[Development] static constexpr in exported classes needs out-of-line definitions

[Marc Mutz]

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.mutz at qt.io>
Principal Software Engineer

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