public inbox for gcc-bugs@sourceware.org help / color / mirror / Atom feed
From: "redi at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org> To: gcc-bugs@gcc.gnu.org Subject: [Bug c++/53225] static operator new in multiple inheritance carries incorrect type information for the class Date: Fri, 04 May 2012 21:30:00 -0000 [thread overview] Message-ID: <bug-53225-4-hAF1flKd0R@http.gcc.gnu.org/bugzilla/> (raw) In-Reply-To: <bug-53225-4@http.gcc.gnu.org/bugzilla/> http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53225 --- Comment #20 from Jonathan Wakely <redi at gcc dot gnu.org> 2012-05-04 21:29:43 UTC --- (In reply to comment #18) > This code compiles: > > #include <cstddef> > #include <stdlib.h> > typedef unsigned int uint; > > class C{ // just here to be faithful to the original code > int y; > }; > > class A{ > public: > typedef A this_type; > > void* operator new(size_t enfacia_size, uint count){ > size_t total_size > = enfacia_size > + sizeof(int) * count; // the 'tail' > ; > this_type *new_pt = (this_type *)malloc(total_size); > new_pt->count = count; The bug is here. > return new_pt; > }; > uint count; > }; > > class B : public C, public A{ > public: > int i; > }; > > int main(){ > B *b_pt = new(5) B; > uint j=0; > j++; > }; > > And this is the debugger output: > > (gdb) r > Starting program try_offsets > Breakpoint 1, main () at try_offsets.cc:32 > (gdb) n > (gdb) p &(b_pt->count) > $1 = (uint *) 0x804a00c > (gdb) x/10 b_pt > 0x804a008: 5 0 0 0 > 0x804a018: 0 0 0 0 > 0x804a028: 0 135129 > (gdb) p b_pt > $2 = (B *) 0x804a008 > (gdb) > > Compare this to the prior code and debugger output of the same form, and you > will notice that operator new, from a point of view of type, behaves > differently than other method. Though it was inherited, its understanding of > this_type has not changed, as it does in the case for other methods. No it doesn't! That never happens for any type or any member function! #include <iostream> class C{ // just here to be faithful to the original code int y; }; class A{ public: void method(void* p){ std::cout << "this=" << this << " p=" << p << '\n'; } int count; }; class B : public C, public A{ }; int main(){ A a; a.method(&a); B b; b.method(&b); } this=0x7fffe08b1640 p=0x7fffe08b1640 this=0x7fffe08b1634 p=0x7fffe08b1630 Note that when calling b.method(&b) the "this" pointer is not the same as &b The member count is always at the same address within the A object, but the address of that A object might not be the same as the address of the B object. A B object looks like this in memory: ----- <------ C* <---- B* | C | |---| <------ A* | A | ----- When you call A::f() the 'this' pointer is adjusted to point to the A sub-object, which is not at the same address as the complete B object that contains it. > In the least, in a strongly typed system I would expect there to be a warning > about inheriting operator new when the type was not going to be updated as it > is for other opertors and methods, though I suspect it is a small matter for > the compiler to give it the right type info. The type is never "updated" when a function is inherited! The base class function is just visible and accessible in the derived class. It's not copied or update, the same, original function is used unaltered. Please, go and learn C++ somewhere else. I don't know what language your mental model corresponds to, but it's not C++.
next prev parent reply other threads:[~2012-05-04 21:30 UTC|newest] Thread overview: 37+ messages / expand[flat|nested] mbox.gz Atom feed top 2012-05-04 3:30 [Bug c++/53225] New: " dimitrisdad at gmail dot com 2012-05-04 3:35 ` [Bug c++/53225] " dimitrisdad at gmail dot com 2012-05-04 3:41 ` pinskia at gcc dot gnu.org 2012-05-04 5:21 ` dimitrisdad at gmail dot com 2012-05-04 8:55 ` redi at gcc dot gnu.org 2012-05-04 9:44 ` redi at gcc dot gnu.org 2012-05-04 18:12 ` dimitrisdad at gmail dot com 2012-05-04 18:32 ` redi at gcc dot gnu.org 2012-05-04 18:57 ` dimitrisdad at gmail dot com 2012-05-04 19:25 ` dimitrisdad at gmail dot com 2012-05-04 19:45 ` redi at gcc dot gnu.org 2012-05-04 19:59 ` dimitrisdad at gmail dot com 2012-05-04 20:02 ` dimitrisdad at gmail dot com 2012-05-04 20:10 ` daniel.kruegler at googlemail dot com 2012-05-04 20:18 ` dimitrisdad at gmail dot com 2012-05-04 20:35 ` redi at gcc dot gnu.org 2012-05-04 20:56 ` dimitrisdad at gmail dot com 2012-05-04 20:59 ` redi at gcc dot gnu.org 2012-05-04 21:07 ` redi at gcc dot gnu.org 2012-05-04 21:16 ` dimitrisdad at gmail dot com 2012-05-04 21:27 ` dimitrisdad at gmail dot com 2012-05-04 21:30 ` redi at gcc dot gnu.org [this message] 2012-05-04 21:41 ` dimitrisdad at gmail dot com 2012-05-04 21:52 ` redi at gcc dot gnu.org 2012-05-04 21:56 ` redi at gcc dot gnu.org 2012-05-04 22:01 ` dimitrisdad at gmail dot com 2012-05-04 22:05 ` daniel.kruegler at googlemail dot com 2012-05-04 22:07 ` pinskia at gcc dot gnu.org 2012-05-04 22:09 ` redi at gcc dot gnu.org 2012-05-04 22:31 ` dimitrisdad at gmail dot com 2012-05-04 22:55 ` dimitrisdad at gmail dot com 2012-05-04 23:21 ` redi at gcc dot gnu.org 2012-05-04 23:37 ` dimitrisdad at gmail dot com 2012-05-04 23:37 ` dimitrisdad at gmail dot com 2012-05-05 0:04 ` pinskia at gcc dot gnu.org 2012-05-05 0:15 ` redi at gcc dot gnu.org 2012-05-05 0:49 ` dimitrisdad at gmail dot com
Reply instructions: You may reply publicly to this message via plain-text email using any one of the following methods: * Save the following mbox file, import it into your mail client, and reply-to-all from there: mbox Avoid top-posting and favor interleaved quoting: https://en.wikipedia.org/wiki/Posting_style#Interleaved_style * Reply using the --to, --cc, and --in-reply-to switches of git-send-email(1): git send-email \ --in-reply-to=bug-53225-4-hAF1flKd0R@http.gcc.gnu.org/bugzilla/ \ --to=gcc-bugzilla@gcc.gnu.org \ --cc=gcc-bugs@gcc.gnu.org \ /path/to/YOUR_REPLY https://kernel.org/pub/software/scm/git/docs/git-send-email.html * If your mail client supports setting the In-Reply-To header via mailto: links, try the mailto: linkBe sure your reply has a Subject: header at the top and a blank line before the message body.
This is a public inbox, see mirroring instructions for how to clone and mirror all data and code used for this inbox; as well as URLs for read-only IMAP folder(s) and NNTP newsgroup(s).