Qt Slot Mapper

QDataWidgetMapper can be used to create data-aware widgets by mapping them to sections of an item model. A section is a column of a model if the orientation is horizontal (the default), otherwise a row. Every time the current index changes, each widget is updated with data from the model via the property specified when its mapping was made. From the signals slots documentation: 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.) This means that a signal of the form. Signal(int, int, QString) can only be connected with slots with the following. Signals & Slots 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.

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I still remember times, when Qt didn't have QSignalMapper, and the only solution was setting property on objects connected to same slot and using sender-property(.) – Kamil Klimek Mar 2 '11 at 10:56.

The QSignalMapper class bundles signals from identifiable senders. More...

This class is obsolete.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

Public Functions

• 32 public functions inherited from QObject

Public Slots

• 1 public slot inherited from QObject

Signals

• 2 signals inherited from QObject

Additional Inherited Members

• 1 property inherited from QObject
• 11 static public members inherited from QObject
• 9 protected functions inherited from QObject

Detailed Description

The QSignalMapper class bundles signals from identifiable senders.

This class collects a set of parameterless signals, and re-emits them with integer, string or widget parameters corresponding to the object that sent the signal.

The class supports the mapping of particular strings or integers with particular objects using setMapping(). The objects' signals can then be connected to the map() slot which will emit the mapped() signal with the string or integer associated with the original signalling object. Mappings can be removed later using removeMappings().

Example: Suppose we want to create a custom widget that contains a group of buttons (like a tool palette). One approach is to connect each button's clicked() signal to its own custom slot; but in this example we want to connect all the buttons to a single slot and parameterize the slot by the button that was clicked.

Here's the definition of a simple custom widget that has a single signal, clicked(), which is emitted with the text of the button that was clicked:

The only function that we need to implement is the constructor:

A list of texts is passed to the constructor. A signal mapper is constructed and for each text in the list a QPushButton is created. We connect each button's clicked() signal to the signal mapper's map() slot, and create a mapping in the signal mapper from each button to the button's text. Finally we connect the signal mapper's mapped() signal to the custom widget's clicked() signal. When the user clicks a button, the custom widget will emit a single clicked() signal whose argument is the text of the button the user clicked.

This class was mostly useful before lambda functions could be used as slots. The example above can be rewritten simpler without QSignalMapper by connecting to a lambda function.

See also QObject, QButtonGroup, and QActionGroup.

Member Function Documentation

QSignalMapper::QSignalMapper(QObject *parent = nullptr)

This function is deprecated.

Constructs a QSignalMapper with parent parent.

QSignalMapper::~QSignalMapper()

Destroys the QSignalMapper.

[slot] void QSignalMapper::map()

This slot emits signals based on which object sends signals to it.

[slot] void QSignalMapper::map(QObject *sender)

This slot emits signals based on the sender object.

[signal] void QSignalMapper::mapped(inti)

This signal is emitted when map() is signalled from an object that has an integer mapping set. The object's mapped integer is passed in i.

Note: Signal mapped is overloaded in this class. To connect to this signal by using the function pointer syntax, Qt provides a convenient helper for obtaining the function pointer as shown in this example:

See also setMapping().

[signal] void QSignalMapper::mapped(const QString &text)

This signal is emitted when map() is signalled from an object that has a string mapping set. The object's mapped string is passed in text.

Note: Signal mapped is overloaded in this class. To connect to this signal by using the function pointer syntax, Qt provides a convenient helper for obtaining the function pointer as shown in this example:

See also setMapping().

[signal] void QSignalMapper::mapped(QWidget *widget)

This signal is emitted when map() is signalled from an object that has a widget mapping set. The object's mapped widget is passed in widget.

Note: Signal mapped is overloaded in this class. To connect to this signal by using the function pointer syntax, Qt provides a convenient helper for obtaining the function pointer as shown in this example:

See also setMapping().

[signal] void QSignalMapper::mapped(QObject *object)

This signal is emitted when map() is signalled from an object that has an object mapping set. The object provided by the map is passed in object.

Note: Signal mapped is overloaded in this class. To connect to this signal by using the function pointer syntax, Qt provides a convenient helper for obtaining the function pointer as shown in this example:

See also setMapping().

QObject *QSignalMapper::mapping(intid) const

Returns the sender QObject that is associated with the id.

See also setMapping().

QObject *QSignalMapper::mapping(const QString &id) const

This function overloads mapping().

QObject *QSignalMapper::mapping(QWidget *widget) const

This function overloads mapping().

Returns the sender QObject that is associated with the widget.

QObject *QSignalMapper::mapping(QObject *object) const

This function overloads mapping().

Returns the sender QObject that is associated with the object.

void QSignalMapper::removeMappings(QObject *sender)

Removes all mappings for sender.

This is done automatically when mapped objects are destroyed.

Note: This does not disconnect any signals. If sender is not destroyed then this will need to be done explicitly if required.

void QSignalMapper::setMapping(QObject *sender, intid)

Adds a mapping so that when map() is signalled from the given sender, the signal mapped(id) is emitted.

There may be at most one integer ID for each sender.

