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From: gyro@kestrel.edu (Scott Layson Burson)
Newsgroups: comp.lang.c++
Subject: Re: C++ Limitation
Keywords: C++
Message-ID: <1991May24.171520.17478@kestrel.edu>
Date: 24 May 91 17:15:20 GMT
References: <1991May14.155950.4336@infonode.ingr.com>
Organization: Kestrel Institute, Palo Alto, CA
Lines: 27

In article <1991May14.155950.4336@infonode.ingr.com> allgood@greg.b17a.ingr.com writes:
>   I would like to have the capability of telling some object A, to send
>message M to an object B where M and B are not specifically known to A.
>For example, I would like to create a ScrollBar and tell it "Hey, when 
>you get scrolled to a new location, send a 'move_to' message to this
>instance of class View".  I can do this in C++ if the ScrollBar class knows
>about the View class, but I want to create a generic ScrollBar which
>can send messages to classes which will be defined later.  

A proposal that either has or will soon come before the committee
addresses this concern.  The idea is to generalize the notion of a
pointer to function so that it can carry a `this' pointer along with
it.  Then given an object `x', with member function `f', the value of
`x.f' (or `&x.f') is a functional object which, when invoked, performs
the `f' operation on that particular `x'.  (In some circles this is
called a "closure".)

The reason this is relevant is that the *type* of such a pointer to
function is determined only by the signature of the member function
from which it was created; it is *independent* of the class that
created it (if any).  Therefore it forms a mechanism for allowing
communication between objects that do not know each other's type; all
that is required is for one or more of these generalized pointers to
functions to be passed from one object to the other at runtime.

-- Scott
Gyro@Reasoning.COM