G::Slot Namespace Reference

A typesafe callback library that isolates event sinks from event sources. More...

Classes
class	SlotImpBase
	Used as a base class to all slot implementation classes (such as G::Slot::SlotImp1), allowing them to be used as bodies to the associated slot reference-counting handle class (eg. More...
class	SignalImp
	A static helper class used by G::Slot signal classes. More...
class	SlotImp0
	A slot implementation class for zero-parameter callbacks. More...
class	SlotCallback0
	Provides a function to down-cast from SlotImpBase to SlotImp0. More...
class	Slot0
	A slot class for zero-parameter callbacks. More...
class	Signal0
	A signal class for zero-parameter callbacks. More...
class	SlotImp1
	A slot implementation class for one-parameter callbacks. More...
class	SlotCallback1
	Provides a function to down-cast from SlotImpBase to SlotImp1. More...
class	Slot1
	A slot class for one-parameter callbacks. More...
class	Signal1
	A signal class for one-parameter callbacks. More...
class	SlotImp2
	A slot implementation class for two-parameter callbacks. More...
class	SlotCallback2
	Provides a function to down-cast from SlotImpBase to SlotImp2. More...
class	Slot2
	A slot class for two-parameter callbacks. More...
class	Signal2
	A signal class for two-parameter callbacks. More...
class	SlotImp3
	A slot implementation class for three-parameter callbacks. More...
class	SlotCallback3
	Provides a function to down-cast from SlotImpBase to SlotImp3. More...
class	Slot3
	A slot class for three-parameter callbacks. More...
class	Signal3
	A signal class for three-parameter callbacks. More...

Functions
template<typename T >
Slot0	slot (T &object, void(T::*fn)())
	A slot factory function overloaded for a zero-parameter callback.
template<typename T , typename P >
Slot1< P >	slot (T &object, void(T::*fn)(P))
	A slot factory function overloaded for a one-parameter callback.
template<typename T , typename P1 , typename P2 >
Slot2< P1, P2 >	slot (T &object, void(T::*fn)(P1, P2))
	A slot factory function overloaded for a two-parameter callback.
template<typename T , typename P1 , typename P2 , typename P3 >
Slot3< P1, P2, P3 >	slot (T &object, void(T::*fn)(P1, P2, P3))
	A slot factory function overloaded for a three-parameter callback.

Detailed Description

A typesafe callback library that isolates event sinks from event sources.

The slot/signal pattern is used in several C++ libraries including libsigc++, Qt and boost; it is completely unrelated to ANSI-C or POSIX signals (signal(), sigaction(2)).

This implementation was inspired by libsigc++, but simplified by:

• not doing multicast
• not detecting dangling references
• not supporting global function callbacks
• using only void returns

Event-generating classes expose a "signal" object which client objects can connect() to in order to receive events. The client receives events through a "slot" member function.

The key to the implementation is that the slot implementation class (eg. G::Slot::SlotImp1) is templated on the callback parameter P and the callback sink class T, whereas the slot public interface (eg. G::Slot::Slot1) is templated only on P. This means that the event source, using the public interface, does not need to know the type of the event sink. It is the slot callback classes (eg. G::Slot::Callback1) that resolve the mismatch, by doing a down-cast to the T-specific slot implementation class.

A slot is a reference-counting handle to a slot implementation body, with the G::Slot::SlotImpBase class doing the reference counting. Slots also hold a function pointer pointing to the relevant downcasting method provided by one of the callback classes.

The overloaded factory function G::Slot::slot() creates a slot handle, passing it a suitable SlotImpBase body and callback casting function. The dynamic type of the SlotImpBase pointer is a slot implementation class that knows about the specific sink class T, and the callback casting function knows how to access the derived class. The combination of SlotImpBase polymorphism and casting function isolates the slot class from the sink class.

The signal classes contain a slot that is initially uninitialised; the connect() method is used to initialise the slot so that it points to the sink class's event handling function.

Usage:

class Source
{
public:
  G::Slot::Signal1<int> m_signal ;
private:
  void Source::raiseEvent()
  {
    int n = 123 ;
    m_signal.emit( n ) ;
  }
} ;

class Sink
{
public:
  void onEvent( int n ) ;
  Sink( Source & source ) 
  { 
     source.m_signal.connect( G::Slot::slot(*this,&Sink::onEvent) ) ; 
  }
} ;