{"id":11738,"url":"https://patchwork.libcamera.org/api/patches/11738/?format=json","web_url":"https://patchwork.libcamera.org/patch/11738/","project":{"id":1,"url":"https://patchwork.libcamera.org/api/projects/1/?format=json","name":"libcamera","link_name":"libcamera","list_id":"libcamera_core","list_email":"libcamera-devel@lists.libcamera.org","web_url":"","scm_url":"","webscm_url":""},"msgid":"<20210328025741.30246-1-laurent.pinchart@ideasonboard.com>","date":"2021-03-28T02:57:41","name":"[libcamera-devel,v2] libcamera: pipeline: uvcvideo: Avoid reference to temporary object","commit_ref":"299e8ef563a83ce6e9624bb21e11ecb2ad5a228f","pull_url":null,"state":"accepted","archived":false,"hash":"3cbd7e7b9633cf45149783117b752ed962be04de","submitter":{"id":2,"url":"https://patchwork.libcamera.org/api/people/2/?format=json","name":"Laurent Pinchart","email":"laurent.pinchart@ideasonboard.com"},"delegate":null,"mbox":"https://patchwork.libcamera.org/patch/11738/mbox/","series":[{"id":1847,"url":"https://patchwork.libcamera.org/api/series/1847/?format=json","web_url":"https://patchwork.libcamera.org/project/libcamera/list/?series=1847","date":"2021-03-28T02:57:41","name":"[libcamera-devel,v2] libcamera: pipeline: uvcvideo: Avoid reference to temporary object","version":2,"mbox":"https://patchwork.libcamera.org/series/1847/mbox/"}],"comments":"https://patchwork.libcamera.org/api/patches/11738/comments/","check":"pending","checks":"https://patchwork.libcamera.org/api/patches/11738/checks/","tags":{},"headers":{"Return-Path":"","X-Original-To":"parsemail@patchwork.libcamera.org","Delivered-To":"parsemail@patchwork.libcamera.org","Received":["from lancelot.ideasonboard.com (lancelot.ideasonboard.com\n\t[92.243.16.209])\n\tby patchwork.libcamera.org (Postfix) with ESMTPS id CCA6DC32ED\n\tfor ;\n\tSun, 28 Mar 2021 02:58:32 +0000 (UTC)","from lancelot.ideasonboard.com (localhost [IPv6:::1])\n\tby lancelot.ideasonboard.com (Postfix) with ESMTP id C53DE68781;\n\tSun, 28 Mar 2021 04:58:31 +0200 (CEST)","from perceval.ideasonboard.com (perceval.ideasonboard.com\n\t[213.167.242.64])\n\tby lancelot.ideasonboard.com (Postfix) with ESMTPS id B5A886877C\n\tfor ;\n\tSun, 28 Mar 2021 04:58:29 +0200 (CEST)","from pendragon.lan (62-78-145-57.bb.dnainternet.fi [62.78.145.57])\n\tby perceval.ideasonboard.com (Postfix) with ESMTPSA id 11F12332;\n\tSun, 28 Mar 2021 04:58:28 +0200 (CEST)"],"Authentication-Results":"lancelot.ideasonboard.com;\n\tdkim=fail reason=\"signature verification failed\" (1024-bit key;\n\tunprotected) header.d=ideasonboard.com header.i=@ideasonboard.com\n\theader.b=\"MniKswyT\"; dkim-atps=neutral","DKIM-Signature":"v=1; a=rsa-sha256; c=relaxed/simple; d=ideasonboard.com;\n\ts=mail; t=1616900309;\n\tbh=BwCg4h9N9fJysbMT1Yjbazbp7zTImD9mkWJSjCKzCmg=;\n\th=From:To:Cc:Subject:Date:From;\n\tb=MniKswyT+aUtPgeMFeLqW6cRMFsPRN4W1XtVDA6pqI0QTSSx5koEuzSfEybjs6Qbp\n\tZdQar1r5nBIRCg9uq85YoLKoFeT7WN4PrwnRYPwaYGAFHfV44rDrq4RBe3VOIb+85W\n\ttKn4HEzjlFelrImVbeymBsgpVhITveSVH1MdYRsE=","From":"Laurent Pinchart ","To":"libcamera-devel@lists.libcamera.org","Date":"Sun, 