QFontMetrics Class

Qt 3.0.5

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QFontMetrics Class Reference

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

#include <qfontmetrics.h>

List of all member functions.

Public Members

Detailed Description

The QFontMetrics class provides font metrics information.

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.

  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. The font metrics object is automatically updated if you set a new painter font.

* If you use a printer font the values returned will almost certainly be inaccurate. Printer fonts are not always accessible so the nearest screen font is used if a printer font is supplied.

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(), strikeOutPos() and lineWidth() functions, return the properties of the line that underlines 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 width(), 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 width(), 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.width( "What's the width of this text?" );
    int pixelsHigh = fm.height();

See also QFont, QFontInfo, QFontDatabase, Graphics Classes and Implicitly and Explicitly Shared Classes.

Member Function Documentation

QFontMetrics::QFontMetrics ( const QFont & font )

Constructs a font metrics object for font.

The font must be screen-compatible, i.e. a font you use when drawing text in widgets or pixmaps, not 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.

Use QPainter::fontMetrics() to get the font metrics when painting. This is a little slower than using this constructor, but it always gives correct results because the font info data is updated.

QFontMetrics::QFontMetrics ( const QFontMetrics & fm )

Constructs a copy of 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().

Examples: drawdemo/drawdemo.cpp and scrollview/scrollview.cpp.

QRect QFontMetrics::boundingRect ( const QString & str, int len = -1 ) const

Returns the bounding rectangle of the first len characters of str, which is the set of pixels the text would cover if drawn at (0, 0).

If len is negative (the default), the entire string is used.

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 normal characters, not as linebreaks.

Due to the different actual character heights, the height of the bounding rectangle of e.g. "Yes" and "yes" may be different.

See also width() and QPainter::boundingRect().

Example: xform/xform.cpp.

QRect QFontMetrics::boundingRect ( QChar ch ) const

This is an overloaded member function, provided for convenience. It behaves essentially like the above function.

Returns the bounding rectangle of the character ch relative to the left-most point on the base line.

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.

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

See also width().

QRect QFontMetrics::boundingRect ( int x, int y, int w, int h, int flgs, const QString & str, int len = -1, int tabstops = 0, int * tabarray = 0, QTextParag ** intern = 0 ) const

This is an overloaded member function, provided for convenience. It behaves essentially like the above function.

Returns the bounding rectangle of the first len characters of str, 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 (x, y, w, h).

If len is negative (which is the default), the entire string is used.

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

  • AlignAuto aligns to the left border for all languages except Arabic and Hebrew where it aligns to the right.
  • AlignLeft aligns to the left border.
  • AlignRight aligns to the right border.
  • AlignJustify produces justified text.
  • AlignHCenter aligns horizontally centered.
  • AlignTop aligns to the top border.
  • AlignBottom aligns to the bottom border.
  • AlignVCenter aligns vertically centered
  • AlignCenter (== AlignHCenter | AlignVCenter)
  • SingleLine ignores newline characters in the text.
  • ExpandTabs expands tabs (see below)
  • ShowPrefix interprets "&x" as "x", i.e. underlined.
  • WordBreak breaks the text to fit the rectangle.

Horizontal alignment defaults to AlignAuto and vertical alignment defaults to AlignTop.

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

These flags are defined in qnamespace.h.

If ExpandTabs is set in flgs, 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 given 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.

The intern argument should not be used.

See also width(), QPainter::boundingRect() and Qt::AlignmentFlags.

int QFontMetrics::charWidth ( const QString & str, int pos ) const

Returns the width of the character at position pos in the string str.

The whole string is needed, as the glyph drawn may change depending on the context (the letter before and after the current one) for some languages (e.g. Arabic).

This function also takes non spacing marks and ligatures into account.

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. (Note that this is different from X, which adds 1 pixel.) 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().

Examples: drawdemo/drawdemo.cpp and hello/hello.cpp.

int QFontMetrics::height () const

Returns the height of the font.

This is always equal to ascent()+descent()+1 (the 1 is for the base line).

See also leading() and lineSpacing().

Examples: grapher/grapher.cpp, hello/hello.cpp and qfd/fontdisplayer.cpp.

bool QFontMetrics::inFont ( QChar ch ) const

Returns TRUE if character ch is a valid character in the font; otherwise returns FALSE.

Example: qfd/fontdisplayer.cpp.

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 width(QChar) for a graphical description of this metric.

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

Example: qfd/fontdisplayer.cpp.

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().

Examples: action/application.cpp, application/application.cpp, mdi/application.cpp, qfd/fontdisplayer.cpp, qwerty/qwerty.cpp and scrollview/scrollview.cpp.

int QFontMetrics::lineWidth () const

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

See also underlinePos() and strikeOutPos().

int QFontMetrics::maxWidth () const

Returns the width of the widest character in the font.

Example: qfd/fontdisplayer.cpp.

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().

Example: qfd/fontdisplayer.cpp.

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().

Example: qfd/fontdisplayer.cpp.

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

Assigns the font metrics fm.

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 width() of the character.

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

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

Example: qfd/fontdisplayer.cpp.

QSize QFontMetrics::size ( int flgs, const QString & str, int len = -1, int tabstops = 0, int * tabarray = 0, QTextParag ** intern = 0 ) const

Returns the size in pixels of the first len characters of str.

If len is negative (the default), the entire string is used.

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

  • SingleLine ignores newline characters.
  • ExpandTabs expands tabs (see below)
  • ShowPrefix interprets "&x" as "x", i.e. underlined.
  • WordBreak breaks the text to fit the rectangle.

These flags are defined in qnamespace.h.

If ExpandTabs is set in flgs, 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.

The intern argument should not be used.

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() and lineWidth().

int QFontMetrics::underlinePos () const

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

See also strikeOutPos() and lineWidth().

int QFontMetrics::width ( const QString & str, int len = -1 ) const

Returns the width in pixels of the first len characters of str. If len is negative (the default), the entire string is used.

Note that this value is not equal to boundingRect().width(); boundingRect() returns a rectangle describing the pixels this string will cover whereas width() returns the distance to where the next string should be drawn.

See also boundingRect().

Examples: drawdemo/drawdemo.cpp, hello/hello.cpp, movies/main.cpp, qfd/fontdisplayer.cpp and scrollview/scrollview.cpp.

int QFontMetrics::width ( QChar ch ) const

This is an overloaded member function, provided for convenience. It behaves essentially like the above function.

Returns the logical width 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 to the right. The central dark rectangles cover the logical width() 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. Use charWidth() instead if you aren't looking for the width of isolated characters.

See also boundingRect() and charWidth().

int QFontMetrics::width ( char c ) const

This is an overloaded member function, provided for convenience. It behaves essentially like the above function.

This function is obsolete. It is provided to keep old source working. We strongly advise against using it in new code.

Provided to aid porting from Qt 1.x.

This file is part of the Qt toolkit. Copyright © 1995-2002 Trolltech. All Rights Reserved.

Copyright © 2002 TrolltechTrademarks
Qt version 3.0.5

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