Signals and slots are used for communication between objects. The signals and slots mechanism is a central feature of Qt and probably the part that differs most from the features provided by other frameworks. Signals and slots are made possible by Qt's meta-object system .

1. Virtual Function In Oop
2. Qt Slot Virtual Function System
3. Virtual Function In Cpp
4. Qt Virtual Slot

Introduction

In GUI programming, when we change one widget, we often want another widget to be notified. More generally, we want objects of any kind to be able to communicate with one another. For example, if a user clicks a Close button, we probably want the window's close() function to be called.

Other toolkits achieve this kind of communication using callbacks. A callback is a pointer to a function, so if you want a processing function to notify you about some event you pass a pointer to another function (the callback) to the processing function. The processing function then calls the callback when appropriate. While successful frameworks using this method do exist, callbacks can be unintuitive and may suffer from problems in ensuring the type-correctness of callback arguments.

The Qt signals/slots and property system are based on the ability to introspect the objects at runtime. Introspection means being able to list the methods and properties of an object and have all kinds of information about them such as the type of their arguments. QtScript and QML would have hardly been possible without that ability. Qt Slot Virtual Function, steam friends 5 slot level bonus, roulette wrecking ball, como ganar en texas holdem poker 3. A slot is a function that is called in response to a particular signal. Qt's widgets have many pre-defined slots, but it is common practice to subclass widgets and add your own slots so that you can handle the signals that you are interested in. Virtual Qt::ItemFlags: flags. Virtual function on the model to reorder the data in the model.

Signals and Slots

In Qt, we have an alternative to the callback technique: We use signals and slots. A signal is emitted when a particular event occurs. Qt's widgets have many predefined signals, but we can always subclass widgets to add our own signals to them. A slot is a function that is called in response to a particular signal. Qt's widgets have many pre-defined slots, but it is common practice to subclass widgets and add your own slots so that you can handle the signals that you are interested in.

Signals and slots in Qt

The signals and slots mechanism is type safe: The signature of a signal must match the signature of the receiving slot. (In fact a slot may have a shorter signature than the signal it receives because it can ignore extra arguments.) Since the signatures are compatible, the compiler can help us detect type mismatches when using the function pointer-based syntax. The string-based SIGNAL and SLOT syntax will detect type mismatches at runtime. Signals and slots are loosely coupled: A class which emits a signal neither knows nor cares which slots receive the signal. Qt's signals and slots mechanism ensures that if you connect a signal to a slot, the slot will be called with the signal's parameters at the right time. Signals and slots can take any number of arguments of any type. They are completely type safe.

All classes that inherit from QObject or one of its subclasses (e.g., QWidget ) can contain signals and slots. Signals are emitted by objects when they change their state in a way that may be interesting to other objects. This is all the object does to communicate. It does not know or care whether anything is receiving the signals it emits. This is true information encapsulation, and ensures that the object can be used as a software component.

Slots can be used for receiving signals, but they are also normal member functions. Just as an object does not know if anything receives its signals, a slot does not know if it has any signals connected to it. This ensures that truly independent components can be created with Qt.

You can connect as many signals as you want to a single slot, and a signal can be connected to as many slots as you need. It is even possible to connect a signal directly to another signal. (This will emit the second signal immediately whenever the first is emitted.)

Together, signals and slots make up a powerful component programming mechanism.

Signals

Signals are emitted by an object when its internal state has changed in some way that might be interesting to the object's client or owner. Signals are public access functions and can be emitted from anywhere, but we recommend to only emit them from the class that defines the signal and its subclasses.

When a signal is emitted, the slots connected to it are usually executed immediately, just like a normal function call. When this happens, the signals and slots mechanism is totally independent of any GUI event loop. Execution of the code following the emit statement will occur once all slots have returned. The situation is slightly different when using queued connections ; in such a case, the code following the emit keyword will continue immediately, and the slots will be executed later.

If several slots are connected to one signal, the slots will be executed one after the other, in the order they have been connected, when the signal is emitted.

