[Interest] QStringList, takeFirst and order of execution
Hi, I have been experiencing a strange behavior with QStringList and the order of execution of takeFirst. The following code: QString line = file.readLine(); // line contains 46 71\n QStringList lineData = line.split(); // lineData contains (46, 71\n) qDebug() lineData.takeFirst() lineData.takeFirst(); behaves differently on two computers. The first one is a MacOS X 10.8.5 with Xcode 4.6.3 installed and using the 5.2.0 alpha package with gcc The second one is a MacOS X 10.8.5 with Xcode 5.0.1 installed and a developer build (no special configuration) of the latest stable branches from git with clang The first computer outputs 71\n 46 and the second one 46 71\n The same happens if I build e.g. a QSize using: QSize mySize = QSize(lineData.takeFirst().toInt(), lineData.takeFirst().toInt()); on the other hand, if I call qDebug() lineData.takeFirst(); qDebug() lineData.takeFirst(); The order is correct on both of them. What could explain the difference ? ___ Interest mailing list Interest@qt-project.org http://lists.qt-project.org/mailman/listinfo/interest
Re: [Interest] QStringList, takeFirst and order of execution
On segunda-feira, 11 de novembro de 2013 13:37:20, Samuel Gaist wrote: Hi, I have been experiencing a strange behavior with QStringList and the order of execution of takeFirst. qDebug() lineData.takeFirst() lineData.takeFirst(); The problem is not takeFirst. behaves differently on two computers. Correct. You have undefined behaviour. Both calls to takeFirst() are in the same sequence-point. That means the compiler is allowed to execute in any order. It would be the same if you had written: f(lineData.takeFirst(), lineData.takeFirst()); qDebug() lineData.takeFirst(); qDebug() lineData.takeFirst(); The order is correct on both of them. What could explain the difference ? In the second case, there are two sequence points (they are separated by a ;). In C++11-speak, the first line of the second example is guaranteed to happens- before the second line. In the first example, there's no happens-before, therefore any one can happen before the other, or after. -- Thiago Macieira - thiago.macieira (AT) intel.com Software Architect - Intel Open Source Technology Center signature.asc Description: This is a digitally signed message part. ___ Interest mailing list Interest@qt-project.org http://lists.qt-project.org/mailman/listinfo/interest
Re: [Interest] QStringList, takeFirst and order of execution
Il 11/11/2013 13:37, Samuel Gaist ha scritto: Hi, I have been experiencing a strange behavior with QStringList and the order of execution of takeFirst. The following code: QString line = file.readLine(); // line contains 46 71\n QStringList lineData = line.split(); // lineData contains (46, 71\n) qDebug() lineData.takeFirst() lineData.takeFirst(); I've already explained this in the past but I can't find the relevant email. This last line is parsed as operator( operator( qDebug(), lineData.takeFirst() ) , lineData.takeFirst() ) (operator is left associative). Take the outer operator. C++ does not define the order of evaluating its parameters. That means that either lineData.takeFirst() can be called before evaluating the inner operator, or vice-versa. Depending on what the compiler chooses to do, you'll have a different result. HTH, -- Giuseppe D'Angelo | giuseppe.dang...@kdab.com | Software Engineer KDAB (UK) Ltd., a KDAB Group company Tel. UK +44-1738-450410, Sweden (HQ) +46-563-540090 KDAB - Qt Experts - Platform-independent software solutions smime.p7s Description: Firma crittografica S/MIME ___ Interest mailing list Interest@qt-project.org http://lists.qt-project.org/mailman/listinfo/interest
Re: [Interest] QStringList, takeFirst and order of execution
On 11 nov. 2013, at 13:42, Thiago Macieira thiago.macie...@intel.com wrote: On segunda-feira, 11 de novembro de 2013 13:37:20, Samuel Gaist wrote: Hi, I have been experiencing a strange behavior with QStringList and the order of execution of takeFirst. qDebug() lineData.takeFirst() lineData.takeFirst(); The problem is not takeFirst. behaves differently on two computers. Correct. You have undefined behaviour. Both calls to takeFirst() are in the same sequence-point. That means the compiler is allowed to execute in any order. It would be the same if you had written: f(lineData.takeFirst(), lineData.takeFirst()); qDebug() lineData.takeFirst(); qDebug() lineData.takeFirst(); The order is correct on both of them. What could explain the difference ? In the second case, there are two sequence points (they are separated by a ;). In C++11-speak, the first line of the second example is guaranteed to happens- before the second line. In the first example, there's no happens-before, therefore any one can happen before the other, or after. I see, thanks for the explanation ! ___ Interest mailing list Interest@qt-project.org http://lists.qt-project.org/mailman/listinfo/interest
Re: [Interest] QStringList, takeFirst and order of execution
On 11 nov. 2013, at 13:50, Giuseppe D'Angelo giuseppe.dang...@kdab.com wrote: Il 11/11/2013 13:37, Samuel Gaist ha scritto: Hi, I have been experiencing a strange behavior with QStringList and the order of execution of takeFirst. The following code: QString line = file.readLine(); // line contains 46 71\n QStringList lineData = line.split(); // lineData contains (46, 71\n) qDebug() lineData.takeFirst() lineData.takeFirst(); I've already explained this in the past but I can't find the relevant email. This last line is parsed as operator( operator( qDebug(), lineData.takeFirst() ) , lineData.takeFirst() ) (operator is left associative). Take the outer operator. C++ does not define the order of evaluating its parameters. That means that either lineData.takeFirst() can be called before evaluating the inner operator, or vice-versa. Depending on what the compiler chooses to do, you'll have a different result. Ok, with that and the explanation of Thiago, it's crystal clear. Thanks ! ___ Interest mailing list Interest@qt-project.org http://lists.qt-project.org/mailman/listinfo/interest
