make one static library from whole project with cmake - linux

Question

c++-project, say, foo is maintained by the cmake.
One wants to create one library libfoo.a (with all classes/methods/functions created at the whole source-tree) to make possible creating programs that could linked to the library with -lfoo.
ok, let's consider now a toy example, and the prolbem will be clear. Directory foo (root of the project) contains directories a, and b. Two CmakeLists.txt are created:
# a/CMakeLists.txt
add_library(A <a_sources>)
# b/CMakeLists.txt
add_library(B <b_sources>)
And one CMakeLists.txt for root directory:
add_subdirectory(a)
add_subdirectory(b)
add_library(foo <foo_sources>
target_link_libraries(foo A B)
That was a surprise for me: after building libfoo.a contains only methods from foo_sources, and a_sources,b_sources are excluded.
That is ok in the case when executables are built with the same project: while creating executables cmake "guesses" that a and b must be linked if it is linked to foo.
But in the case executable is created "outside" project to use library foo one must link with -lfoo -la -lb, now imagine a project with lots of subdirectories - how to deal with it? so question is "how to create one library, aggregating methods from whole project with means of cmake?"
Googling led me to relatively recently embedded (appeared in 2.8.8) OBJECT library opportunity. Nice example of using it is shown here. Now the problem above can be solved with that:
# a/CMakeLists.txt
add_library(A OBJECT <a_sources>)
# b/CMakeLists.txt
add_library(B OBJECT <b_sources>)
# foo/CMakeLists.txt
add_subdirectory(a)
add_subdirectory(b)
add_library(foo <foo_sources> $<TARGET_OBJECTS:A> $<TARGET_OBJECTS:B>)
problem seems to be solved, unfortunately, not quite.
if dependency chain is longer than 2, for example, foo depends on A, which depends on B, problem still remains.
That is because,
Object libraries may contain only sources (and headers) that compile to object files.
and
Object libraries cannot be imported, exported, installed, or linked.
(quotes are taken from the same link)
I've tried several combinations of target_link_library(), add_library(), add_library(... OBJECT ..) trying to link A and B to foo without success (error during cmake-process.)
I must be loosing something simple, please help, thank you!
I am not sure is it important: project is maintained at the linux.

Answer 1

I think you're getting tangled up in the term "depends on". If you're building a library named foo and it has two parts, A and B, it doesn't matter whether A depends on B; the library should contain both. The CMake code you've shown will build foo properly.

Answer 2

Yep, I support answer #Pete Becker# . But it should be said as well that those libraries a $<TARGET_OBJECTS:A> and $<TARGET_OBJECTS:B> actually not a libraries at all, but rather cmake internal list of object modules. There is no dependencies between compilation of object modules (except auto-generated sources) so they can be done in any order and in parallel.
I guess more correct term for your intention is gathering together several TARGET_OBJECTS under single object library. That's really bad that you can't write add_library(B OBJECT b.cpp $<TARGET_OBJECTS:A>). But you always can implement this by yourself:
add_library(A OBJECT a.cpp)
set(A_OBJECTS $<TARGET_OBJECTS:A>)
add_library(B OBJECT b.cpp)
set(B_OBJECTS $<TARGET_OBJECTS:B> ${A_OBJECTS})
add_library(foo ${B_OBJECTS})
I.e just create special variables _OBJECTS to use them whenever you want to include those object libraries in library, executable or as part of other object library with that _OBJECTS flavor.