Signals are automatically generated by the moc and must not be implemented in the .cpp file. They can never have return types (i.e. use void ).

A note about arguments: Our experience shows that signals and slots are more reusable if they do not use special types. If QScrollBar::valueChanged () were to use a special type such as the hypothetical QScrollBar::Range, it could only be connected to slots designed specifically for QScrollBar . Connecting different input widgets together would be impossible.

Slots

A slot is called when a signal connected to it is emitted. Slots are normal C++ functions and can be called normally; their only special feature is that signals can be connected to them.

Since slots are normal member functions, they follow the normal C++ rules when called directly. However, as slots, they can be invoked by any component, regardless of its access level, via a signal-slot connection. This means that a signal emitted from an instance of an arbitrary class can cause a private slot to be invoked in an instance of an unrelated class.

You can also define slots to be virtual, which we have found quite useful in practice.

Compared to callbacks, signals and slots are slightly slower because of the increased flexibility they provide, although the difference for real applications is insignificant. In general, emitting a signal that is connected to some slots, is approximately ten times slower than calling the receivers directly, with non-virtual function calls. This is the overhead required to locate the connection object, to safely iterate over all connections (i.e. checking that subsequent receivers have not been destroyed during the emission), and to marshall any parameters in a generic fashion. While ten non-virtual function calls may sound like a lot, it's much less overhead than any new or delete operation, for example. As soon as you perform a string, vector or list operation that behind the scene requires new or delete , the signals and slots overhead is only responsible for a very small proportion of the complete function call costs. The same is true whenever you do a system call in a slot; or indirectly call more than ten functions. The simplicity and flexibility of the signals and slots mechanism is well worth the overhead, which your users won't even notice.

Note that other libraries that define variables called signals or slots may cause compiler warnings and errors when compiled alongside a Qt-based application. To solve this problem, #undef the offending preprocessor symbol.

Connecting the signal to the slot

Prior to the fifth version of Qt to connect the signal to the slot through the recorded macros, whereas in the fifth version of the recording has been applied, based on the signs.

Writing with macros:

Writing on the basis of indicators:

The advantage of the second option is that it is possible to determine the mismatch of signatures and the wrong slot or signal name of another project compilation stage, not in the process of testing applications.

An example of using signals and slots

For example, the use of signals and slots project was created, which in the main window contains three buttons, each of which is connected to the slot and these slots already transmit a signal in a single slot with the pressed button number.

Project Structure

Project Structure

According to the tradition of conducting lessons enclosing structure of the project, which is absolutely trivial and defaulted to the disgrace that will not even describe members of her classes and files.

mainwindow.h

Thus, the following three buttons - three slots, one signal at all three buttons, which is fed into the slot button and transmits the number buttons into a single slot that displays a message with the number buttons.

mainwindow.cpp

A file in this logic is configured as described in the preceding paragraphs. Just check the code and go to the video page, there is shown in detail the whole process, demonstrated the application, and also shows what happens if we make coding a variety of errors.

Video

The QAbstractSpinBox class provides a spinbox and a line edit to display values. More...

Public Types

Properties

• 59 properties inherited from QWidget
• 1 property inherited from QObject

Public Functions

Reimplemented Public Functions

• 214 public functions inherited from QWidget
• 32 public functions inherited from QObject
• 14 public functions inherited from QPaintDevice

Public Slots

• 19 public slots inherited from QWidget
• 1 public slot inherited from QObject

Signals

• 3 signals inherited from QWidget
• 2 signals inherited from QObject

Protected Functions

Reimplemented Protected Functions

• 35 protected functions inherited from QWidget
• 9 protected functions inherited from QObject
• 1 protected function inherited from QPaintDevice

Additional Inherited Members

• 5 static public members inherited from QWidget
• 11 static public members inherited from QObject
• 1 protected slot inherited from QWidget

Detailed Description

The QAbstractSpinBox class provides a spinbox and a line edit to display values.

