Path: utzoo!attcan!uunet!jarthur!usc!cs.utexas.edu!milano!cadillac!dsouza
From: dsouza@optima.cad.mcc.com (Desmond Dsouza)
Newsgroups: comp.lang.c++
Subject: Re: virtual static class functions
Message-ID: <DSOUZA.90Mar15110351@optima.cad.mcc.com>
Date: 15 Mar 90 17:03:50 GMT
References: <38579@cornell.UUCP> <2380@dataio.Data-IO.COM>
Sender: news@cadillac.CAD.MCC.COM
Organization: MCC CAD Program, Austin, Texas
Lines: 46
In-reply-to: bright@Data-IO.COM's message of 14 Mar 90 21:06:37 GMT



   In article <38579@cornell.UUCP> deb@svax.cs.cornell.edu (David Baraff) writes:
   <Why can't static class functions be virtual? That is,
   <is there some reason why allowing static virtual class functions
   <would be bad? I can't see that the implementation would pose
   <any problems.
I agree with this observation. To elaborate:

  1. Static member functions can be invoked with OR without an object
     of that class. i.e. both of these are legal:
          (a)  Class::static_func(); // explicit "Class::" or by context
	  (b)  obj.static_func(); // or through a pointer p->static_func();
     Clearly, static_func() cannot access any non_static data or function
     members of Class because of case (a).

  2. The above restriction does NOT rule out permitting static_func()
     to be VIRTUAL.

     If the invocation is as in (a) above, the call is
     resolved statically, like any other member function call --
     virtual or non-virtual.

     If the invocation is as in (b) above, the call gets resolved
     dynamically through the virtual function table. Only VIRTUAL
     static member functions need be entered in this table.

     Whether or not am *implmentation* chooses to pass an implicit "this" pointer
     as a first argument to VIRTUAL static member functions is unimportant
     and may be decided by implementation convenience : the function
     cannot use any non-static members anyway so the value of "this"
     is irrelevant.

In summary, VIRTUAL static member functions are entered in the virtual
function table, may only access static data and functions, and may or
may not be actually passed a "this" pointer. Non-virtual static member
functions would remain unchanged.

It is easy to come up with examples where this is useful. 

Desmond.
--

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