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QLabel Class

The QLabel widget provides a text or image display. More...

Header: #include <QLabel>
CMake: find_package(Qt6 COMPONENTS Widgets REQUIRED) target_link_libraries(mytarget PRIVATE Qt6::Widgets)
qmake: QT += widgets
Inherits: QFrame

Properties

Public Functions

QLabel(const QString &text, QWidget *parent = nullptr, Qt::WindowFlags f = Qt::WindowFlags())
QLabel(QWidget *parent = nullptr, Qt::WindowFlags f = Qt::WindowFlags())
virtual ~QLabel()
Qt::Alignment alignment() const
QWidget * buddy() const
bool hasScaledContents() const
bool hasSelectedText() const
int indent() const
int margin() const
QMovie * movie() const
bool openExternalLinks() const
QPicture picture() const
QPixmap pixmap() const
QTextDocument::ResourceProvider resourceProvider() const
QString selectedText() const
int selectionStart() const
void setAlignment(Qt::Alignment)
void setBuddy(QWidget *buddy)
void setIndent(int)
void setMargin(int)
void setOpenExternalLinks(bool open)
void setResourceProvider(const QTextDocument::ResourceProvider &provider)
void setScaledContents(bool)
void setSelection(int start, int length)
void setTextFormat(Qt::TextFormat)
void setTextInteractionFlags(Qt::TextInteractionFlags flags)
void setWordWrap(bool on)
QString text() const
Qt::TextFormat textFormat() const
Qt::TextInteractionFlags textInteractionFlags() const
bool wordWrap() const

Reimplemented Public Functions

virtual int heightForWidth(int w) const override
virtual QSize minimumSizeHint() const override
virtual QSize sizeHint() const override

Public Slots

void clear()
void setMovie(QMovie *movie)
void setNum(double num)
void setNum(int num)
void setPicture(const QPicture &picture)
void setPixmap(const QPixmap &)
void setText(const QString &)

Signals

void linkActivated(const QString &link)
void linkHovered(const QString &link)

Reimplemented Protected Functions

virtual void changeEvent(QEvent *ev) override
virtual void contextMenuEvent(QContextMenuEvent *ev) override
virtual bool event(QEvent *e) override
virtual void focusInEvent(QFocusEvent *ev) override
virtual bool focusNextPrevChild(bool next) override
virtual void focusOutEvent(QFocusEvent *ev) override
virtual void keyPressEvent(QKeyEvent *ev) override
virtual void mouseMoveEvent(QMouseEvent *ev) override
virtual void mousePressEvent(QMouseEvent *ev) override
virtual void mouseReleaseEvent(QMouseEvent *ev) override
virtual void paintEvent(QPaintEvent *) override

Detailed Description

QLabel is used for displaying text or an image. No user interaction functionality is provided. The visual appearance of the label can be configured in various ways, and it can be used for specifying a focus mnemonic key for another widget.

A QLabel can contain any of the following content types:

Content Setting
Plain text Pass a QString to setText().
Rich text Pass a QString that contains rich text to setText().
A pixmap Pass a QPixmap to setPixmap().
A movie Pass a QMovie to setMovie().
A number Pass an int or a double to setNum(), which converts the number to plain text.
Nothing The same as an empty plain text. This is the default. Set by clear().

Warning: When passing a QString to the constructor or calling setText(), make sure to sanitize your input, as QLabel tries to guess whether it displays the text as plain text or as rich text, a subset of HTML 4 markup. You may want to call setTextFormat() explicitly, e.g. in case you expect the text to be in plain format but cannot control the text source (for instance when displaying data loaded from the Web).

When the content is changed using any of these functions, any previous content is cleared.

By default, labels display left-aligned, vertically-centered text and images, where any tabs in the text to be displayed are automatically expanded. However, the look of a QLabel can be adjusted and fine-tuned in several ways.

The positioning of the content within the QLabel widget area can be tuned with setAlignment() and setIndent(). Text content can also wrap lines along word boundaries with setWordWrap(). For example, this code sets up a sunken panel with a two-line text in the bottom right corner (both lines being flush with the right side of the label):

QLabel *label = new QLabel(this);
label->setFrameStyle(QFrame::Panel | QFrame::Sunken);
label->setText("first line\nsecond line");
label->setAlignment(Qt::AlignBottom | Qt::AlignRight);

The properties and functions QLabel inherits from QFrame can also be used to specify the widget frame to be used for any given label.

