns true if this layout's preferred height depends on its width; otherwise returns false. The default implementation returns false.

Reimplement this function in layout managers that support height for width.

See also heightForWidth() and QWidget::heightForWidth().

### int QLayoutItem::heightForWidth ( int w ) const `[virtual]`

Returns the preferred height for this layout item, given the width w.

The default implementation returns -1, indicating that the preferred height is independent of the width of the item. Using the function hasHeightForWidth() will typically be much faster than calling this function and testing for -1.

Reimplement this function in layout managers that support height for width. A typical implementation will look like this:

     int MyLayout::heightForWidth(int w) const
     {
         if (cache_dirty || cached_width != w) {
             // not all C++ compilers support "mutable"
             MyLayout *that = (MyLayout*)this;
             int h = calculateHeightForWidth(w);
             that->cached_hfw = h;
             return h;
         }
         return cached_hfw;
     }

Caching is strongly recommended; without it layout will take exponential time.

See also hasHeightForWidth().

### void QLayoutItem::invalidate () `[virtual]`

Invalidates any cached information in this layout item.

### bool QLayoutItem::isEmpty () const `[pure virtual]`

Implemented in subclasses to return whether this item is empty, i.e. whether it contains any widgets.

### QLayout * QLayoutItem::layout () `[virtual]`

If this item is a QLayout, it is returned as a QLayout; otherwise 0 is returned. This function provides type-safe casting.

### QSize QLayoutItem::maximumSize () const `[pure virtual]`

Implemented in subclasses to return the maximum size of this item.

### int QLayoutItem::minimumHeightForWidth ( int w ) const `[virtual]`

Returns the minimum height this widget needs for the given width, w. The default implementation simply returns heightForWidth(w).

### QSize QLayoutItem::minimumSize () const `[pure virtual]`

Implemented in subclasses to return the minimum size of this item.

### void QLayoutItem::setAlignment ( Qt::Alignment alignment )

Sets the alignment of this item to alignment.

Note: Item alignment is only supported by QLayoutItem subclasses where it would have a visual effect. Except for QSpacerItem, which provides blank space for layouts, all public Qt classes that inherit QLayoutItem support item alignment.

See also alignment().

### void QLayoutItem::setGeometry ( const QRect & r ) `[pure virtual]`

Implemented in subclasses to set this item's geometry to r.

See also geometry().

### QSize QLayoutItem::sizeHint () const `[pure virtual]`

Implemented in subclasses to return the preferred size of this item.

### QSpacerItem * QLayoutItem::spacerItem () `[virtual]`

If this item is a QSpacerItem, it is returned as a QSpacerItem; otherwise 0 is returned. This function provides type-safe casting.

### QWidget * QLayoutItem::widget () `[virtual]`

If this item is a QWidget, it is returned as a QWidget; otherwise 0 is returned. This function provides type-safe casting.