🚀 go-pugleaf

Anders Lindback (alindbac@sw.seisy.abb.se) wrote:
: In article <4gsi2p$905@bcarh8ab.bnr.ca>,
: brian (b.c.) white <bcwhite@bnr.ca> wrote:
: >
: >>What is needed is the ability to use member functions as callbacks
: >>without any constraint on the type of the object they are invoked on.
: >>This is possible in C++ only by using various implementation-dependent
: >>hacks to escape the type system, because the language does not provide
: >>any way to express such a construct.
: >
:
: There exist a rather simple solution using a static member function. 
: Instead of having the callback call a normal member function it can 
: call a static callback member function with the object as a 
: argument. Then this callback function can call the real callback function.
: This will involve an extra callback function for each callback one has,
: but on the other hand it involves no changes to the base class if
: one want to override the real callback function in a derived class.

: class X
: {
: public:
:   static void callback(void* to)
:   virtual void real_callback();
: };

: void X::callback(void* to)
: { 
:   X* me = (X*)to;
:   me ->real_callback()
: }

: Motif widget callbacks and event handlers are examples where this pattern
: works without problems. 

: If the desired class Y can not be modified it is possible that X is a helper
: class that knows which member function on the real class Y it shall call.
: I.e. this solution is portable and can be used for all possible classes.

Granted, this works but it is still not typesafe since you are casting
the object pointer to a void.  It also forces you to clutter your class
definitions with extra functions in order to allow for the possibility
of callbacks (you can also define global functions which do the same
thing but then you clutter the global namespace).

The point we have been making is that there is a trivial way to extend
the C++ language to provide for completely typesafe callbacks which
are simple to use and which will dramatically improve the clarity
of programs.

Ben Jones
Hughes Information Technology
bjones@eos.hitc.com
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