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From: steve@taumet.com (Stephen Clamage)
Newsgroups: comp.lang.c++
Subject: Re: cast of a bound pointer: why ?
Message-ID: <764@taumet.com>
Date: 13 Jun 91 16:53:09 GMT
Article-I.D.: taumet.764
References: <1991Jun11.191016.9873@beaver.cs.washington.edu> <763@taumet.com> <1991Jun12.180414.16718@beaver.cs.washington.edu>
Organization: Taumetric Corporation, San Diego
Lines: 43

pauld@stowe.cs.washington.edu (Paul Barton-Davis) writes:

>This is the heart of the matter. C++ assumes that EVERY function
>*requires* a "this" pointer. If one is using C++ for "systems"
>programming, there are a number of cases where this is not true. The
>particular one I'm facing involves the use of a much lower level call
>(via direct asm macros) to a function whose address is already known.
>The stack and frame pointers are set up *by the C++ code itself*
>before calling, and it knows exactly what to pass the function.

First of all, only non-static class member functions have a "this"
pointer.  You can declare a static member function, or a friend
function, or even an ordinary function which takes a parameter
of class pointer type and achieve the same effect.  For example,
instead of this:
	class C {
	    ...
	    foo(int);
	};
use this:
	class C {
	    ...
	    friend foo(C*, int);
	};
Now you can take foo's address and call it as any ordinary function,
which it is.  This is appropriate when interfacing to low-level
system calls, but not necessarily for high-level C++ programming.

What you describe is very system-specific, and far from being portable
to other systems, may not even survive the next release of the same
C++ compiler on the same system.

It is not appropriate, IMHO, to mix low-level assembler interface code
with C++.  If you have to do work in assembler, write that code in
assembler, but give it a standard function interface so it can be
called from C++.  Then your C++ code remains portable.  The non-
portable bits are well-isolated in assembler files.

You can also use C functions similarly.  For example, function foo
above coulde be declared extern "C" and do whatever you want.
-- 

Steve Clamage, TauMetric Corp, steve@taumet.com