The class is designed as a common super class for widgets like QSpinBox, QDoubleSpinBox and QDateTimeEdit

Here are the main properties of the class:

1. text: The text that is displayed in the QAbstractSpinBox.
2. alignment: The alignment of the text in the QAbstractSpinBox.
3. wrapping: Whether the QAbstractSpinBox wraps from the minimum value to the maximum value and vica versa.

QAbstractSpinBox provides a virtual stepBy() function that is called whenever the user triggers a step. This function takes an integer value to signify how many steps were taken. E.g. Pressing Qt::Key_Down will trigger a call to stepBy(-1).

QAbstractSpinBox also provide a virtual function stepEnabled() to determine whether stepping up/down is allowed at any point. This function returns a bitset of StepEnabled.

See also QAbstractSlider, QSpinBox, QDoubleSpinBox, QDateTimeEdit, and Spin Boxes Example.

Member Type Documentation

enum QAbstractSpinBox::ButtonSymbols

This enum type describes the symbols that can be displayed on the buttons in a spin box.

See also QAbstractSpinBox::buttonSymbols.

enum QAbstractSpinBox::CorrectionMode

This enum type describes the mode the spinbox will use to correct an Intermediate value if editing finishes.

See also correctionMode.

enum QAbstractSpinBox::StepEnabledFlag
flags QAbstractSpinBox::StepEnabled

The StepEnabled type is a typedef for QFlags<StepEnabledFlag>. It stores an OR combination of StepEnabledFlag values.

Property Documentation

accelerated : bool

This property holds whether the spin box will accelerate the frequency of the steps when pressing the step Up/Down buttons.

If enabled the spin box will increase/decrease the value faster the longer you hold the button down.

This property was introduced in Qt 4.2.

Access functions:

acceptableInput : const bool

This property holds whether the input satisfies the current validation

This property was introduced in Qt 4.2.

Access functions:

See also validate(), fixup(), and correctionMode.

alignment : Qt::Alignment

This property holds the alignment of the spin box

Possible Values are Qt::AlignLeft, Qt::AlignRight, and Qt::AlignHCenter.

By default, the alignment is Qt::AlignLeft

Attempting to set the alignment to an illegal flag combination does nothing.

Access functions:

See also Qt::Alignment.

buttonSymbols : ButtonSymbols

This property holds the current button symbol mode

The possible values can be either UpDownArrows or PlusMinus. The default is UpDownArrows.

Note that some styles might render PlusMinus and UpDownArrows identically.

Access functions:

See also ButtonSymbols.

correctionMode : CorrectionMode

This property holds the mode to correct an Intermediate value if editing finishes

The default mode is QAbstractSpinBox::CorrectToPreviousValue.

This property was introduced in Qt 4.2.

Access functions:

See also acceptableInput, validate(), and fixup().

frame : bool

This property holds whether the spin box draws itself with a frame

If enabled (the default) the spin box draws itself inside a frame, otherwise the spin box draws itself without any frame.

Access functions:

keyboardTracking : bool

This property holds whether keyboard tracking is enabled for the spinbox.

If keyboard tracking is enabled (the default), the spinbox emits the valueChanged() signal while the new value is being entered from the keyboard.

E.g. when the user enters the value 600 by typing 6, 0, and 0, the spinbox emits 3 signals with the values 6, 60, and 600 respectively.

If keyboard tracking is disabled, the spinbox doesn't emit the valueChanged() signal while typing. It emits the signal later, when the return key is pressed, when keyboard focus is lost, or when other spinbox functionality is used, e.g. pressing an arrow key.

This property was introduced in Qt 4.3.

Access functions:

readOnly : bool

Virtual Function In Oop

This property holds whether the spin box is read only.

In read-only mode, the user can still copy the text to the clipboard, or drag and drop the text; but cannot edit it.

The QLineEdit in the QAbstractSpinBox does not show a cursor in read-only mode.

