This will not work in every case because you cannot guarantee that your
hash function generates a unique value for every string. In hash based
containers you still have to compare against the valueof the string.
Am 24.08.2015 01:27 schrieb "mark diener" <rpzrpz...@gmail.com>:
> Hello List:
>
> Have you ever wanted an efficient switch statement on QString values?
>
> There are many references to the subject with various algorithms, but they
> all have some major flaw in the implementation.
>
> For those looking for a way to have compile and run time hash on their
> strings, I spent a little while doing some research on the subject and
> wanted to share back to Qt community:
>
> The tests ran successfully with the C++11 example for OSX/IOS Xcode 6.3.2
> and Android NDK 10 r10e,
>
> I went ahead and tested the function on MSVC 2013 with CTP November and it
> failed. MSVC 2015 worked, but I could not build the QT 5.5.0 source code
> successfully, so I just built a console app in MSVC2015 and the C++11
> function worked.
>
> The C++14 example did not work on OSX or Android, but it did work on IOS.
> With 2 out of 3 failures, I did not feel motivated to test for MSVC 2015 on
> Windows 10.
>
> So for those wanting to verify and make use of it, here are the required
> code snippets:
>
> First, in your .PRO file:
>
> #CONFIG += c++14
>
> CONFIG += c++11
>
>
> Now for a source sample:
>
>
> //main.cpp ######################//VALID C++14 Constant Expression Function
> (Compiles for IOS on Qt5.5.0 and XCode 6.3.2, but does not Compile for OSX
> Desktop with Clang)/*constexpr unsigned int fnhash(const char *gpstr){
> unsigned int hash = 0; size_t gidx = 0 ; while (gpstr[gidx] > 0) {
> hash = 65599 * hash + gpstr[gidx]; if (gidx > 15) break ;
> gidx++ ; } return hash ^ (hash >> 16);}*///
> ######################//C++11 Constant Expression Function
> //Forward declarationconstexpr unsigned int fnhashrec(unsigned int hash,
> const char* str) ;
> constexpr unsigned int fnhash(const char* str){ return ( !str ? 0 :
> fnhashrec(5381, str) );}
> constexpr unsigned int fnhashrec(unsigned int hash, const char* str){
> return ( !*str ? hash : fnhashrec(((hash << 5) + hash) + *str, str + 1));}//
> ######################
> int main(int argc, char *argv[]){ QGuiApplication gapp(argc, argv);
> QQmlApplicationEngine geng;
> QString gval = "JOKER" ;
>
> //NOW look at the run time eval from QString -> char*
>
> switch (fnhash(gval.toUtf8().data())) { case fnhash("PETE"):
> //Compile time optimized qDebug() << "POOR PETE IS NOT A JOKER" <<
> endl ; break ; case fnhash("JOKER"): //Compile time optimized
> qDebug() << "BLOODY JOKER WORKED -> SUCCESS" << endl ; break ; }
> geng.load(QUrl(QStringLiteral("qrc:/main.qml"))); return gapp.exec();}
>
> Cheers,
>
> md
>
>
>
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