A QLabel is often used as a label for an interactive widget. For this use QLabel provides a useful mechanism for adding an mnemonic (see QKeySequence) that will set the keyboard focus to the other widget (called the QLabel's "buddy"). For example:

QLineEdit *phoneEdit = new QLineEdit(this);
QLabel *phoneLabel = new QLabel("&Phone:", this);
phoneLabel->setBuddy(phoneEdit);

In this example, keyboard focus is transferred to the label's buddy (the QLineEdit) when the user presses Alt+P. If the buddy was a button (inheriting from QAbstractButton), triggering the mnemonic would emulate a button click.

See also QLineEdit, QTextEdit, QPixmap, QMovie, and GUI Design Handbook: Label.

Property Documentation

alignment : Qt::Alignment

This property holds the alignment of the label's contents

By default, the contents of the label are left-aligned and vertically-centered.

Access functions:

Qt::Alignment alignment() const
void setAlignment(Qt::Alignment)

See also text.

[read-only] hasSelectedText : const bool

This property holds whether there is any text selected

hasSelectedText() returns true if some or all of the text has been selected by the user; otherwise returns false.

By default, this property is false.

Note: The textInteractionFlags set on the label need to include either TextSelectableByMouse or TextSelectableByKeyboard.

Access functions:

bool hasSelectedText() const

See also selectedText().

indent : int

This property holds the label's text indent in pixels

If a label displays text, the indent applies to the left edge if alignment() is Qt::AlignLeft, to the right edge if alignment() is Qt::AlignRight, to the top edge if alignment() is Qt::AlignTop, and to the bottom edge if alignment() is Qt::AlignBottom.

If indent is negative, or if no indent has been set, the label computes the effective indent as follows: If frameWidth() is 0, the effective indent becomes 0. If frameWidth() is greater than 0, the effective indent becomes half the width of the "x" character of the widget's current font().

By default, the indent is -1, meaning that an effective indent is calculating in the manner described above.

Access functions:

int indent() const
void setIndent(int)

See also alignment, margin, frameWidth(), and font().

margin : int

This property holds the width of the margin

The margin is the distance between the innermost pixel of the frame and the outermost pixel of contents.

The default margin is 0.

Access functions:

int margin() const
void setMargin(int)

See also indent.

Specifies whether QLabel should automatically open links using QDesktopServices::openUrl() instead of emitting the linkActivated() signal.

Note: The textInteractionFlags set on the label need to include either LinksAccessibleByMouse or LinksAccessibleByKeyboard.

The default value is false.

Access functions:

bool openExternalLinks() const
void setOpenExternalLinks(bool open)

See also textInteractionFlags().

pixmap : QPixmap

This property holds the label's pixmap.

Setting the pixmap clears any previous content. The buddy shortcut, if any, is disabled.

Access functions:

QPixmap pixmap() const
void setPixmap(const QPixmap &)

scaledContents : bool

This property holds whether the label will scale its contents to fill all available space.

When enabled and the label shows a pixmap, it will scale the pixmap to fill the available space.

This property's default is false.

Access functions:

bool hasScaledContents() const
void setScaledContents(bool)

[read-only] selectedText : const QString

This property holds the selected text

If there is no selected text this property's value is an empty string.

By default, this property contains an empty string.

Note: The textInteractionFlags set on the label need to include either TextSelectableByMouse or TextSelectableByKeyboard.

Access functions:

QString selectedText() const

See also hasSelectedText().

text : QString

This property holds the label's text

If no text has been set this will return an empty string. Setting the text clears any previous content.

The text will be interpreted either as plain text or as rich text, depending on the text format setting; see setTextFormat(). The default setting is Qt::AutoText; i.e. QLabel will try to auto-detect the format of the text set. See Supported HTML Subset for the definition of rich text.

If a buddy has been set, the buddy mnemonic key is updated from the new text.