Access functions:

See also QLineEdit::readOnly.

showGroupSeparator : bool

This property holds whether a thousands separator is enabled. By default this property is false.

This property was introduced in Qt 5.3.

Access functions:

specialValueText : QString

This property holds the special-value text

If set, the spin box will display this text instead of a numeric value whenever the current value is equal to minimum(). Typical use is to indicate that this choice has a special (default) meaning.

For example, if your spin box allows the user to choose a scale factor (or zoom level) for displaying an image, and your application is able to automatically choose one that will enable the image to fit completely within the display window, you can set up the spin box like this:

The user will then be able to choose a scale from 1% to 1000% or select 'Auto' to leave it up to the application to choose. Your code must then interpret the spin box value of 0 as a request from the user to scale the image to fit inside the window.

All values are displayed with the prefix and suffix (if set), except for the special value, which only shows the special value text. This special text is passed in the QSpinBox::valueChanged() signal that passes a QString.

To turn off the special-value text display, call this function with an empty string. The default is no special-value text, i.e. the numeric value is shown as usual.

If no special-value text is set, specialValueText() returns an empty string.

Access functions:

text : const QString

This property holds the spin box's text, including any prefix and suffix

There is no default text.

Access functions:

wrapping : bool

This property holds whether the spin box is circular.

If wrapping is true stepping up from maximum() value will take you to the minimum() value and vica versa. Wrapping only make sense if you have minimum() and maximum() values set.

Access functions:

See also QSpinBox::minimum() and QSpinBox::maximum().

Member Function Documentation

QAbstractSpinBox::QAbstractSpinBox(QWidget *parent = Q_NULLPTR)

Constructs an abstract spinbox with the given parent with default wrapping, and alignment properties.

QAbstractSpinBox::~QAbstractSpinBox()

Called when the QAbstractSpinBox is destroyed.

[override virtual protected] void QAbstractSpinBox::changeEvent(QEvent *event)

Reimplemented from QWidget::changeEvent().

[virtual slot] void QAbstractSpinBox::clear()

Clears the lineedit of all text but prefix and suffix.

[override virtual protected] void QAbstractSpinBox::closeEvent(QCloseEvent *event)

Reimplemented from QWidget::closeEvent().

[override virtual protected] void QAbstractSpinBox::contextMenuEvent(QContextMenuEvent *event)

Reimplemented from QWidget::contextMenuEvent().

[signal] void QAbstractSpinBox::editingFinished()

This signal is emitted editing is finished. This happens when the spinbox loses focus and when enter is pressed.

[override virtual] bool QAbstractSpinBox::event(QEvent *event)

Reimplemented from QObject::event().

[virtual] void QAbstractSpinBox::fixup(QString &input) const

This virtual function is called by the QAbstractSpinBox if the input is not validated to QValidator::Acceptable when Return is pressed or interpretText() is called. It will try to change the text so it is valid. Reimplemented in the various subclasses.

[override virtual protected] void QAbstractSpinBox::focusInEvent(QFocusEvent *event)

Reimplemented from QWidget::focusInEvent().

[override virtual protected] void QAbstractSpinBox::focusOutEvent(QFocusEvent *event)

Reimplemented from QWidget::focusOutEvent().

[override virtual protected] void QAbstractSpinBox::hideEvent(QHideEvent *event)

Reimplemented from QWidget::hideEvent().

[protected] void QAbstractSpinBox::initStyleOption(QStyleOptionSpinBox *option) const

Initialize option with the values from this QSpinBox. This method is useful for subclasses when they need a QStyleOptionSpinBox, but don't want to fill in all the information themselves.

See also QStyleOption::initFrom().

[override virtual] QVariant QAbstractSpinBox::inputMethodQuery(Qt::InputMethodQueryquery) const

Reimplemented from QWidget::inputMethodQuery().

void QAbstractSpinBox::interpretText()

This function interprets the text of the spin box. If the value has changed since last interpretation it will emit signals.

