Shared library on windows fails when using moc

[Mesrine]

Hi everyone,

I have a library that export a class that derive from QObject,
The class is called 'nonceFinder'. The class use Q_OBJECT macro on its declaration because uses QT signals.

The library compiles well for ubuntu, windows, macos for static library and shared library.

Then I have another library called 'qclient' that uses the previous library 'nonceFinder'.

The compilation of 'qclient' works for ubuntu, macos for shared and static library, and for windows static library, but fails for windows shared library.
The error that gives is :

qclient.cpp.obj : error LNK2019: unresolved external symbol "public: static struct QMetaObject const qiota::qpow::nonceFinder::staticMetaObject" (?staticMetaObject@nonceFinder@qpow@qiota@@2UQMetaObject@@B) referenced in function "public: static class QMetaObject::Connection __cdecl QObject::connect<void (__cdecl qiota::qpow::nonceFinder::*)(void),class `public: __cdecl `public: __cdecl `public: void __cdecl qiota::Client::send_block(class qiota::qblocks::Block const &)'::`2'::<lambda_2>::operator()(void)const '::`2'::<lambda_1>::operator()(void)const '::`5'::<lambda_2> >(class qiota::qpow::nonceFinder const *,void (__cdecl qiota::qpow::nonceFinder::*)(void),class QObject const *,class `public: __cdecl `public: __cdecl `public: void __cdecl qiota::Client::send_block(class qiota::qblocks::Block const &)'::`2'::<lambda_2>::operator()(void)const '::`2'::<lambda_1>::operator()(void)const '::`5'::<lambda_2>,enum Qt::ConnectionType)" (??$connect@P8nonceFinder@qpow@qiota@@EAAXXZV<lambda_2>@?4???R<lambda_1>@?1???R4?1??send_block@Client@3@QEAAXAEBVBlock@qblocks@3@@Z@QEBA@XZ@QEBA@XZ@@QObject@@SA?AVConnection@QMetaObject@@PEBVnonceFinder@qpow@qiota@@P8345@EAAXXZPEBV0@V<lambda_2>@?4???R<lambda_1>@?1???R6?1??send_block@Client@5@QEAAXAEBVBlock@qblocks@5@@Z@QEBA@XZ@QEBA@XZ@W4ConnectionType@Qt@@@Z)

I have check is a problem with

Methods like metaObject(), qt_metacall(), qt_metacast(), etc, are defined in the generated moc_ files. So the cause of the unresolved external symbol error is that the moc_ files are not compiled/linked to the project. If, in Visual Studio, deleting and reinserting Q_OBJECT does not work, check that the moc_ files are correctly generated and included in the project.

I have also try https://doc.qt.io/qt-6/sharedlibrary.html macros.
I use CMake, i do not understand when should I use :

• Q_DECL_EXPORT - Should be at compilation time of the library?

• Q_DECL_IMPORT - Should be at linking time of the client and the shared library?

Also if i am compiling a static libraries can i keep Q_DECL_IMPORT in my class declaration?

If you have managed to compile a shared library on Windows that uses MOC generated sources I will appreciate your help on this.
If you now how to set this on CMake better.

[Christian Ehrlicher]

I did not see (and not use by myself) qt_standard_project_setup() so you're correct with the automoc.

You use the same define for the export definition for both libraries - how should this work?

[Christian Ehrlicher]

Please show a minimal, compilable example opf your problem. You most likely do not export the classes you want to use from outside correct.

[Mesrine]

@Christian-Ehrlicher Thanks for the interest.
the client library that use the shared library:
https://github.com/EddyTheCo/Qclient-IOTA
the shared library:
https://github.com/EddyTheCo/Qpow-IOTA
You can open a PR with the solution to develop branch :).

[Christian Ehrlicher]

But there is no shared libary at all nor import/export macros.

[Mesrine]

@Christian-Ehrlicher
I tried all combinations of export import macros but did not work either.

Why my others shared library works on windows without explicit export macros ?
Why the compilation only fails when using MOC?

[Christian Ehrlicher]

@Mesrine said in Shared library on windows fails when using moc:

Why my others shared library works on windows without explicit export macros ?

None will work if you don't properly export the symbols - I don't see any shared library in your github repos

[Mesrine]

@Christian-Ehrlicher

To build shared library you just say to cmake

qt-cmake -G Ninja -DCMAKE_BUILD_TYPE="release"   -DCMAKE_INSTALL_PREFIX="../install" -DBUILD_SHARED_LIBS=ON ../

All my shared libraries work for windows it only fail when using MOC. The later is not a question is an affirmation.