Note that QLabel is well-suited to display small rich text documents, such as small documents that get their document specific settings (font, text color, link color) from the label's palette and font properties. For large documents, use QTextEdit in read-only mode instead. QTextEdit can also provide a scroll bar when necessary.

Note: This function enables mouse tracking if text contains rich text.

Access functions:

QString text() const
void setText(const QString &)

See also setTextFormat(), setBuddy(), and alignment.

textFormat : Qt::TextFormat

This property holds the label's text format

See the Qt::TextFormat enum for an explanation of the possible options.

The default format is Qt::AutoText.

Access functions:

Qt::TextFormat textFormat() const
void setTextFormat(Qt::TextFormat)

See also text().

textInteractionFlags : Qt::TextInteractionFlags

Specifies how the label should interact with user input if it displays text.

If the flags contain Qt::LinksAccessibleByKeyboard the focus policy is also automatically set to Qt::StrongFocus. If Qt::TextSelectableByKeyboard is set then the focus policy is set to Qt::ClickFocus.

The default value is Qt::LinksAccessibleByMouse.

Access functions:

Qt::TextInteractionFlags textInteractionFlags() const
void setTextInteractionFlags(Qt::TextInteractionFlags flags)

wordWrap : bool

This property holds the label's word-wrapping policy

If this property is true then label text is wrapped where necessary at word-breaks; otherwise it is not wrapped at all.

By default, word wrap is disabled.

Access functions:

bool wordWrap() const
void setWordWrap(bool on)

See also text.

Member Function Documentation

QLabel::QLabel(const QString &text, QWidget *parent = nullptr, Qt::WindowFlags f = Qt::WindowFlags())

Constructs a label that displays the text, text.

The parent and widget flag f, arguments are passed to the QFrame constructor.

See also setText(), setAlignment(), setFrameStyle(), and setIndent().

QLabel::QLabel(QWidget *parent = nullptr, Qt::WindowFlags f = Qt::WindowFlags())

Constructs an empty label.

The parent and widget flag f, arguments are passed to the QFrame constructor.

See also setAlignment(), setFrameStyle(), and setIndent().

[slot] void QLabel::clear()

Clears any label contents.

[signal] void QLabel::linkActivated(const QString &link)

This signal is emitted when the user clicks a link. The URL referred to by the anchor is passed in link.

See also linkHovered().

[signal] void QLabel::linkHovered(const QString &link)

This signal is emitted when the user hovers over a link. The URL referred to by the anchor is passed in link.

See also linkActivated().

[slot] void QLabel::setMovie(QMovie *movie)

Sets the label contents to movie. Any previous content is cleared. The label does NOT take ownership of the movie.

The buddy shortcut, if any, is disabled.

See also movie() and setBuddy().

[slot] void QLabel::setNum(double num)

This is an overloaded function.

Sets the label contents to plain text containing the textual representation of double num. Any previous content is cleared. Does nothing if the double's string representation is the same as the current contents of the label.

The buddy shortcut, if any, is disabled.

See also setText(), QString::setNum(), and setBuddy().

[slot] void QLabel::setNum(int num)

Sets the label contents to plain text containing the textual representation of integer num. Any previous content is cleared. Does nothing if the integer's string representation is the same as the current contents of the label.

The buddy shortcut, if any, is disabled.

See also setText(), QString::setNum(), and setBuddy().

[slot] void QLabel::setPicture(const QPicture &picture)

Sets the label contents to picture. Any previous content is cleared.

The buddy shortcut, if any, is disabled.

See also picture() and setBuddy().

[virtual] QLabel::~QLabel()

Destroys the label.

QWidget *QLabel::buddy() const

Returns this label's buddy, or nullptr if no buddy is currently set.

See also setBuddy().

[override virtual protected] void QLabel::changeEvent(QEvent *ev)

Reimplements: QFrame::changeEvent(QEvent *ev).

[override virtual protected] void QLabel::contextMenuEvent(QContextMenuEvent *ev)

Reimplements: QWidget::contextMenuEvent(QContextMenuEvent *event).

[override virtual protected] bool QLabel::event(QEvent *e)

Reimplements: QFrame::event(QEvent *e).

