[Digikam-devel] digikam libs questions

[Andi Clemens]

Thanks for all the explanation. I guess that helped a lot, if something is 
still missing I'm asking again :D

Andi


On Saturday 25 October 2008 16:17:12 Marcel Wiesweg wrote:
> > Gilles asked me to send this mail to the ML...
> >
> >
> > Hi,
> >
> > I used to program in python all the time, so I'm still quite confused by
> > the libs mechanism in C++.
> >
> > I understand that some of the .o files are packed into library files (to
> > save space and make things easier) and some header files are stored
> > in /usr/include/digikam to actually access those libs from the program
> > the libs will be used in.
> >
> > But what are those DIGIKAM_EXPORT macros for? They seem to be in nearly
> > every class although not every class seems to be exported into libs (or
> > at least no header files exist in /usr/include/digikam).
> >
> > Do they signal gcc to pack those object files into libraries?
> > I found out that in the main CMakeLists.txt (in digikam root) there are a
> > lot of libs defined, but in the end only a few of them are actually
> > installed in /usr/lib. For example we define a libhaar, but I can't find
> > it anywhere.
>
> This is a very valid question.
> Take CMakeLists.txt in the main project directory and digikam/CMakeList.txt
> in the digikam/ subdir.
> The former one defines a large number of "libs", like libcurves_SRCS,
> libdimg_SRCS, libdimgloaders_SRCS etc. These are indeed not really libs,
> but we just use this to package the huge list of source files in chunks.
> (They were libs, static libs, with KDE3 and automake, so the lib... prefix
> is there for historic reasons).
>
> Now on to digikam/CMakeList.txt. This one defines the real binary objects
> that we have: libdigikam, libdigikamdatabase and the digikam application
> binary (the kioslaves and showfoto binaries are defined in their own
> CMakeLists.txt). You see that we now use all the lib... defines from above
> to group the source files into these binaries.
> Now the layout here actually has a rationale:
> libdigikam is the base library and contains all code shared between
> digikam, showfoto, the ioslaves and the image plugins.
> libdigikamdatabase obviously contains database code. It depends on
> libdigikam and is used by digikam and the ioslaves, not by showfoto and the
> image plugins.
> Non-shared code is kept in the digikam binary.
> There is no source file compiled twice in either of the binaries, with a
> few small exceptions with digikam/showfoto.
>
> Now to your DIGIKAM_EXPORT question: Any code that is contained in a
> library and used from another binary must contain the DIGIKAM_EXPORT macro,
> or it will result in "undefined reference" errors when linking.
> So if code is contained in a library and does not carry the macro, it is
> not intended to be used publicly (e.g. ImageInfoCache in libs/database/).
> Code in an application binary, e.g. AlbumIconView, does not need the macro
> as it will never be linked in a shared lib.
>
> Marcel
>
> > Maybe those questions are stupid, but I want to understand completely
> > what we are doing here (in Cmake and in the code as well).
> > For example my histogrambox widget: I just used copy and paste for it,
> > because I didn't know what to do at this moment. So it has an EXPORT
> > macro in front of the class and it also becomes installed in
> > /usr/include, but I guess we don't need this widget the be accessed from
> > outside of digiKam. So I could remove it from the INSTALL rule in the
> > CMake file. But if I do so, do I have to remove the EXPORT macro as well
> > or do I have to keep it.
> >
> >
> > Andi
> > _______________________________________________
> > Digikam-devel mailing list
> > Digikam-devel at kde.org
> > https://mail.kde.org/mailman/listinfo/digikam-devel
>
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