QFontMetrics Class

The QFontMetrics class provides font metrics information. More...

Header:	#include <QFontMetrics>
qmake:	QT += gui

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

Note: All functions in this class are reentrant.

Public Functions

	QFontMetrics(const QFontMetrics &fm)
	QFontMetrics(const QFont &font, const QPaintDevice *paintdevice)
	QFontMetrics(const QFont &font)
QFontMetrics &	operator=(QFontMetrics &&other)
QFontMetrics &	operator=(const QFontMetrics &fm)
	~QFontMetrics()
int	ascent() const
int	averageCharWidth() const
QRect	boundingRect(QChar ch) const
QRect	boundingRect(const QString &text) const
QRect	boundingRect(const QRect &rect, int flags, const QString &text, int tabStops = 0, int *tabArray = nullptr) const
QRect	boundingRect(int x, int y, int width, int height, int flags, const QString &text, int tabStops = 0, int *tabArray = nullptr) const
int	capHeight() const
int	descent() const
QString	elidedText(const QString &text, Qt::TextElideMode mode, int width, int flags = 0) const
qreal	fontDpi() const
int	height() const
int	horizontalAdvance(const QString &text, int len = -1) const
int	horizontalAdvance(QChar ch) const
bool	inFont(QChar ch) const
bool	inFontUcs4(uint ucs4) const
int	leading() const
int	leftBearing(QChar ch) const
int	lineSpacing() const
int	lineWidth() const
int	maxWidth() const
int	minLeftBearing() const
int	minRightBearing() const
int	overlinePos() const
int	rightBearing(QChar ch) const
QSize	size(int flags, const QString &text, int tabStops = 0, int *tabArray = nullptr) const
int	strikeOutPos() const
void	swap(QFontMetrics &other)
QRect	tightBoundingRect(const QString &text) const
int	underlinePos() const
int	xHeight() const
bool	operator!=(const QFontMetrics &other) const
bool	operator==(const QFontMetrics &other) const

Detailed Description

QFontMetrics functions calculate the size of characters and strings for a given font. There are three ways you can create a QFontMetrics object:

1. Calling the QFontMetrics constructor with a QFont creates a font metrics object for a screen-compatible font, i.e. the font cannot be a printer font. If the font is changed later, the font metrics object is not updated.

   (Note: If you use a printer font the values returned may be inaccurate. Printer fonts are not always accessible so the nearest screen font is used if a printer font is supplied.)

2. QWidget::fontMetrics() returns the font metrics for a widget's font. This is equivalent to QFontMetrics(widget->font()). If the widget's font is changed later, the font metrics object is not updated.
3. QPainter::fontMetrics() returns the font metrics for a painter's current font. If the painter's font is changed later, the font metrics object is not updated.

Once created, the object provides functions to access the individual metrics of the font, its characters, and for strings rendered in the font.

There are several functions that operate on the font: ascent(), descent(), height(), leading() and lineSpacing() return the basic size properties of the font. The underlinePos(), overlinePos(), strikeOutPos() and lineWidth() functions, return the properties of the line that underlines, overlines or strikes out the characters. These functions are all fast.

There are also some functions that operate on the set of glyphs in the font: minLeftBearing(), minRightBearing() and maxWidth(). These are by necessity slow, and we recommend avoiding them if possible.

For each character, you can get its horizontalAdvance(), leftBearing(), and rightBearing(), and find out whether it is in the font using inFont(). You can also treat the character as a string, and use the string functions on it.

The string functions include horizontalAdvance(), to return the width of a string in pixels (or points, for a printer), boundingRect(), to return a rectangle large enough to contain the rendered string, and size(), to return the size of that rectangle.

Example:

QFont font("times", 24);
QFontMetrics fm(font);
int pixelsWide = fm.horizontalAdvance("What's the width of this text?");
int pixelsHigh = fm.height();

See also QFont, QFontInfo, QFontDatabase, and Character Map Example.

Member Function Documentation

QFontMetrics::QFontMetrics(const QFontMetrics &fm)

Constructs a copy of fm.

QFontMetrics::QFontMetrics(const QFont &font, const QPaintDevice *paintdevice)

Constructs a font metrics object for font and paintdevice.

The font metrics will be compatible with the paintdevice passed. If the paintdevice is nullptr, the metrics will be screen-compatible, ie. the metrics you get if you use the font for drawing text on a widgets or pixmaps, not on a QPicture or QPrinter.

