Thanks! Added to https://wiki.qt.io/Things_To_Look_Out_For_In_Reviews#Variables
On 18.01.24 04:35, Thiago Macieira wrote: > See http://bugreports.qt.io/browse/QTBUG-121135 for the issue and > https://codereview.qt-project.org/c/qt/qtbase/+/531251 for the solution. > > TL;DR: if you define a static constexpr (which is implicitly inline) member > variable in a class with export macro, you MUST add the definition of tha > variable to the .cpp. > > Alternatives: > * move to a class that is not exported > trick: add an empty base with the variable > * change to enum (if primitive) > > But this happens even for plain int variables. For example: > > #include <vector> > #include <QtCore/qlocale.h> > void f(std::vector<int> &v) > { > v.push_back(QLocale::FirstTwoDigitYear); > } > > This produced: with GCC 13 at -O2 optimisations: > > _Z1fRSt6vectorIiSaIiEE: > movq 8(%rcx), %rdx > cmpq 16(%rcx), %rdx > je .L29 > movl $1900, (%rdx) > addq $4, %rdx > movq %rdx, 8(%rcx) > ret > .L29: > movq __imp__ZN7QLocale17FirstTwoDigitYearE(%rip), %r8 > jmp > _ZNSt6vectorIiSaIiEE17_M_realloc_insertIJRKiEEEvN9__gnu_cxx17__normal_iteratorIPiS1_EEDpOT_ > > Interpretation: if the vector has enough pre-allocated space, it simply moves > 1900 to the end of te array and adjusts the end. But if not, it calls > std::vector::::_M_realloc_insert(), which takes the integer by reference, so > the compiler emits an import from the DLL. > > This appears to be a GCC bug/shortcoming. Clang and MSVC apparently > automatically emit and export the variable for you: > https://mingw.godbolt.org/z/q4dYdosjh > > -- Marc Mutz <marc.m...@qt.io> Principal Software Engineer The Qt Company Erich-Thilo-Str. 10 12489 Berlin, Germany www.qt.io Geschäftsführer: Mika Pälsi, Juha Varelius, Jouni Lintunen Sitz der Gesellschaft: Berlin, Registergericht: Amtsgericht Charlottenburg, HRB 144331 B -- Development mailing list Development@qt-project.org https://lists.qt-project.org/listinfo/development
