[Qt-interest] Q_OBJECT in DLL - Release build warnings

[Catalin Roman]

Thanks a lot for you quick reply.
Funny thing is that when I followed your suggestions I realized that all the
defines were already in place ! And, of course, the warnings were gone :).
Anyhow, you gave me a very good hint to export only the necessary functions.
Thanks again.

On Mon, Feb 8, 2010 at 12:55 PM, <Oliver.Knoll at comit.ch> wrote:

> Catalin Roman wrote on Monday, February 08, 2010 10:50 AM:
>
> > This is my first post, so please let me know gently if this is the
> > wrong place to seek answers
>
> Hi there and welcome :)
>
> >  to my following problem.
> > I'm building a DLL using Qt 4.6 and Qt Creator 1.3.0. The main class
> > of the DLL has the Q_OBJECT macro declared in the .h file because I'm
> > using signals/slots. When I build in Release mode,
>
> So I read it implicitly that it works in debug mode, right?
>
> > I get 4 warnings
> > concerning staticMetaObject, metaObject(), qt_metacast() and
> > qt_metacall() saying "redeclared without dllimport attribute after
> > being referenced with dll linkage/previous dllimport ignored".
>
> These "redeclared without dllimport" warnings are typical for wrong "DLL
> export/import" declarations, typically due to a missing or wrong DEFINE in
> the project settings.
>
> So without knowing the exact problem in your case I am giving you a rather
> generic answer how it /should/ work:
>
> First, for each DLL project, I define a header such as (for library Foo):
>
> // FooDLL.h
> #ifndef __FOO_DLL_H_
> #define __FOO_DLL_H_
>
> #include <QtGlobal>
>
> #ifdef FOO_EXPORT
> # define FOO_API Q_DECL_EXPORT
> #else
> # define FOO_API Q_DECL_IMPORT
> #endif
>
> #endif // __FOO_DLL_H_
>
> Note the usage of the Qt Q_DECL_EXPORT / Q_DECL_IMPORT macros: they expand
> to the __declspec(dllimport/export) thing on Windows (using the Visual
> Studio compiler) and to "nothing" everywhere else (IIRC). So they come quite
> handy here.
>
>
> Next, let's assume we want to export the *whole* following class from our
> Foo library:
>
> #include <QtCore/QObject>
> #include "FooDLL.h"
> ...
>
> class FOO_API Foo : public QObject {
>  Q_OBJECT
> public:
>  Foo();
>  virtual ~Foo();
>
>  void doFoo();
>  void doBar();
>  ...
> signals:
>  void signalFoo();
>  ...
> private:
>  void doSomething();
> };
>
>
> Now the crucial thing here is off course that you DEFINE FOO_EXPORT within
> your "Foo" project, but that you DON'T DEFINE it for every other project
> (that is using the Foo library). This way the FOO_API will become the proper
> Q_DECL_EXPORT (when building the library) or Q_DECL_IMPORT respective (when
> "importing" / using the library from any other project).
>
> Typically this is done in your Foo.pro like:
>
> DEFINE += FOO_EXPORT
>
> (or in Visual Studio, right-click on your project -> Properties -> C/C++ ->
> Preprocessor and define it there for BOTH DEBUG and RELEASE configurations!)
>
> If however - my preferred way - you only want to export /certain/ methods,
> this should (and does, in my case) also work:
>
> class Foo : public QObject {
>  Q_OBJECT
> public:
>  FOO_API Foo();
>  FOO_API virtual ~Foo();
>
>  FOO_API void doFoo();
>  void doBar();
>  ...
> signals:
>  void signalFoo();
>  ...
> };
>
>
> That is, only the (public) methods Foo(), ~Foo() and doFoo() are exported
> and hence can be called from another library/*.exe. In the above example
> doBar() however can *not* be called. Well, technically your code would
> compile if you would call doBar() from any other place than the Foo library,
> but you would get a linker error! This gives you an additional "package
> visibility" level :) And since you are only exporting the really needed
> methods this also results in a *slightly* smaller DLL ;)
>
>
> To answer your concrete question again, I assume that you either
>
> - Did not properly export your class or
> - You forgot to DEFINE the corresponding "EXPORT" value in your release
> configuration
>
>
> Cheers, Oliver
> --
> Oliver Knoll
> Dipl. Informatik-Ing. ETH
> COMIT AG - ++41 79 520 95 22
> _______________________________________________
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> Qt-interest at trolltech.com
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>
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