Qt is well known for its signals and slots mechanism. But how does it work? In this blog post, we will explore the internals of QObject and QMetaObject and discover how signals and slot work under the hood. In this blog article, I show portions of Qt5 code, sometimes edited for formatting and brevity. A slot refers to the method containing the code that you want to be executed on the occurrence of a signal. Most widgets have predefined slots; you don't have to write code to connect a predefined signal to a predefined slot. You can even edit a signal/slot by navigating to the Edit Edit Signals/Slots tool in the toolbar. First of all. It is good to have a IDE like Eclipse. I use Oxygen 4.7.0 on a windows 10 host. Usually it works very well. But the installation of the CDT new qt form wizard does not work. When I try to install it then there are some messages: The following solutions are not available: CDT New Qt Form Wizard1.0.0. Proceed with the installation.

qt-signal-tools is a collection of utility classes related to signal and slots in Qt. It includes:

QtCallback - Package up a receiver and slot arguments into an object for invoking later.
QtSignalForwarder - Connect signals and events from objects to QtCallback or arbitrary functions.
QtMetacallAdapter - Low-level interface for calling a function using a list of QGenericArgument() arguments.
safe_bind() - Create a wrapper around a method call which does nothing and returns a default value ifthe object is destroyed before the wrapper is called.

Requirements

Qt 4.6 (earliest version tested, may also work with older Qt 4.x releases) or Qt 5.x
The TR1 standard library (for C++03 compilers) or the C++11 standard library(for newer compilers when C++11 support is enabled).

Classes

QtCallback

QtCallback is a binder class which provides a way to create callbacks that invoke a signal or slotwhen invoked, using a mixture of pre-bound arguments and arguments passed to QtCallback::invoke().

Usage:

QtSignalForwarder

QtSignalForwarder provides a way to invoke callbacks when an object emits a signal or receivesa particular type of event. The callbacks can be signals and slots(via QtCallback) or arbitrary functions using tr1::function, std::function, boost::function ora similar wrapper.

Qt 5 provides support for connecting signals to arbitrary functions out of the box and to lambdaswhen using C++11. QtSignalForwarder emulates this for Qt 4.

As well as being able to connect signals to functions that are not slots, this also providesa way to pass additional arguments to the receiver other than those from the signal using QtCallback::bind()or std::tr1::bind().

Usage:

Connecting a signal to a slot with pre-bound arguments:

Connecting a signal to an arbitrary function:

Automatic disconnection

For standard signal-slot connections, Qt automatically removes the connection if either the senderor receiver objects are destroyed.

When using QtSignalForwarder::connect(), the connection is automatically removed if the senderis destroyed. However there is no receiver since the callback is a function object - which maycall a method on a QObject or it may call a function which is not a QObject method at all.

QtSignalTools provides two solutions to this:

QtSignalForwarder::connect() accepts an optional context QObject*. The signal will automatically bedisconnected if either the sender or the context object is destroyed. This is the approach that should beused if the callback is a method on a QObject. This behaves the same as the context argumentto QObject::connect() in Qt 5.2 and later.
A more generic facility is to use the safe_bind() function which creates a wrapper around an objectand a method call. The wrapper can then be called with the same arguments as the wrapped method.When a call happens, either the wrapped method is called with the provided arguments, or if the object hasbeen destroyed, nothing happens and a default value is returned. The wrapper created by safe_bind()can be used with bind() and function and can be used together with QtSignalForwarder toautomatically 'disconnect' if the receiver is destroyed.

QtMetacallAdapter

QtMetacallAdapter is a low-level wrapper around a function or function object (eg. std::function)which can be used to invoke the function with a list of QGenericArgument (created by the Q_ARG() macro)and introspect the function's argument types at runtime.

License

qt-signal-tools is licensed under the BSD license.

Related Projects & Reading

Qt Signal Adapters - Library for connecting signals to Boost function objects: http://sourceforge.net/projects/qtsignaladapter/
sigfwd - Library for connecting signals to function objects. Uses Boost. https://bitbucket.org/edd/sigfwd/wiki/Home
Qt 5 meta-object system changes - http://blog.qt.digia.com/blog/2012/06/22/changes-to-the-meta-object-system-in-qt-5/

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.

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.

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.

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 pu

Eclipse Qt Signal Slot Editor