[Christian Ehrlicher]

@Mesrine said in Shared library on windows fails when using moc:

-DBUILD_SHARED_LIBS=ON ..

I don't see how this should affect any code in e.g. https://github.com/EddyTheCo/Qpow-IOTA/blob/main/CMakeLists.txt apart from the naming of the library.

[Mesrine]

@Christian-Ehrlicher
Cmake will let know the compiler that I want a shared library.

[Christian Ehrlicher]

@Mesrine said in Shared library on windows fails when using moc:

Cmake will let know the compiler that I want a shared library.

ah, correct

But you should add the correct defines then as described in the Qt documentation.

[Mesrine]

@Christian-Ehrlicher
I have tried but did not work either, maybe is another problem not related with the export macros.

So if someone has been able to produce shared libraries on Windows while using MOC compiled sources it will be good to know how to do it in CMake.

[Christian Ehrlicher]

@Mesrine said in Shared library on windows fails when using moc:

I have tried but did not work either, maybe is another problem not related with the export macros.

Then finally show us what you tried...

[JoeCFD]

@Mesrine It does not matter what you have done. You have to add MYSHAREDLIB_EXPORT into your class definition
class MYSHAREDLIB_EXPORT MyClass

But it does not exist in your classes.

#include <QtCore/QtGlobal>

#if defined(MYSHAREDLIB_LIBRARY)  
# define MYSHAREDLIB_EXPORT Q_DECL_EXPORT
#else
#define MYSHAREDLIB_EXPORT Q_DECL_IMPORT
#endif

When you build your lib, add target_compile_definitions(mysharedlib PRIVATE MYSHAREDLIB_LIBRARY). This means your lib exports these classes
while MYSHAREDLIB_EXPORT=Q_DECL_EXPORT since MYSHAREDLIB_LIBRARY is defined.

But in your main application, do not add it. However, the headers of all classes in your main application which use the lib have to have MYSHAREDLIB_EXPORT as well. That means these classes import the lib while MYSHAREDLIB_EXPORT=Q_DECL_IMPORT
since MYSHAREDLIB_LIBRARY is not defined .

[Christian Ehrlicher]

@JoeCFD said in Shared library on windows fails when using moc:

#if defined(_WIN32) || defined(WIN32)

This is not needed and even counter-productive when you want to hide the symbols on linux.

[JoeCFD]

@Christian-Ehrlicher Got it, thanks. Q_DECL_EXPORT and Q_DECL_IMPORT have it already. But for non Qt code, this is needed.

[Christian Ehrlicher]

@JoeCFD said in Shared library on windows fails when using moc:

But for non Qt code, this is needed.

Even then I would use the correct attribution. Hiding the symbols by default on linux will help the linker and startup time and (iirc) also the optimizer in the linker since it can remove unused functions / inline stuff when it's not visible from outside.

[Mesrine]

@JoeCFD said in Shared library on windows fails when using moc:

MYSHAREDLIB_LIBRARY

Yes, I tried that and give the same error.

Then my question is when should i define MYSHAREDLIB_LIBRARY in cmake?
When compiling the library?
If compiling the client should i set MYSHAREDLIB_LIBRARY?

In my case compiling the client download the source code of the library and compile the source code.
The target resulting from that shared library is linked to the client using cmake .

Maybe i found my problem while writing this.
I will do what Qt says(exactly :)) and try again, I llet you know the result and the code.

Thanks.

[Christian Ehrlicher]

@Mesrine said in Shared library on windows fails when using moc:

Then my question is when should i define MYSHAREDLIB_LIBRARY in cmake?
When compiling the library?

add target_compile_definitions(mysharedlib PRIVATE MYSHAREDLIB_LIBRARY)

And instead 'MYSHAREDLIB' you should use your library name...

[JoeCFD]

@Christian-Ehrlicher
Good to know, Thanks. I have not used Windows for ages.

  #if __GNUC__ >= 4
    #define DLL_PUBLIC __attribute__ ((visibility ("default")))
    #define DLL_LOCAL  __attribute__ ((visibility ("hidden")))
  #else
    #define DLL_PUBLIC
    #define DLL_LOCAL
  #endif

[Christian Ehrlicher]

@JoeCFD You have then compile with -fvisibility-inlines-hidden -fvisibility=hiddenand get the same behavior like the (default) windows behavior. See CMAKE_CXX_VISIBILITY_PRESET for more information on how to use it platform independent with cmake.