See also mapping().

void QSignalMapper::setMapping(QObject *sender, const QString &text)

Adds a mapping so that when map() is signalled from the sender, the signal mapped(text ) is emitted.

There may be at most one text for each sender.

void QSignalMapper::setMapping(QObject *sender, QWidget *widget)

Adds a mapping so that when map() is signalled from the sender, the signal mapped(widget ) is emitted.

There may be at most one widget for each sender.

void QSignalMapper::setMapping(QObject *sender, QObject *object)

Adds a mapping so that when map() is signalled from the sender, the signal mapped(object ) is emitted.

There may be at most one object for each sender.

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This page was used to describe the new signal and slot syntax during its development. The feature is now released with Qt 5.

• Differences between String-Based and Functor-Based Connections (Official documentation)
• Introduction (Woboq blog)
• Implementation Details (Woboq blog)

Note: This is in addition to the old string-based syntax which remains valid.

• 1Connecting in Qt 5
• 2Disconnecting in Qt 5
• 4Error reporting
• 5Open questions

Connecting in Qt 5

There are several ways to connect a signal in Qt 5.

Old syntax

Qt 5 continues to support the old string-based syntax for connecting signals and slots defined in a QObject or any class that inherits from QObject (including QWidget)

New: connecting to QObject member

Here's Qt 5's new way to connect two QObjects and pass non-string objects:

Pros

• Compile time check of the existence of the signals and slot, of the types, or if the Q_OBJECT is missing.
• Argument can be by typedefs or with different namespace specifier, and it works.
• Possibility to automatically cast the types if there is implicit conversion (e.g. from QString to QVariant)
• It is possible to connect to any member function of QObject, not only slots.

Cons

• More complicated syntax? (you need to specify the type of your object)
• Very complicated syntax in cases of overloads? (see below)
• Default arguments in slot is not supported anymore.

New: connecting to simple function

The new syntax can even connect to functions, not just QObjects:

Pros

• Can be used with std::bind:

• Can be used with C++11 lambda expressions:

Cons

• There is no automatic disconnection when the 'receiver' is destroyed because it's a functor with no QObject. However, since 5.2 there is an overload which adds a 'context object'. When that object is destroyed, the connection is broken (the context is also used for the thread affinity: the lambda will be called in the thread of the event loop of the object used as context).

Disconnecting in Qt 5

As you might expect, there are some changes in how connections can be terminated in Qt 5, too.

Old way

You can disconnect in the old way (using SIGNAL, SLOT) but only if

• You connected using the old way, or
• If you want to disconnect all the slots from a given signal using wild card character

Symetric to the function pointer one

Only works if you connected with the symmetric call, with function pointers (Or you can also use 0 for wild card)In particular, does not work with static function, functors or lambda functions.

New way using QMetaObject::Connection

Works in all cases, including lambda functions or functors.

Asynchronous made easier

With C++11 it is possible to keep the code inline

Here's a QDialog without re-entering the eventloop, and keeping the code where it belongs:

Another example using QHttpServer : http://pastebin.com/pfbTMqUm

Error reporting

Tested with GCC.

Fortunately, IDEs like Qt Creator simplifies the function naming

Missing Q_OBJECT in class definition

Type mismatch

Open questions

Default arguments in slot

If you have code like this:

The old method allows you to connect that slot to a signal that does not have arguments.But I cannot know with template code if a function has default arguments or not.So this feature is disabled.

There was an implementation that falls back to the old method if there are more arguments in the slot than in the signal.This however is quite inconsistent, since the old method does not perform type-checking or type conversion. It was removed from the patch that has been merged.

Overload

As you might see in the example above, connecting to QAbstractSocket::error is not really beautiful since error has an overload, and taking the address of an overloaded function requires explicit casting, e.g. a connection that previously was made as follows:

connect(mySpinBox, SIGNAL(valueChanged(int)), mySlider, SLOT(setValue(int));

cannot be simply converted to:

...because QSpinBox has two signals named valueChanged() with different arguments. Instead, the new code needs to be:

Unfortunately, using an explicit cast here allows several types of errors to slip past the compiler. Adding a temporary variable assignment preserves these compile-time checks:

Some macro could help (with C++11 or typeof extensions). A template based solution was introduced in Qt 5.7: qOverload

Qt Slot Mapper Free

The best thing is probably to recommend not to overload signals or slots …

… but we have been adding overloads in past minor releases of Qt because taking the address of a function was not a use case we support. But now this would be impossible without breaking the source compatibility.

Disconnect

Qt Slot Mapper Software

Should QMetaObject::Connection have a disconnect() function?

The other problem is that there is no automatic disconnection for some object in the closure if we use the syntax that takes a closure.One could add a list of objects in the disconnection, or a new function like QMetaObject::Connection::require

Callbacks

Function such as QHostInfo::lookupHost or QTimer::singleShot or QFileDialog::open take a QObject receiver and char* slot.This does not work for the new method.If one wants to do callback C++ way, one should use std::functionBut we cannot use STL types in our ABI, so a QFunction should be done to copy std::function.In any case, this is irrelevant for QObject connections.

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