The font metrics object holds the information for the font that is passed in the constructor at the time it is created, and is not updated if the font's attributes are changed later.

This function was introduced in Qt 5.13.

QFontMetrics::QFontMetrics(const QFont &font)

Constructs a font metrics object for font.

The font metrics will be compatible with the paintdevice used to create font.

The font metrics object holds the information for the font that is passed in the constructor at the time it is created, and is not updated if the font's attributes are changed later.

Use QFontMetrics(const QFont &, QPaintDevice *) to get the font metrics that are compatible with a certain paint device.

QFontMetrics &QFontMetrics::operator=(QFontMetrics &&other)

Move-assigns other to this QFontMetrics instance.

This function was introduced in Qt 5.2.

QFontMetrics &QFontMetrics::operator=(const QFontMetrics &fm)

Assigns the font metrics fm.

QFontMetrics::~QFontMetrics()

Destroys the font metrics object and frees all allocated resources.

int QFontMetrics::ascent() const

Returns the ascent of the font.

The ascent of a font is the distance from the baseline to the highest position characters extend to. In practice, some font designers break this rule, e.g. when they put more than one accent on top of a character, or to accommodate an unusual character in an exotic language, so it is possible (though rare) that this value will be too small.

See also descent().

int QFontMetrics::averageCharWidth() const

Returns the average width of glyphs in the font.

This function was introduced in Qt 4.2.

QRect QFontMetrics::boundingRect(QChar ch) const

Returns the rectangle that is covered by ink if character ch were to be drawn at the origin of the coordinate system.

Note that the bounding rectangle may extend to the left of (0, 0) (e.g., for italicized fonts), and that the text output may cover all pixels in the bounding rectangle. For a space character the rectangle will usually be empty.

Note that the rectangle usually extends both above and below the base line.

Warning: The width of the returned rectangle is not the advance width of the character. Use boundingRect(const QString &) or horizontalAdvance() instead.

See also horizontalAdvance().

QRect QFontMetrics::boundingRect(const QString &text) const

Returns the bounding rectangle of the characters in the string specified by text. The bounding rectangle always covers at least the set of pixels the text would cover if drawn at (0, 0).

Note that the bounding rectangle may extend to the left of (0, 0), e.g. for italicized fonts, and that the width of the returned rectangle might be different than what the horizontalAdvance() method returns.

If you want to know the advance width of the string (to lay out a set of strings next to each other), use horizontalAdvance() instead.

Newline characters are processed as normal characters, not as linebreaks.

The height of the bounding rectangle is at least as large as the value returned by height().

See also horizontalAdvance(), height(), QPainter::boundingRect(), and tightBoundingRect().

QRect QFontMetrics::boundingRect(const QRect &rect, int flags, const QString &text, int tabStops = 0, int *tabArray = nullptr) const

This is an overloaded function.

Returns the bounding rectangle of the characters in the string specified by text, which is the set of pixels the text would cover if drawn at (0, 0). The drawing, and hence the bounding rectangle, is constrained to the rectangle rect.

The flags argument is the bitwise OR of the following flags:

• Qt::AlignLeft aligns to the left border, except for Arabic and Hebrew where it aligns to the right.
• Qt::AlignRight aligns to the right border, except for Arabic and Hebrew where it aligns to the left.
• Qt::AlignJustify produces justified text.
• Qt::AlignHCenter aligns horizontally centered.
• Qt::AlignTop aligns to the top border.
• Qt::AlignBottom aligns to the bottom border.
• Qt::AlignVCenter aligns vertically centered
• Qt::AlignCenter (== Qt::AlignHCenter | Qt::AlignVCenter)
• Qt::TextSingleLine ignores newline characters in the text.
• Qt::TextExpandTabs expands tabs (see below)
• Qt::TextShowMnemonic interprets "&x" as x; i.e., underlined.
• Qt::TextWordWrap breaks the text to fit the rectangle.

Qt::Horizontal alignment defaults to Qt::AlignLeft and vertical alignment defaults to Qt::AlignTop.

If several of the horizontal or several of the vertical alignment flags are set, the resulting alignment is undefined.

If Qt::TextExpandTabs is set in flags, then: if tabArray is non-null, it specifies a 0-terminated sequence of pixel-positions for tabs; otherwise if tabStops is non-zero, it is used as the tab spacing (in pixels).

