From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (qmail 11029 invoked by alias); 19 Jun 2017 16:55:50 -0000 Mailing-List: contact gdb-patches-help@sourceware.org; run by ezmlm Precedence: bulk List-Id: List-Subscribe: List-Archive: List-Post: List-Help: , Sender: gdb-patches-owner@sourceware.org Received: (qmail 10948 invoked by uid 89); 19 Jun 2017 16:55:49 -0000 Authentication-Results: sourceware.org; auth=none X-Virus-Found: No X-Spam-SWARE-Status: No, score=-1.9 required=5.0 tests=BAYES_00,SPF_HELO_PASS,T_RP_MATCHES_RCVD autolearn=ham version=3.3.2 spammy= X-HELO: mx1.redhat.com Received: from mx1.redhat.com (HELO mx1.redhat.com) (209.132.183.28) by sourceware.org (qpsmtpd/0.93/v0.84-503-g423c35a) with ESMTP; Mon, 19 Jun 2017 16:55:47 +0000 Received: from smtp.corp.redhat.com (int-mx06.intmail.prod.int.phx2.redhat.com [10.5.11.16]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id 9F2714E4CB; Mon, 19 Jun 2017 16:55:51 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mx1.redhat.com 9F2714E4CB Authentication-Results: ext-mx09.extmail.prod.ext.phx2.redhat.com; dmarc=none (p=none dis=none) header.from=redhat.com Authentication-Results: ext-mx09.extmail.prod.ext.phx2.redhat.com; spf=pass smtp.mailfrom=palves@redhat.com DKIM-Filter: OpenDKIM Filter v2.11.0 mx1.redhat.com 9F2714E4CB Received: from [127.0.0.1] (ovpn04.gateway.prod.ext.ams2.redhat.com [10.39.146.4]) by smtp.corp.redhat.com (Postfix) with ESMTP id F025D82746; Mon, 19 Jun 2017 16:55:48 +0000 (UTC) Subject: Re: [PATCH v6] C++ify gdb/common/environ.c To: Simon Marchi References: <20170413040455.23996-1-sergiodj@redhat.com> <20170619043531.32394-1-sergiodj@redhat.com> <87k248y3zp.fsf@redhat.com> <8aabc6fabb04f4e3e8b08e6fa1b0eacc@polymtl.ca> <816a5744-b3b4-855c-5f2e-4c9f0d255512@redhat.com> <1cff1a8055c0d770fef7171b8394e86d@polymtl.ca> <7cf7b53f-600a-32f5-c9d0-2f45a8bb2b46@redhat.com> <50ea9e5c05e31e1e459f22901ee86527@polymtl.ca> Cc: Sergio Durigan Junior , GDB Patches From: Pedro Alves Message-ID: <9be4fd14-279e-3dde-656d-3ea6b9aab148@redhat.com> Date: Mon, 19 Jun 2017 16:55:00 -0000 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.4.0 MIME-Version: 1.0 In-Reply-To: <50ea9e5c05e31e1e459f22901ee86527@polymtl.ca> Content-Type: text/plain; charset=windows-1252 Content-Transfer-Encoding: 7bit X-SW-Source: 2017-06/txt/msg00529.txt.bz2 On 06/19/2017 05:26 PM, Simon Marchi wrote: > On 2017-06-19 17:44, Pedro Alves wrote: >> If we take the "always push a NULL on construction" approach, and >> we want moved-from gdb_environs to be valid, then yes. Note how this >> results in extra heap allocations when e.g., returning a >> gdb_environ from functions by value, and makes std::vector >> much less efficient when it decides it needs to reallocate/move >> elements. Representing the empty state with a cleared internal >> vector would avoid this. > > Given the move case, since the goal is to be efficient, then yeah I > would agree > that it would make sense to make a little bit of efforts to avoid > allocating > memory for an objects we are almost certainly throwing away. > > But still, in order to leave environ objects in a valid state after a > move and > to pedantically comply with the STL spec which says that the vector is > left in > an unspecified state, shouldn't we do a .clear () on the moved-from > vector after > the move? See accepted answer at: https://stackoverflow.com/questions/17730689/is-a-moved-from-vector-always-empty So the only case where it'd be needed would be in op=, and iff the vectors had different allocators, which is not the case here. So no, it's not necessary. But I'd be fine with calling it. > >> Note BTW, that we need to be careful with self-move leaving the >> *this object in a valid state. > > Should we just do > > if (&other == this) > return *this; Might not be necessary if without that the object ends up valid anyway. But what you wrote is a safe bet. Thanks, Pedro Alves