You are right. I also just verified that: qSwap(str1, str2) and swap(str1, str2)
produce both the same code. while std::swap(str1, str2) is different and produce far more assembly code. As you say, this is a big gotcha, and using the qSwap is more obvious. Therefore you should consider removing qSwap from the Qt official obsolete list... Philippe On Sun, 25 Oct 2015 23:52:37 -0700 Thiago Macieira <thiago.macie...@intel.com> wrote: > On Monday 26 October 2015 07:44:55 Philippe wrote: > > In the Qt doc, it is said that qSwap is obsolete and should never be used > > in new code: > > http://doc.qt.io/qt-5/qtalgorithms-obsolete.html > > > > But in this recent video from the Qt World Summit 2015, Marc Mutz says the > > exact opposite (around time 20:30) > > https://www.youtube.com/watch?v=Y3YMA1Ip3ek > > > > A quick test with Qt 5.5.1 on qSwap with QString tends to show Marc is > > right (much less assembly code for this call). > > > > So, is it a mistake from the Qt documentation or?... > > Both are correct, since one is implemented in terms of the other. That means > they MUST expand to exactly the same assembly (and they do, I've just tested). > > If you're seeing something different, the problem is probably your code. And > that's why Marc is recommending qSwap: because people don't know how to use > std::swap. There's a big gotcha: you MUST NOT write the "std::" part of > std::swap. > > This is the correct way to swap two QStrings using std::swap: > > swap(s1, s2); > > Or, for a given type that you don't know about, you must write: > > using namespace std; > swap(t1, t2); > > This is independent of your coding policies. You MUST use the namespace in > order to call std::swap. So if your coding policies is not to use it, then > you > need to wrap it in { } to avoid the namespace usage leaking. > > -- > Thiago Macieira - thiago.macieira (AT) intel.com > Software Architect - Intel Open Source Technology Center > > _______________________________________________ > Interest mailing list > Interest@qt-project.org > http://lists.qt-project.org/mailman/listinfo/interest _______________________________________________ Interest mailing list Interest@qt-project.org http://lists.qt-project.org/mailman/listinfo/interest