Note that the bounding rectangle may extend to the left of (0, 0), e.g. for italicized fonts, and that the text output may cover all pixels in the bounding rectangle.

Newline characters are processed as linebreaks.

Despite the different actual character heights, the heights of the bounding rectangles of "Yes" and "yes" are the same.

The bounding rectangle returned by this function is somewhat larger than that calculated by the simpler boundingRect() function. This function uses the maximum left and right font bearings as is necessary for multi-line text to align correctly. Also, fontHeight() and lineSpacing() are used to calculate the height, rather than individual character heights.

See also horizontalAdvance(), QPainter::boundingRect(), and Qt::Alignment.

QRect QFontMetrics::boundingRect(int x, int y, int width, int height, int flags, const QString &text, int tabStops = 0, int *tabArray = nullptr) const

This is an overloaded function.

Returns the bounding rectangle for the given text within the rectangle specified by the x and y coordinates, width, and height.

If Qt::TextExpandTabs is set in flags and tabArray is non-null, it specifies a 0-terminated sequence of pixel-positions for tabs; otherwise, if tabStops is non-zero, it is used as the tab spacing (in pixels).

int QFontMetrics::capHeight() const

Returns the cap height of the font.

The cap height of a font is the height of a capital letter above the baseline. It specifically is the height of capital letters that are flat - such as H or I - as opposed to round letters such as O, or pointed letters like A, both of which may display overshoot.

This function was introduced in Qt 5.8.

See also ascent().

int QFontMetrics::descent() const

Returns the descent of the font.

The descent is the distance from the base line to the lowest point characters extend to. In practice, some font designers break this rule, e.g. to accommodate an unusual character in an exotic language, so it is possible (though rare) that this value will be too small.

See also ascent().

QString QFontMetrics::elidedText(const QString &text, Qt::TextElideMode mode, int width, int flags = 0) const

If the string text is wider than width, returns an elided version of the string (i.e., a string with "..." in it). Otherwise, returns the original string.

The mode parameter specifies whether the text is elided on the left (e.g., "...tech"), in the middle (e.g., "Tr...ch"), or on the right (e.g., "Trol...").

The width is specified in pixels, not characters.

The flags argument is optional and currently only supports Qt::TextShowMnemonic as value.

The elide mark follows the layoutdirection. For example, it will be on the right side of the text for right-to-left layouts if the mode is Qt::ElideLeft, and on the left side of the text if the mode is Qt::ElideRight.

This function was introduced in Qt 4.2.

qreal QFontMetrics::fontDpi() const

Returns the font DPI.

This function was introduced in Qt 5.14.

int QFontMetrics::height() const

Returns the height of the font.

This is always equal to ascent()+descent().

See also leading() and lineSpacing().

int QFontMetrics::horizontalAdvance(const QString &text, int len = -1) const

Returns the horizontal advance in pixels of the first len characters of text. If len is negative (the default), the entire string is used.

This is the distance appropriate for drawing a subsequent character after text.

This function was introduced in Qt 5.11.

See also boundingRect().

int QFontMetrics::horizontalAdvance(QChar ch) const

This is an overloaded function.

Returns the horizontal advance of character ch in pixels. This is a distance appropriate for drawing a subsequent character after ch.

Some of the metrics are described in the image. The central dark rectangles cover the logical horizontalAdvance() of each character. The outer pale rectangles cover the leftBearing() and rightBearing() of each character. Notice that the bearings of "f" in this particular font are both negative, while the bearings of "o" are both positive.

Warning: This function will produce incorrect results for Arabic characters or non-spacing marks in the middle of a string, as the glyph shaping and positioning of marks that happens when processing strings cannot be taken into account. When implementing an interactive text control, use QTextLayout instead.

This function was introduced in Qt 5.11.

See also boundingRect().

bool QFontMetrics::inFont(QChar ch) const

Returns true if character ch is a valid character in the font; otherwise returns false.

bool QFontMetrics::inFontUcs4(uint ucs4) const

Returns true if the character ucs4 encoded in UCS-4/UTF-32 is a valid character in the font; otherwise returns false.

int QFontMetrics::leading() const

Returns the leading of the font.

This is the natural inter-line spacing.

See also height() and lineSpacing().

int QFontMetrics::leftBearing(QChar ch) const

Returns the left bearing of character ch in the font.

