NetNews Usenet Archive 1992 #16/NN_1992_16.iso/spool/comp/sys/sgi/11561

Text File | 1992-07-28 | 2.8 KB | 88 lines

Newsgroups: comp.sys.sgi
Path: sparky!uunet!stanford.edu!CSD-NewsHost.Stanford.EDU!news
From: [e-mail] (Philip Machanick)
Subject: Re: C++ member pointer error
Message-ID: <[e-mail]>
Sender: [e-mail]
Reply-To: [e-mail] (Philip Machanick)
Organization: CS Department, Stanford University, California, USA
References: <[e-mail]>
Date: Wed, 29 Jul 1992 03:06:51 GMT
Lines: 75

In article <[e-mail]> [e-mail]
(Tim McBrayer) writes:
> I'm trying to initialize a pointer to a class method and then call a function
> through said pointer.  I'm working in C++ on a SGI IRIS.  As far as I can
> tell, I'm accessing the pointer properly, according to Stroustrup's book.
> However, when I compile with "CC junk.cc", I get:
>
> "junk.cc", line 21: error:  object missing in call through pointer to
> member function
>
> The most frustrating part of this is that the code works under g++ on a
> Sparcstation.  Are there any known bugs with SGI C++ with IRIX 4.0.1? Does
> anyone have any idea what 'object' is being referred to in the error
> message?

I don't think this is a bug. I had the same results on a DECstation with cfront
ported directly from AT&T source. A call to a member function (unless it is
declared static) must be made through an object of the class _where the
member function is defined_. Although it appears you are calling the function
from an object of the appropriate class, this is not in fact so because under
other circumstances e.fn () may be called from a different place or even be
called after the original object is destroyed. (Do you think the semantics of
the line

   e.fn = (fnptr) &V::print;

should depend on the fact that this has happened inside a member function of
class V?)

If you want to ask a language lawyer for a more definitive answer post to
comp.lang.c++.

See modifications below for syntax (I found this in first edition of Lippman's
C++ primer p. 214) - I don't usually use this feature:

> #include <iostream.h>
>
> class V{
> public:
>   int i;
>   V() { i = 0;};
>   void run();
>   char * print(int x) {cout << "x is " << x << endl; return "Foo!";};
> };
>
> struct E {
>   typedef char * (V::*fnptr)(int);
>   fnptr fn;
>   E() { fn = 0;};
> };
> void V::run() {
>   E e;
>   e.fn = (fnptr) &V::print;
>   i = 42;
>   cout << (e.fn)(i) << endl;  //ERROR: (*(e.fn))(i) gives the same error
// this doesn't work because a non-static member function needs an object
// of its own class (the "this" pointer)

// replace the above by:
  cout << (this->*(e.fn))(i) << endl;
> };
>
> int main(){
>   V v;
>   v.run();
>   cout << v.print(5) << endl;
> }
output:
  x is 42
  Foo!
  x is 5
  Foo!
--
Philip Machanick
[e-mail]