28 Mar 2021 05:57:41 +0300","Message-Id":"<20210328025741.30246-1-laurent.pinchart@ideasonboard.com>","X-Mailer":"git-send-email 2.28.1","MIME-Version":"1.0","Subject":"[libcamera-devel] [PATCH v2] libcamera: pipeline: uvcvideo: Avoid\n\treference to temporary object","X-BeenThere":"libcamera-devel@lists.libcamera.org","X-Mailman-Version":"2.1.29","Precedence":"list","List-Id":"","List-Unsubscribe":",\n\t","List-Archive":"","List-Post":"","List-Help":"","List-Subscribe":",\n\t","Content-Type":"text/plain; charset=\"us-ascii\"","Content-Transfer-Encoding":"7bit","Errors-To":"libcamera-devel-bounces@lists.libcamera.org","Sender":"\"libcamera-devel\" "},"content":"From: Khem Raj \n\nA range-based for loop whose range expression is an array of char\npointers and range variable declaration is a const reference to a\nstd::string creates a temporary string from the char pointer and binds\nthe range variable reference to it. This creates a const reference to a\ntemporary, which is valid in C++, and extends the lifetime of the\ntemporary to the lifetime of the reference.\n\nHowever, lifetime extension in range-based for loops is considered as a\nsign of a potential issue, as a temporary is created for every\niteration, which can be costly, and the usage of a reference in the\nrange declaration doesn't make it obvious that the code isn't simply\nbinding a reference to an existing object. gcc 11, with the\n-Wrange-loop-construct option, flags this:\n\nuvcvideo.cpp:432:33: error: loop variable 'name' of type 'const string&' {aka 'const std::__cxx11::basic_string&'} binds to a temporary constructed from type 'const char* const' [-Werror=range-loop-construct]\n| 432 | for (const std::string &name : { \"idVendor\", \"idProduct\" }) {\n| | ^~~~\n\nTo please the compiler, make the range variable a const char *. This may\nbring a tiny performance improvement, as the name is only used once, in\na location where the compiler can use\n\n\toperator+(const std::string &, const char *)\n\ninstead of\n\n\toperator+(const std::string &, const std::string &)\n\nSigned-off-by: Khem Raj \nReviewed-by: Laurent Pinchart \n\nUse a const char * type instead of auto, and update the commit message\naccordingly.\n\nSigned-off-by: Laurent Pinchart \n---\n src/libcamera/pipeline/uvcvideo/uvcvideo.cpp | 2 +-\n 1 file changed, 1 insertion(+), 1 deletion(-)","diff":"diff --git a/src/libcamera/pipeline/uvcvideo/uvcvideo.cpp b/src/libcamera/pipeline/uvcvideo/uvcvideo.cpp\nindex 031f96e28449..b6c6ade5ebaf 100644\n--- a/src/libcamera/pipeline/uvcvideo/uvcvideo.cpp\n+++ b/src/libcamera/pipeline/uvcvideo/uvcvideo.cpp\n@@ -429,7 +429,7 @@ std::string PipelineHandlerUVC::generateId(const UVCCameraData *data)\n \n \t/* Creata a device ID from the USB devices vendor and product ID. */\n \tstd::string deviceId;\n-\tfor (const std::string &name : { \"idVendor\", \"idProduct\" }) {\n+\tfor (const char *name : { \"idVendor\", \"idProduct\" }) {\n \t\tstd::ifstream file(path + \"/../\" + name);\n \n \t\tif (!file.is_open())\n","prefixes":["libcamera-devel","v2"]}