From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <gcc-help-return-21007-listarch-gcc-help=gcc.gnu.org@gcc.gnu.org>
Received: (qmail 11844 invoked by alias); 28 Jul 2005 09:21:09 -0000
Mailing-List: contact gcc-help-help@gcc.gnu.org; run by ezmlm
Precedence: bulk
List-Archive: <http://gcc.gnu.org/ml/gcc-help/>
List-Post: <mailto:gcc-help@gcc.gnu.org>
List-Help: <mailto:gcc-help-help@gcc.gnu.org>
Sender: gcc-help-owner@gcc.gnu.org
Received: (qmail 11590 invoked by uid 22791); 28 Jul 2005 09:20:51 -0000
Received: from smtp06.wanadoo.nl (HELO smtp06.wanadoo.nl) (194.134.35.146)
    by sourceware.org (qpsmtpd/0.30-dev) with ESMTP; Thu, 28 Jul 2005 09:20:51 +0000
Received: from [10.1.1.5] (dc5146ff27.adsl.wanadoo.nl [81.70.255.39])
	by smtp6.wanadoo.nl (Postfix) with ESMTP id 949811D59C
	for <gcc-help@gcc.gnu.org>; Thu, 28 Jul 2005 11:20:49 +0200 (CEST)
Message-ID: <42E8A370.4040602@wanadoo.nl>
Date: Thu, 28 Jul 2005 09:21:00 -0000
From: Daan Oosterveld <d.oosterveld@wanadoo.nl>
User-Agent: Mozilla Thunderbird 1.0 (Windows/20041206)
MIME-Version: 1.0
To: gcc-help@gcc.gnu.org
Subject: Stack allocation skips vtable, optimalisation option?
Content-Type: multipart/mixed;
 boundary="------------000904000702050406000005"
X-SW-Source: 2005-07/txt/msg00311.txt.bz2

This is a multi-part message in MIME format.
--------------000904000702050406000005
Content-Type: text/plain; charset=ISO-8859-1; format=flowed
Content-Transfer-Encoding: 7bit
Content-length: 1292


Hi all,

I have a problem finding the right switch to eliminate the compiler to 
jump over the vtable and calling the wrong virtual  function. When 
allocating a class on the stack and copying a subclass over it virtual 
functions don't behave as expected. As a pointer the code calls the 
virtual function of the subclass, but when called directly it always 
calls the function in the base class.

The compiler assumes the pointer to the vtable is to Foo, and then 
assumes to call Foo::foo() in stead of looking in the object and see the 
vtable is pointing at Bar and calling Bar::foo.

The output of this is:

$ ./a.out
Foo!
Bar!

It should be in my opinion:

$ ./a.out
Bar!
Bar!

The source is listed below. It is compiled without optimalisations (-O0)

Thanks

Daan Oosterveld


---- source:
#include <stdio.h>

class Foo
{
protected:
    char aap[12];
public:
    Foo() {}
    Foo(const Foo & foo) {
        char * d = (char*)this;
        char * s = (char*)&foo;
        
        for(int i = 0; i < sizeof(Foo); i ++) {
            d[i] = s[i];
        }
    }

    virtual void foo() { printf("Foo!\n"); }
};

class Bar : public Foo
{
public:
    virtual void foo() { printf("Bar!\n"); }
};

int main()
{
    Foo foo = Bar();
    Foo * bar = &foo;
    foo.foo();
    bar->foo();
}


--------------000904000702050406000005
Content-Type: text/x-vcard; charset=utf-8;
 name="d.oosterveld.vcf"
Content-Transfer-Encoding: 7bit
Content-Disposition: attachment;
 filename="d.oosterveld.vcf"
Content-length: 153

begin:vcard
fn:Daan Oosterveld
n:Oosterveld;Daan
email;internet:d.oosterveld@wanadoo.nl
tel;home:053-4323699
tel;cell:06-24732100
version:2.1
end:vcard


--------------000904000702050406000005--