[cmake-developers] Installation of imported targets

[Elie Roudninski]

I'm a CMake user for while now, and i found some limitation with the
current state of imported targets.
>From what i understand, they were introduced to manipulate _already
installed_ binary/library inside CMake in order to handle linking, etc, and
is used for example in exported project configuration file.

Another regular use case that i encounter frequently, is when you need to
build an ExternalProject (because there is no build system, or because the
buildsystem is autotools and there is no package available) for a library,
and you use imported targets to declare the library target and make it
depend on the ExternalProject target. You can then properly link with it
and build order dependency is respected.

Everything works fine except when you need to install the imported target.
You can't use the regular:
install(TARGETS myimportedlib LIBRARY DESTINATION lib)
It is not permitted for imported targets. At the moment, i usually use:
install(FILES ${CMAKE_CURRENT_BINARY_DIR}/path/to/myimportedlib.so
DESTINATION lib)
to install it.
The problem now, is when you need to export the library ... At the moment,
you just can't do it properly. There is an ugly-tricky way that i thought
of (without testing it ...) using a second fake target, but an interface
one this time, linking with the futur full path of installed library ...
Installing it with install(TARGETS myinterfacelib EXPORT myproj DESTINATION
lib) will do nothing but you should be able to do an install(EXPORT myproj)

You can track the issue here:
https://gitlab.kitware.com/cmake/cmake/issues/14311

To solve the problem, there are many solutions, with one of them might be
being able to install imported targets and use them pretty much like
regular targets. Maybe that would make interface targets a bit useless ? In
fact mixing INTERFACE and IMPORTED targets "should" do it.
This solution could work with ExternalProject as decribed before, and even
for other use cases, like providing closed-source library already built
with your project as it is done by PF_RING for example (
https://github.com/ntop/PF_RING in userland/lib/libs)

I did my best to explain myself, but if it is still confusing or if my
english is bad, do not hesitate to respond, i'll try to do better :)

Regards,

marmeladema
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