From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (qmail 11701 invoked by alias); 24 Apr 2002 13:36:45 -0000 Mailing-List: contact gcc-help@gcc.gnu.org; run by ezmlm Precedence: bulk List-Archive: List-Post: List-Help: Sender: gcc-owner@gcc.gnu.org Received: (qmail 11604 invoked from network); 24 Apr 2002 13:36:41 -0000 Received: from unknown (HELO Cantor.suse.de) (213.95.15.193) by sources.redhat.com with SMTP; 24 Apr 2002 13:36:41 -0000 Received: from Hermes.suse.de (Charybdis.suse.de [213.95.15.201]) by Cantor.suse.de (Postfix) with ESMTP id 793481E8E7; Wed, 24 Apr 2002 15:36:40 +0200 (MEST) X-Authentication-Warning: sykes.suse.de: schwab set sender to schwab@suse.de using -f To: mike stump Cc: dann@godzilla.ICS.UCI.EDU, jason@redhat.com, gcc@gcc.gnu.org, libstdc++@gcc.gnu.org Subject: Re: C++ aliasing rules References: <200204240235.TAA23236@kankakee.wrs.com> X-Yow: Yow! We're going to a new disco! From: Andreas Schwab Date: Wed, 24 Apr 2002 06:43:00 -0000 In-Reply-To: <200204240235.TAA23236@kankakee.wrs.com> (mike stump's message of "Tue, 23 Apr 2002 19:35:47 -0700 (PDT)") Message-ID: User-Agent: Gnus/5.090005 (Oort Gnus v0.05) Emacs/21.2.50 (ia64-suse-linux) MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Transfer-Encoding: 8bit X-SW-Source: 2002-04/txt/msg01238.txt.bz2 mike stump writes: |> No, before you implement this, you need to be aware that some |> prefixing is probably ok: |> |> 6.3.2.3 Structure and union members FYI, this is how the section reads in the final standard. Especially [#5] is different and there is one more example. 6.5.2.3 Structure and union members Constraints [#1] The first operand of the . operator shall have a qualified or unqualified structure or union type, and the second operand shall name a member of that type. [#2] The first operand of the -> operator shall have type pointer to qualified or unqualified structure or pointer to qualified or unqualified union, and the second operand shall name a member of the type pointed to. Semantics [#3] A postfix expression followed by the . operator and an identifier designates a member of a structure or union object. The value is that of the named member, and is an lvalue if the first expression is an lvalue. If the first expression has qualified type, the result has the so-qualified version of the type of the designated member. [#4] A postfix expression followed by the -> operator and an identifier designates a member of a structure or union object. The value is that of the named member of the object to which the first expression points, and is an lvalue.79) If the first expression is a pointer to a qualified type, the result has the so-qualified version of the type of the designated member. [#5] One special guarantee is made in order to simplify the use of unions: if a union contains several structures that share a common initial sequence (see below), and if the union object currently contains one of these structures, it is permitted to inspect the common initial part of any of them anywhere that a declaration of the complete type of the union is visible. Two structures share a common initial sequence if corresponding members have compatible types (and, for bit-fields, the same widths) for a sequence of one or more initial members. [#6] EXAMPLE 1 If f is a function returning a structure or union, and x is a member of that structure or union, f().x is a valid postfix expression but is not an lvalue. [#7] EXAMPLE 2 In: struct s { int i; const int ci; }; struct s s; const struct s cs; volatile struct s vs; the various members have the types: s.i int s.ci const int cs.i const int cs.ci const int vs.i volatile int vs.ci volatile const int [#8] EXAMPLE 3 The following is a valid fragment: union { struct { int alltypes; } n; struct { int type; int intnode; } ni; struct { int type; double doublenode; } nf; } u; u.nf.type = 1; u.nf.doublenode = 3.14; /* ... */ if (u.n.alltypes == 1) if (sin(u.nf.doublenode) == 0.0) /* ... */ The following is not a valid fragment (because the union type is not visible within function f): struct t1 { int m; }; struct t2 { int m; }; int f(struct t1 * p1, struct t2 * p2) { if (p1->m < 0) p2->m = -p2->m; return p1->m; } int g() { union { struct t1 s1; struct t2 s2; } u; /* ... */ return f(&u.s1, &u.s2); } Forward references: address and indirection operators (6.5.3.2), structure and union specifiers (6.7.2.1). -------------------- 79) If &E is a valid pointer expression (where & is the address-of operator, which generates a pointer to its operand), the expression (&E)->MOS is the same as E.MOS. -- Andreas Schwab, SuSE Labs, schwab@suse.de SuSE GmbH, Deutschherrnstr. 15-19, D-90429 Nürnberg Key fingerprint = 58CA 54C7 6D53 942B 1756 01D3 44D5 214B 8276 4ED5 "And now for something completely different."