- Qt 6.8
- Qt GUI
- C++ Classes
- QPolygon
- Qt 6.8.3 Reference Documentation

# QPolygon Class

The QPolygon class provides a list of points using integer precision. More...

Header: | `#include <QPolygon>`
CMake: | `find_package(Qt6 REQUIRED COMPONENTS Gui)` `target_link_libraries(mytarget PRIVATE Qt6::Gui)`
qmake: | `QT += gui`
Inherits: | QList

- List of all members, including inherited members
- QPolygon is part of Painting Classes and Implicitly Shared Classes.

Note: All functions in this class are reentrant.

## Public Functions

| QPolygon()
| QPolygon(const QList<QPoint> &points)
| QPolygon(const QRect &rectangle, bool closed = false)
QRect | boundingRect() const
bool | containsPoint(const QPoint &point, Qt::FillRule fillRule) const
QPolygon | intersected(const QPolygon &r) const
bool | intersects(const QPolygon &p) const
void | point(int index, int *x, int *y) const
QPoint | point(int index) const
void | putPoints(int index, int nPoints, int firstx, int firsty, ...)
void | putPoints(int index, int nPoints, const QPolygon &fromPolygon, int fromIndex = 0)
void | setPoint(int index, int x, int y)
void | setPoint(int index, const QPoint &point)
void | setPoints(int nPoints, const int *points)
void | setPoints(int nPoints, int firstx, int firsty, ...)
QPolygon | subtracted(const QPolygon &r) const
void | swap(QPolygon &other)
`(since 6.4)` QPolygonF | toPolygonF() const
void | translate(int dx, int dy)
void | translate(const QPoint &offset)
QPolygon | translated(int dx, int dy) const
QPolygon | translated(const QPoint &offset) const
QPolygon | united(const QPolygon &r) const
QVariant | operator QVariant() const

## Related Non-Members

QDataStream & | operator<<(QDataStream &stream, const QPolygon &polygon)
QDataStream & | operator>>(QDataStream &stream, QPolygon &polygon)

## Detailed Description

A QPolygon object is a QList<QPoint>. The easiest way to add points to a QPolygon is to use QList's streaming operator, as illustrated below:

     QPolygon polygon;
     polygon << QPoint(10, 20) << QPoint(20, 30);

In addition to the functions provided by QList, QPolygon provides some point-specific functions.

Each point in a polygon can be retrieved by passing its index to the point() function. To populate the polygon, QPolygon provides the setPoint() function to set the point at a given index, the setPoints() function to set all the points in the polygon (resizing it to the given number of points), and the putPoints() function which copies a number of given points into the polygon from a specified index (resizing the polygon if necessary).

QPolygon provides the boundingRect() and translate() functions for geometry functions. Use the QTransform::map() function for more general transformations of QPolygons.

The QPolygon class is implicitly shared.

See also QList, QPolygonF, and QLine.

## Member Function Documentation

### `[constexpr noexcept]` QPolygon::QPolygon()

Constructs a polygon with no points.

See also QList::isEmpty().

### QPolygon::QPolygon(const QList<QPoint> &points)

Constructs a polygon containing the specified points.

See also setPoints().

### QPolygon::QPolygon(const QRect &rectangle, bool closed = false)

Constructs a polygon from the given rectangle. If closed is false, the polygon just contains the four points of the rectangle ordered clockwise, otherwise the polygon's fifth point is set to rectangle.topLeft().

Note that the bottom-right corner of the rectangle is located at (rectangle.x() + rectangle.width(), rectangle.y() + rectangle.height()).

See also setPoints().

### QRect QPolygon::boundingRect() const

Returns the bounding rectangle of the polygon, or QRect(0, 0, 0, 0) if the polygon is empty.

See also QList::isEmpty().

### bool QPolygon::containsPoint(const QPoint &point, Qt::FillRule fillRule) const

Returns `true` if the given point is inside the polygon according to the specified fillRule; otherwise returns `false`.

### QPolygon QPolygon::intersected(const QPolygon &r) const

Returns a polygon which is the intersection of this polygon and r.

Set operations on polygons will treat the polygons as areas. Non-closed polygons will be treated as implicitly closed.

See also intersects().

### bool QPolygon::intersects(const QPolygon &p) const

Returns `true` if the current polygon intersects at any point the given polygon p. Also returns `true` if the current polygon contains or is contained by any part of p.

Set operations on polygons will treat the polygons as areas. Non-closed polygons will be treated as implicitly closed.

See also intersected().

### void QPolygon::point(int index, int *x, int *y) const

Extracts the coordinates of the point at the given index to *x and *y (if they are valid pointers).