[override virtual protected] void QAbstractSpinBox::keyPressEvent(QKeyEvent *event)

Reimplemented from QWidget::keyPressEvent().

This function handles keyboard input.

The following keys are handled specifically:

[override virtual protected] void QAbstractSpinBox::keyReleaseEvent(QKeyEvent *event)

Reimplemented from QWidget::keyReleaseEvent().

[protected] QLineEdit *QAbstractSpinBox::lineEdit() const

This function returns a pointer to the line edit of the spin box.

See also setLineEdit().

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

Reimplemented from QWidget::minimumSizeHint().

[override virtual protected] void QAbstractSpinBox::mouseMoveEvent(QMouseEvent *event)

Reimplemented from QWidget::mouseMoveEvent().

[override virtual protected] void QAbstractSpinBox::mousePressEvent(QMouseEvent *event)

Reimplemented from QWidget::mousePressEvent().

[override virtual protected] void QAbstractSpinBox::mouseReleaseEvent(QMouseEvent *event)

Reimplemented from QWidget::mouseReleaseEvent().

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

Reimplemented from QWidget::paintEvent().

[override virtual protected] void QAbstractSpinBox::resizeEvent(QResizeEvent *event)

Reimplemented from QWidget::resizeEvent().

[slot] void QAbstractSpinBox::selectAll()

Selects all the text in the spinbox except the prefix and suffix.

[protected] void QAbstractSpinBox::setLineEdit(QLineEdit *lineEdit)

Sets the line edit of the spinbox to be lineEdit instead of the current line edit widget. lineEdit can not be 0.

QAbstractSpinBox takes ownership of the new lineEdit

If QLineEdit::validator() for the lineEdit returns 0, the internal validator of the spinbox will be set on the line edit.

See also lineEdit().

[override virtual protected] void QAbstractSpinBox::showEvent(QShowEvent *event)

Reimplemented from QWidget::showEvent().

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

Reimplemented from QWidget::sizeHint().

[virtual] void QAbstractSpinBox::stepBy(intsteps)

Virtual function that is called whenever the user triggers a step. The steps parameter indicates how many steps were taken, e.g. Pressing Qt::Key_Down will trigger a call to stepBy(-1), whereas pressing Qt::Key_Prior will trigger a call to stepBy(10).

If you subclass QAbstractSpinBox you must reimplement this function. Note that this function is called even if the resulting value will be outside the bounds of minimum and maximum. It's this function's job to handle these situations.

[slot] void QAbstractSpinBox::stepDown()

Steps down by one linestep Calling this slot is analogous to calling stepBy(-1);

See also stepBy() and stepUp().

[virtual protected] StepEnabled QAbstractSpinBox::stepEnabled() const

Virtual function that determines whether stepping up and down is legal at any given time.

The up arrow will be painted as disabled unless (stepEnabled() & StepUpEnabled) != 0.

The default implementation will return (StepUpEnabled StepDownEnabled) if wrapping is turned on. Else it will return StepDownEnabled if value is > minimum() or'ed with StepUpEnabled if value < maximum().

If you subclass QAbstractSpinBox you will need to reimplement this function.

See also QSpinBox::minimum(), QSpinBox::maximum(), and wrapping().

[slot] void QAbstractSpinBox::stepUp()

Steps up by one linestep Calling this slot is analogous to calling stepBy(1);

See also stepBy() and stepDown().

[override virtual protected] void QAbstractSpinBox::timerEvent(QTimerEvent *event)

Reimplemented from QObject::timerEvent().

[virtual] QValidator::State QAbstractSpinBox::validate(QString &input, int &pos) const

This virtual function is called by the QAbstractSpinBox to determine whether input is valid. The pos parameter indicates the position in the string. Reimplemented in the various subclasses.

Qt Slot Virtual Function System

[override virtual protected] void QAbstractSpinBox::wheelEvent(QWheelEvent *event)

Virtual Function In Cpp

Reimplemented from QWidget::wheelEvent().

Qt Virtual Slot

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