Signals and slots is a language construct introduced in Qt, which makes it easy to implement the Observer pattern while avoiding boilerplate code. The concept is that controls (also known as widgets) can send signals containing event information (e.g. the text "toto" was selected, the scrollbar has been adjusted to value 37) which can be received by other controls using special functions known as slots. The signal/slot system fits well with the way Graphical User Interfaces are designed. Similarly, the signal/slot system can be used for asynchronous I/O (including sockets, pipes, serial devices, etc.) event notification or to associate timeout events with appropriate object instances and methods or functions. No registration/deregistration/invocation code need be written, because Qt's Meta Object Compiler (MOC) automatically generates the needed infrastructure.
Alternative implementations 
There are some implementations of signal/slot systems based on C++ templates, which don't require the extra Meta Object Compiler, as used by Qt, such as libsigc++, sigslot, nano-signal-slot, neosigslot, Signals, boost.signals, Cpp::Events, Platinum and JBroadcaster. CLI languages such as C# also supports a similar construct although with a different terminology and syntax: events play the role of signals, and delegates are the slots. Another implementation of signals exists for ActionScript 3.0, inspired by C# events and signals/slots in Qt. Additionally, a delegate can be a local variable, much like a function pointer, while a slot in Qt must be a class member declared as such. Due to the language limitation, there isn't a common implementation for ANSI C, but an environment dependent method was given out, see c-sigslot, it has been tested on several platforms, notably the ARM C Compiler, but should support any standard ANSI C compiler. The C based GObject system also provides similar functionality via GSignal.
See also 
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