See also setPoint().

### QPoint QPolygon::point(int index) const

This is an overloaded function.

Returns the point at the given index.

### void QPolygon::putPoints(int index, int nPoints, int firstx, int firsty, ...)

Copies nPoints points from the variable argument list into this polygon from the given index.

The points are given as a sequence of integers, starting with firstx then firsty, and so on. The polygon is resized if `index+nPoints` exceeds its current size.

The example code creates a polygon with three points (4,5), (6,7) and (8,9), by expanding the polygon from 1 to 3 points:

     QPolygon polygon(1);
     polygon[0] = QPoint(4, 5);
     polygon.putPoints(1, 2, 6,7, 8,9);

The following code has the same result, but here the putPoints() function overwrites rather than extends:

     QPolygon polygon(3);
     polygon.putPoints(0, 3, 4,5, 0,0, 8,9);
     polygon.putPoints(1, 1, 6,7);

See also setPoints().

### void QPolygon::putPoints(int index, int nPoints, const QPolygon &fromPolygon, int fromIndex = 0)

This is an overloaded function.

Copies nPoints points from the given fromIndex ( 0 by default) in fromPolygon into this polygon, starting at the specified index. For example:

     QPolygon polygon1;
     polygon1.putPoints(0, 3, 1,2, 0,0, 5,6);
     // polygon1 is now the three-point polygon(1,2, 0,0, 5,6);

     QPolygon polygon2;
     polygon2.putPoints(0, 3, 4,4, 5,5, 6,6);
     // polygon2 is now (4,4, 5,5, 6,6);

     polygon1.putPoints(2, 3, polygon2);
     // polygon1 is now the five-point polygon(1,2, 0,0, 4,4, 5,5, 6,6);

### void QPolygon::setPoint(int index, int x, int y)

Sets the point at the given index to the point specified by (x, y).

See also point(), putPoints(), and setPoints().

### void QPolygon::setPoint(int index, const QPoint &point)

This is an overloaded function.

Sets the point at the given index to the given point.

### void QPolygon::setPoints(int nPoints, const int *points)

Resizes the polygon to nPoints and populates it with the given points.

The example code creates a polygon with two points (10, 20) and (30, 40):

     static const int points[] = { 10, 20, 30, 40 };
     QPolygon polygon;
     polygon.setPoints(2, points);

See also setPoint() and putPoints().

### void QPolygon::setPoints(int nPoints, int firstx, int firsty, ...)

This is an overloaded function.

Resizes the polygon to nPoints and populates it with the points specified by the variable argument list. The points are given as a sequence of integers, starting with firstx then firsty, and so on.

The example code creates a polygon with two points (10, 20) and (30, 40):

     QPolygon polygon;
     polygon.setPoints(2, 10, 20, 30, 40);

### QPolygon QPolygon::subtracted(const QPolygon &r) const

Returns a polygon which is r subtracted from this polygon.

Set operations on polygons will treat the polygons as areas. Non-closed polygons will be treated as implicitly closed.

### `[noexcept]` void QPolygon::swap(QPolygon &other)

Swaps this polygon with other. This operation is very fast and never fails.

### `[since 6.4]` QPolygonF QPolygon::toPolygonF() const

Returns this polygon as a polygon with floating point accuracy.

This function was introduced in Qt 6.4.

See also QPolygonF::toPolygon().

### void QPolygon::translate(int dx, int dy)

Translates all points in the polygon by (dx, dy).

See also translated().

### void QPolygon::translate(const QPoint &offset)

This is an overloaded function.

Translates all points in the polygon by the given offset.

See also translated().

### QPolygon QPolygon::translated(int dx, int dy) const

Returns a copy of the polygon that is translated by (dx, dy).

See also translate().

### QPolygon QPolygon::translated(const QPoint &offset) const

This is an overloaded function.

Returns a copy of the polygon that is translated by the given offset.

See also translate().

### QPolygon QPolygon::united(const QPolygon &r) const

Returns a polygon which is the union of this polygon and r.

Set operations on polygons, will treat the polygons as areas, and implicitly close the polygon.

See also intersected() and subtracted().

### QVariant QPolygon::operator QVariant() const

Returns the polygon as a QVariant

## Related Non-Members

### QDataStream &operator<<(QDataStream &stream, const QPolygon &polygon)

Writes the given polygon to the given stream, and returns a reference to the stream.

See also Serializing Qt Data Types.

### QDataStream &operator>>(QDataStream &stream, QPolygon &polygon)

Reads a polygon from the given stream into the given polygon, and returns a reference to the stream.

See also Serializing Qt Data Types.