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# QLayoutItem Class Reference

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

    #include <QLayoutItem>

Inherited by QSpacerItem, QWidgetItem, and QLayout.

- List of all members, including inherited members

### Public Functions

- QLayoutItem ( Qt::Alignment alignment = 0 )
- virtual ~QLayoutItem ()
- Qt::Alignment alignment () const
- virtual QSizePolicy::ExpandData expanding () 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 is empty. iterator() returns an iterator for the layout's children. 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.

### QSizePolicy::ExpandData QLayoutItem::expanding () const `[pure virtual]`

Implemented in subclasses to return the direction(s) this item "wants" to expand in (if any).

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

Returns the rectangle covered by this layout item.

### 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. Not all subclasses support alignment.

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

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

### 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.