[CMake] Avoiding error when using add_subdirectory twice on same path (CMake 2.6.4)

[Tyler Roscoe]

On Tue, Jun 16, 2009 at 06:49:38PM -0400, David Cole wrote:
> What is your CMake code for this "diamond dependency" case?

The case as layed out is hypothetical, of course, but I'm thinking
something like this:

myApp:

project(myApp)

add_subdirectory(lib1)
add_subdirectory(lib2)

add_executable(myApp ${files})
target_link_libraries(myApp lib1 lib2)


lib1/lib2:

project(lib1)

add_subdirectory(commonLib)

add_library(lib1 ${files})
target_link_libraries(lib1 commonLib)

> Is there a separate directory for each of the libs involved?

Yes.

> Whichever CMakeLists file glues them all together should be able to
> add_subdirectory just once on each of them.... Why do you need two
> add_subdirectory calls for commonLib?

1. It makes sense to me if lib1 and lib2, both of which have
dependencies on commonLib, both make that dependency explicit in their
CMakeLists. It is less clear to me if some other orchestrator "happens
to" add_subdirectory(commonLib) and lib1 and lib2 "happen to" be invoked
from that orchestrator.

2. It allows lib1 and lib2 to be independent projects. Imagine that
myApp is something big and complex; call it KDE (Komplex Development
Environment :)). Just because I want to build lib1 doesn't necessarily
mean that I want to build all of KDE. Or at least I may not want to let
CMake build the build system for all of KDE since this is
time-consuming.

In my project, we use a sort of object model where each directory (we
call them "buildables") is its own independent thing that knows how to
build itself and its dependencies. I am coming to understand that this
model is a little strange for CMake but it makes sense to us and for now
(on CMake 2.6.2) it works.

I hope this illustrates where I'm coming from.

Thanks,
tyler