- Qt 6.8
- Qt Core
- C++ Classes
- QLineF
- Qt 6.8.3 Reference Documentation

# QLineF Class

The QLineF class provides a two-dimensional vector using floating point precision. More...

Header: | `#include <QLineF>`
CMake: | `find_package(Qt6 REQUIRED COMPONENTS Core)` `target_link_libraries(mytarget PRIVATE Qt6::Core)`
qmake: | `QT += core`

- List of all members, including inherited members
- Deprecated members
- QLineF is part of Painting Classes.

This class is equality-comparable.

This class is equality-comparable with QLine.

## Public Types

enum | IntersectionType { NoIntersection, UnboundedIntersection, BoundedIntersection }

## Public Functions

| QLineF()
| QLineF(const QLine &line)
| QLineF(const QPointF &p1, const QPointF &p2)
| QLineF(qreal x1, qreal y1, qreal x2, qreal y2)
QPointF | p1() const
QPointF | p2() const
qreal | x1() const
qreal | x2() const
qreal | y1() const
qreal | y2() const
qreal | angle() const
qreal | angleTo(const QLineF &line) const
QPointF | center() const
qreal | dx() const
qreal | dy() const
QLineF::IntersectionType | intersects(const QLineF &line, QPointF *intersectionPoint = nullptr) const
bool | isNull() const
qreal | length() const
QLineF | normalVector() const
QPointF | pointAt(qreal t) const
void | setP1(const QPointF &p1)
void | setP2(const QPointF &p2)
void | setAngle(qreal angle)
void | setLength(qreal length)
void | setLine(qreal x1, qreal y1, qreal x2, qreal y2)
void | setPoints(const QPointF &p1, const QPointF &p2)
QLine | toLine() const
void | translate(const QPointF &offset)
void | translate(qreal dx, qreal dy)
QLineF | translated(const QPointF &offset) const
QLineF | translated(qreal dx, qreal dy) const
QLineF | unitVector() const

## Static Public Members

QLineF | fromPolar(qreal length, qreal angle)

## Related Non-Members

`(since 6.8)` bool | qFuzzyCompare(const QLineF &lhs, const QLineF &rhs)
`(since 6.8)` bool | qFuzzyIsNull(const QLineF &line)
bool | operator!=(const QLineF &lhs, const QLineF &rhs)
QDataStream & | operator<<(QDataStream &stream, const QLineF &line)
bool | operator==(const QLineF &lhs, const QLineF &rhs)
QDataStream & | operator>>(QDataStream &stream, QLineF &line)

## Detailed Description

A QLineF describes a finite length line (or line segment) on a two-dimensional surface. QLineF defines the start and end points of the line using floating point accuracy for coordinates. Use the toLine() function to retrieve an integer-based copy of this line.

The positions of the line's start and end points can be retrieved using the p1(), x1(), y1(), p2(), x2(), and y2() functions. The dx() and dy() functions return the horizontal and vertical components of the line, respectively.

The line's length can be retrieved using the length() function, and altered using the setLength() function. Similarly, angle() and setAngle() are respectively used for retrieving and altering the angle of the line. Use the isNull() function to determine whether the QLineF represents a valid line or a null line.

The intersects() function determines the IntersectionType for this line and a given line, while the angleTo() function returns the angle between the lines. In addition, the unitVector() function returns a line that has the same starting point as this line, but with a length of only 1, while the normalVector() function returns a line that is perpendicular to this line with the same starting point and length.

Finally, the line can be translated a given offset using the translate() function, and can be traversed using the pointAt() function.

### Constraints

QLine is limited to the minimum and maximum values for the `int` type. Operations on a QLine that could potentially result in values outside this range will result in undefined behavior.

See also QLine, QPolygonF, and QRectF.

## Member Type Documentation

### enum QLineF::IntersectionType

Describes the intersection between two lines.

QLineF::UnboundedIntersection | QLineF::BoundedIntersection

Constant | Value | Description
--- | --- | ---
`QLineF::NoIntersection` | `0` | Indicates that the lines do not intersect; i.e. they are parallel.
`QLineF::UnboundedIntersection` | `2` | The two lines intersect, but not within the range defined by their lengths. This will be the case if the lines are not parallel. intersect() will also return this value if the intersect point is within the start and end point of only one of the lines.
`QLineF::BoundedIntersection` | `1` | The two lines intersect with each other within the start and end points of each line.

See also intersects().

## Member Function Documentation

### `[constexpr]` QLineF::QLineF()

Constructs a null line.

### `[constexpr]` QLineF::QLineF(const QLine &line)

Construct a QLineF object from the given integer-based line.

See also toLine() and QLine::toLineF().

### `[constexpr]` QLineF::QLineF(const QPointF &p1, const QPointF &p2)

Constructs a line object that represents the line between p1 and p2.

### `[constexpr]` QLineF::QLineF(qreal x1, qreal y1, qreal x2, qreal y2)

Constructs a line object that represents the line between (x1, y1) and (x2, y2).

### `[constexpr]` QPointF QLineF::p1() const

Returns the line's start point.

See also setP1(), x1(), y1(), and p2().

### `[constexpr]` QPointF QLineF::p2() const

Returns the line's end point.

See also setP2(), x2(), y2(), and p1().

### `[constexpr]` qreal QLineF::x1() const

Returns the x-coordinate of the line's start point.

See also p1().

### `[constexpr]` qreal QLineF::x2() const

Returns the x-coordinate of the line's end point.

See also p2().

### `[constexpr]` qreal QLineF::y1() const

Returns the y-coordinate of the line's start point.

See also p1().

### `[constexpr]` qreal QLineF::y2() const

Returns the y-coordinate of the line's end point.

See also p2().

### qreal QLineF::angle() const

Returns the angle of the line in degrees.

