🚀 go-pugleaf

In article <4gsi2p$905@bcarh8ab.bnr.ca>,
brian (b.c.) white <bcwhite@bnr.ca> wrote:
>In article <pgpmoose.199602221531.14635@isolde.mti.sgi.com>,
>ian (i.) willmott <willmott@bnr.ca> wrote:
>
>>In an article entitled "Q: Generic Callbacks -- Object->*func(...)"
>>(reference <4fti32$p3p@bcarh8ab.bnr.ca>), bcwhite@bnr.ca asks
>
>>"Does the C++ standard allow for a generic callback to be specified?
>>Basically, I'd like to be able to pass an arbitrary object and
>>function of that object to be called at some later time."
>
>>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.
>
>Of course, what we're really trying to with all this is to pass _code_
>(along with its context) as an object.

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.

Anders



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