The left bearing is the right-ward distance of the left-most pixel of the character from the logical origin of the character. This value is negative if the pixels of the character extend to the left of the logical origin.

See horizontalAdvance() for a graphical description of this metric.

See also rightBearing(), minLeftBearing(), and horizontalAdvance().

int QFontMetrics::lineSpacing() const

Returns the distance from one base line to the next.

This value is always equal to leading()+height().

See also height() and leading().

int QFontMetrics::lineWidth() const

Returns the width of the underline and strikeout lines, adjusted for the point size of the font.

See also underlinePos(), overlinePos(), and strikeOutPos().

int QFontMetrics::maxWidth() const

Returns the width of the widest character in the font.

int QFontMetrics::minLeftBearing() const

Returns the minimum left bearing of the font.

This is the smallest leftBearing(char) of all characters in the font.

Note that this function can be very slow if the font is large.

See also minRightBearing() and leftBearing().

int QFontMetrics::minRightBearing() const

Returns the minimum right bearing of the font.

This is the smallest rightBearing(char) of all characters in the font.

Note that this function can be very slow if the font is large.

See also minLeftBearing() and rightBearing().

int QFontMetrics::overlinePos() const

Returns the distance from the base line to where an overline should be drawn.

See also underlinePos(), strikeOutPos(), and lineWidth().

int QFontMetrics::rightBearing(QChar ch) const

Returns the right bearing of character ch in the font.

The right bearing is the left-ward distance of the right-most pixel of the character from the logical origin of a subsequent character. This value is negative if the pixels of the character extend to the right of the horizontalAdvance() of the character.

See horizontalAdvance() for a graphical description of this metric.

See also leftBearing(), minRightBearing(), and horizontalAdvance().

QSize QFontMetrics::size(int flags, const QString &text, int tabStops = 0, int *tabArray = nullptr) const

Returns the size in pixels of text.

The flags argument is the bitwise OR of the following flags:

• Qt::TextSingleLine ignores newline characters.
• Qt::TextExpandTabs expands tabs (see below)
• Qt::TextShowMnemonic interprets "&x" as x; i.e., underlined.
• Qt::TextWordWrap breaks the text to fit the rectangle.

If Qt::TextExpandTabs is set in flags, then: if tabArray is non-null, it specifies a 0-terminated sequence of pixel-positions for tabs; otherwise if tabStops is non-zero, it is used as the tab spacing (in pixels).

Newline characters are processed as linebreaks.

Despite the different actual character heights, the heights of the bounding rectangles of "Yes" and "yes" are the same.

See also boundingRect().

int QFontMetrics::strikeOutPos() const

Returns the distance from the base line to where the strikeout line should be drawn.

See also underlinePos(), overlinePos(), and lineWidth().

void QFontMetrics::swap(QFontMetrics &other)

Swaps this font metrics instance with other. This function is very fast and never fails.

This function was introduced in Qt 5.0.

QRect QFontMetrics::tightBoundingRect(const QString &text) const

Returns a tight bounding rectangle around the characters in the string specified by text. The bounding rectangle always covers at least the set of pixels the text would cover if drawn at (0, 0).

Note that the bounding rectangle may extend to the left of (0, 0), e.g. for italicized fonts, and that the width of the returned rectangle might be different than what the horizontalAdvance() method returns.

If you want to know the advance width of the string (to lay out a set of strings next to each other), use horizontalAdvance() instead.

Newline characters are processed as normal characters, not as linebreaks.

Warning: Calling this method is very slow on Windows.

This function was introduced in Qt 4.3.

See also horizontalAdvance(), height(), and boundingRect().

int QFontMetrics::underlinePos() const

Returns the distance from the base line to where an underscore should be drawn.

See also overlinePos(), strikeOutPos(), and lineWidth().

int QFontMetrics::xHeight() const

Returns the 'x' height of the font. This is often but not always the same as the height of the character 'x'.

bool QFontMetrics::operator!=(const QFontMetrics &other) const

Returns true if other is not equal to this object; otherwise returns false.

Two font metrics are considered equal if they were constructed from the same QFont and the paint devices they were constructed for are considered compatible.

See also operator==().

bool QFontMetrics::operator==(const QFontMetrics &other) const

Returns true if other is equal to this object; otherwise returns false.

Two font metrics are considered equal if they were constructed from the same QFont and the paint devices they were constructed for are considered compatible.

See also operator!=().