The return value will be in the range of values from 0.0 up to but not including 360.0. The angles are measured counter-clockwise from a point on the x-axis to the right of the origin (x > 0).

See also setAngle().

### qreal QLineF::angleTo(const QLineF &line) const

Returns the angle (in degrees) from this line to the given line, taking the direction of the lines into account. If the lines do not intersect within their range, it is the intersection point of the extended lines that serves as origin (see QLineF::UnboundedIntersection).

The returned value represents the number of degrees you need to add to this line to make it have the same angle as the given line, going counter-clockwise.

See also intersects().

### `[constexpr]` QPointF QLineF::center() const

Returns the center point of this line. This is equivalent to 0.5 * p1() + 0.5 * p2().

### `[constexpr]` qreal QLineF::dx() const

Returns the horizontal component of the line's vector.

See also dy() and pointAt().

### `[constexpr]` qreal QLineF::dy() const

Returns the vertical component of the line's vector.

See also dx() and pointAt().

### `[static]` QLineF QLineF::fromPolar(qreal length, qreal angle)

Returns a QLineF with the given length and angle.

The first point of the line will be on the origin.

Positive values for the angles mean counter-clockwise while negative values mean the clockwise direction. Zero degrees is at the 3 o'clock position.

### QLineF::IntersectionType QLineF::intersects(const QLineF &line, QPointF *intersectionPoint = nullptr) const

Returns a value indicating whether or not this line intersects with the given line.

The actual intersection point is extracted to intersectionPoint (if the pointer is valid). If the lines are parallel, the intersection point is undefined.

### `[constexpr]` bool QLineF::isNull() const

Returns `true` if the line does not have distinct start and end points; otherwise returns `false`. The start and end points are considered distinct if qFuzzyCompare() can distinguish them in at least one coordinate.

Note: Due to the use of fuzzy comparison, isNull() may return `true` for lines whose length() is not zero.

See also qFuzzyCompare() and length().

### qreal QLineF::length() const

Returns the length of the line.

See also setLength() and isNull().

### `[constexpr]` QLineF QLineF::normalVector() const

Returns a line that is perpendicular to this line with the same starting point and length.

See also unitVector().

### `[constexpr]` QPointF QLineF::pointAt(qreal t) const

Returns the point at the position specified by finite parameter t. The function returns the line's start point if t = 0, and its end point if t = 1.

See also dx() and dy().

### void QLineF::setP1(const QPointF &p1)

Sets the starting point of this line to p1.

See also setP2() and p1().

### void QLineF::setP2(const QPointF &p2)

Sets the end point of this line to p2.

See also setP1() and p2().

### void QLineF::setAngle(qreal angle)

Sets the angle of the line to the given angle (in degrees). This will change the position of the second point of the line such that the line has the given angle.

Positive values for the angles mean counter-clockwise while negative values mean the clockwise direction. Zero degrees is at the 3 o'clock position.

See also angle().

### void QLineF::setLength(qreal length)

Sets the length of the line to the given finite length. QLineF will move the end point - p2() - of the line to give the line its new length, unless length() was previously zero, in which case no scaling is attempted.

See also length() and unitVector().

### void QLineF::setLine(qreal x1, qreal y1, qreal x2, qreal y2)

Sets this line to the start in x1, y1 and end in x2, y2.

See also setP1(), setP2(), p1(), and p2().

### void QLineF::setPoints(const QPointF &p1, const QPointF &p2)

Sets the start point of this line to p1 and the end point of this line to p2.

See also setP1(), setP2(), p1(), and p2().

### `[constexpr]` QLine QLineF::toLine() const

Returns an integer-based copy of this line.

Note that the returned line's start and end points are rounded to the nearest integer.

See also QLineF() and QLine::toLineF().

### void QLineF::translate(const QPointF &offset)

Translates this line by the given offset.

### void QLineF::translate(qreal dx, qreal dy)

This is an overloaded function.

Translates this line the distance specified by dx and dy.

### `[constexpr]` QLineF QLineF::translated(const QPointF &offset) const

Returns this line translated by the given offset.

### `[constexpr]` QLineF QLineF::translated(qreal dx, qreal dy) const

This is an overloaded function.

Returns this line translated the distance specified by dx and dy.

### QLineF QLineF::unitVector() const

Returns the unit vector for this line, i.e a line starting at the same point as this line with a length of 1.0, provided the line is non-null.

See also normalVector() and setLength().

## Related Non-Members

### `[constexpr noexcept, since 6.8]` bool qFuzzyCompare(const QLineF &lhs, const QLineF &rhs)

Returns `true` if line lhs is approximately equal to line rhs; otherwise returns `false`.

The lines are considered approximately equal if their start and end points are approximately equal.

This function was introduced in Qt 6.8.

### `[constexpr noexcept, since 6.8]` bool qFuzzyIsNull(const QLineF &line)

Returns `true` if the start point of line line is approximately equal to its end point; otherwise returns `false`.

This function was introduced in Qt 6.8.

### `[constexpr noexcept]` bool operator!=(const QLineF &lhs, const QLineF &rhs)

Returns `true` if the line lhs is not the same as line rhs.

A line is different from another line if their start or end points differ, or the internal order of the points is different.

### QDataStream &operator<<(QDataStream &stream, const QLineF &line)

Writes the given line to the given stream and returns a reference to the stream.

See also Serializing Qt Data Types.

### `[constexpr noexcept]` bool operator==(const QLineF &lhs, const QLineF &rhs)

Returns `true` if the line lhs is the same as line rhs.

A line is identical to another line if the start and end points are identical, and the internal order of the points is the same.

### QDataStream &operator>>(QDataStream &stream, QLineF &line)

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

See also Serializing Qt Data Types.