[override virtual protected] void QLabel::focusInEvent(QFocusEvent *ev)

Reimplements: QWidget::focusInEvent(QFocusEvent *event).

[override virtual protected] bool QLabel::focusNextPrevChild(bool next)

Reimplements: QWidget::focusNextPrevChild(bool next).

[override virtual protected] void QLabel::focusOutEvent(QFocusEvent *ev)

Reimplements: QWidget::focusOutEvent(QFocusEvent *event).

[override virtual] int QLabel::heightForWidth(int w) const

Reimplements: QWidget::heightForWidth(int w) const.

[override virtual protected] void QLabel::keyPressEvent(QKeyEvent *ev)

Reimplements: QWidget::keyPressEvent(QKeyEvent *event).

[override virtual] QSize QLabel::minimumSizeHint() const

Reimplements an access function for property: QWidget::minimumSizeHint.

[override virtual protected] void QLabel::mouseMoveEvent(QMouseEvent *ev)

Reimplements: QWidget::mouseMoveEvent(QMouseEvent *event).

[override virtual protected] void QLabel::mousePressEvent(QMouseEvent *ev)

Reimplements: QWidget::mousePressEvent(QMouseEvent *event).

[override virtual protected] void QLabel::mouseReleaseEvent(QMouseEvent *ev)

Reimplements: QWidget::mouseReleaseEvent(QMouseEvent *event).

QMovie *QLabel::movie() const

Returns a pointer to the label's movie, or nullptr if no movie has been set.

See also setMovie().

[override virtual protected] void QLabel::paintEvent(QPaintEvent *)

Reimplements: QFrame::paintEvent(QPaintEvent *).

[since 6.0] QPicture QLabel::picture() const

Returns the label's picture.

This function was introduced in Qt 6.0.

[since 6.1] QTextDocument::ResourceProvider QLabel::resourceProvider() const

Returns the resource provider for rich text of this label.

This function was introduced in Qt 6.1.

See also setResourceProvider().

int QLabel::selectionStart() const

selectionStart() returns the index of the first selected character in the label or -1 if no text is selected.

Note: The textInteractionFlags set on the label need to include either TextSelectableByMouse or TextSelectableByKeyboard.

See also selectedText().

void QLabel::setBuddy(QWidget *buddy)

Sets this label's buddy to buddy.

When the user presses the shortcut key indicated by this label, the keyboard focus is transferred to the label's buddy widget.

The buddy mechanism is only available for QLabels that contain text in which one character is prefixed with an ampersand, '&'. This character is set as the shortcut key. See the QKeySequence::mnemonic() documentation for details (to display an actual ampersand, use '&&').

In a dialog, you might create two data entry widgets and a label for each, and set up the geometry layout so each label is just to the left of its data entry widget (its "buddy"), for example:

QLineEdit *nameEdit  = new QLineEdit(this);
QLabel    *nameLabel = new QLabel("&Name:", this);
nameLabel->setBuddy(nameEdit);
QLineEdit *phoneEdit  = new QLineEdit(this);
QLabel    *phoneLabel = new QLabel("&Phone:", this);
phoneLabel->setBuddy(phoneEdit);
// (layout setup not shown)

With the code above, the focus jumps to the Name field when the user presses Alt+N, and to the Phone field when the user presses Alt+P.

To unset a previously set buddy, call this function with buddy set to nullptr.

See also buddy(), setText(), QShortcut, and setAlignment().

[since 6.1] void QLabel::setResourceProvider(const QTextDocument::ResourceProvider &provider)

Sets the provider of resources for rich text of this label.

Note: The label does not take ownership of the provider.

This function was introduced in Qt 6.1.

See also resourceProvider().

void QLabel::setSelection(int start, int length)

Selects text from position start and for length characters.

Note: The textInteractionFlags set on the label need to include either TextSelectableByMouse or TextSelectableByKeyboard.

See also selectedText().

[override virtual] QSize QLabel::sizeHint() const

Reimplements: QFrame::sizeHint() const.

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Licensed under the GNU Free Documentation License, Version 1.3.
https://doc.qt.io/qt-6.2/qlabel.html