[Digikam-devel] digikam libs questions

[Marcel Wiesweg]

> 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
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