[CMake] Proposal: extra option PRIVATE for target_link_libraries

[Alexander Neundorf]

On Wednesday 23 December 2009, Michael Wild wrote:
> On 23. Dec, 2009, at 14:36 , Alexander Neundorf wrote:
> > On Wednesday 23 December 2009, Marcel Loose wrote:
> >> Hi all,
> >>
> >> I suggested this in the quite long thread "third party library
> >> dependencies", but it may have been overlooked. Hence, I started a new
> >> thread.
> >>
> >> Upon (re)reading the Mandriva page
> >> http://wiki.mandriva.com/en/Overlinking, I was thinking: maybe the issue
> >> of overlinking can be solved more or less the same way as pkg-config
> >> does: i.e. by defining private dependencies. This could be an extra
> >> option to target_link_libraries.
> >> Something like:
> >>
> >>  target_link_libraries(mylib public1 public2 PRIVATE private1 private2)
> >
> > Assuming that public1 and public2 are libraries also built with cmake,
> > that can be done already today:
> >
> > add_library(public1 ...)
> > target_link_libraries(public1 private1)
> > target_link_libraries(public1 LINK_INTERFACE_LIBRARIES "")
> >
> > add_library(public2 ...)
> > target_link_libraries(public2 private2)
> > target_link_libraries(public2 LINK_INTERFACE_LIBRARIES "")
> >
> > install(TARGETS public1 public2 ... EXPORT MyPublicLibs)
> > install(EXPORT ...)
> >
> >
> > Then later on, when loading these exported targets, you will get what you
> > want. If public1/2 are shared libs, their "link interface" will be empty.
> > If they are static, you will link against everything they have been
> > "linked" against, independent of the LINK_INTERFACE_LIBRARIES (which is
> > used only for shared libs).
> >
> > Alex
>
> So, the last step in doing
>
> add_library(public1 ...)
> add_library(public2 ...)
> target_link_libraries(public2 public1 private1)
> target_link_libraries(public2 LINK_INTERFACE_LIBRARIES public1)
>
> is equivalent to
>
> set_target_properties(public2 PROPERTIES LINK_INTERFACE_LIBRARIES
> "public1")
>
> Am I right?

Yes, but if you use 
target_link_libraries(public2 LINK_INTERFACE_LIBRARIES public1)
it can handle the "optimized" and "debug" keywords, which may be contained in 
some of the _LIBRARY/_LIBRARIES variables.

> So, this should definitely go somewhere on the Wiki, that when one creates
> a project that does an install(EXPORT ...) and provides a XXX-config.cmake
> file, that one should carefully use target_link_libraries(...
> LINK_INTERFACE_LIBRARIES ...) to make things play nicely.
>
> This doesn't solve the problem for non-cmake built libraries which are
> discovered through FindXXX.cmake modules, though.

Well, partly. The FindXXX.cmake module can also set up the library as an 
imported target and set the respective properties manually.
But I'm not aware of any Findxxx.cmake module which does that currently.

Alex