Cannot find target Boost::boost after add_subdirectory

[q-ycong-p]

My project git checks out Boost's source code to a subdirectory, and adds it through add_subdirectory. I'd like to use Boost::boost in my main project via target_link_libraries(myTarget INTERFACE Boost::boost). However Boost::boost is not recognized while the other individual boost libraries work fine.

I looked into config/BoostConfig.cmake and found that Boost::boost is added as a target here - however BoostConfig.cmake is NOT installed if BOOST_SKIP_INSTALL_RULES or BOOST_SKIP_INSTALL_RULES - in this case is true because I am not installing Boost as project root, but rather added as a subdirectory.

It seems that making Boost::boost available in the add_subdirectory use case would be a correct solution. Can someone (1) help me understand what's the correct way I can workaround it? And (2) Would be make sense to open this as a feature request, or do I miss the intended usage here?

[pdimov]

Boost::boost is a bit of a fake target, because it relies on the fact that after installation, all Boost headers are in the same directory (which it can then add to the include path.)

However, with add_subdirectory use, the headers aren't in the same directory, but in their corresponding libs/<libname>/include directories. So providing a Boost::boost target in this case would necessitate adding all those directories (for each Boost library). It can be done, but I'm not sure why you would want that instead of linking to the corresponding library targets.

[q-ycong-p]

Thanks @pdimov!

In my project's native build system, Boost is already installed, and Boost::boost is available to link. In a cross-compilation scenario, the project downloads Boost from source to re-compile, uses this cmake submodule to add Boost via add_subdirectory. The need for Boost::boost is in hope to unify the project's cmake logic.

What would you suggest as the best way for my use case? Would I best use some if-else logic and explicitly link to individual Boost libraries if detect cross-compilation? Ty!

[pdimov]

I think that you can create the target Boost::boost yourself in the add_subdirectory path, by using something like

  add_library(Boost::boost INTERFACE IMPORTED)
  set_property(TARGET Boost::boost APPEND PROPERTY INTERFACE_LINK_LIBRARIES Boost::lib1 Boost::lib2)

where lib1 and lib2 are the (header-only) Boost libraries your project needs to use.

[q-ycong-p]

Thank you very much @pdimov! As a followup question, Boost::numeric_ublas lib is needed in my project. However i see that boost/libs/numeric/ublas submodule does not have a CMakeLists.txt. This causes the line in boost/tools/cmake/include/BoostRoot.cmake here to not recognize numeric/ublas as a target.

Do you happen to know what's special about it that it lacks a top-level CMakeLists.txt, and what should one do to make Boost::numeric_ublas a target so that I can add it using the method you demonstrate above?

As a reference, numeric/conversion and numeric/interval are both fine, since they have their top-level CMakeLists.txt. Thanks a lot! (I could copy the other numeric/* lib's CMakeLists.txt, just wondering why ublas is different here!)

[pdimov]

I submitted a pull request (https://github.com/boostorg/ublas/pull/142) to add a CMakeLists.txt file to ublas, but the maintainers haven't merged it yet. I'll ask them again in the PR.

[q-ycong-p]

TLDR: Is there a way where a project uses header-only components of the Boost libraries, and avoid building them, with the above Boost::boost approach?

---

Hi @pdimov , sorry that I am extending on this thread. Pls let me know if I'd better take these how-to questions to another thread, and close this issue (since it is not a boostorg/cmake bug!).

First, thanks for pointing me to the ublas PR. I patched with the PR change and unblocked.

However, my project should only need header-only Boost. Due to its cross-compilation nature, I'd like to avoid building the Boost CXX objects since it suffices as long as headers are included. However, with the above approach through Boost::boost, linking happens error-free, but building of the project triggers .cpp.obj being build for a number of Boost libraries.

Below is how I'm using Boost in my project. Does it look like I am doing something obviously off? Ty!!

# Project top-level macro
set(BOOST_INCLUDE_LIBRARIES core system variant numeric/ublas)
add_subdirectory(${NEMORTLIB_BOOST_SOURCE_DIR} ${NEMORTLIB_BOOST_BUILD_DIR} EXCLUDE_FROM_ALL)

if(NOT TARGET Boost::boost)
    add_library(Boost::boost INTERFACE IMPORTED)
    set_property(TARGET Boost::boost APPEND PROPERTY
                    INTERFACE_LINK_LIBRARIES Boost::core
                                            Boost::system
                                            Boost::variant
                                            Boost::numeric_ublas)
endif()

#  Subdirectory A
target_link_libraries(target_a PRIVATE target_b Boost::boost)

My suspicion is that, a bunch of transitive boost deps are being built due to ublas (e.g. file_system). However my project should only needs the header-only component of ublas. Will it make sense if my project manually moves all boost headers into one directory, which mimics the state after a Boost installation? If so, is there a place in boostorg/cmake that's already doing it?

[pdimov]

Your guess seems correct; ublas links both directly to a library that needs building (serialization) and indirectly via compute to e.g. filesystem (see the dependency report).

I'm not sure how this can be fixed; in CMake, if you link to a library, you link to its transitive dependencies, which means they will have to be built. You can in principle patch the CMakeLists.txt file for ublas and remove the unneeded dependencies, if that works in your case.

As for copying the headers somewhere, that's what the installation step does. I don't think it's possible to achieve this effect with add_subdirectory.

[q-ycong-p]

Thank you very much @pdimov! That makes a lot of sense to me.

Given what you said, I experimented with using target_include_directories instead of target_link_libraries to avoid building things transitively. What I struggle is that it now seems to require manually adding all direct AND indirect boost libraries to BOOST_INCLUDE_LIBRARIES:

set(BOOST_INCLUDE_LIBRARIES ...) # Here, each transitive dependency needs to be identified and added
add_subdirectory(${BOOST_SOURCE_DIR} ${BOOST_BUILD_DIR} EXCLUDE_FROM_ALL) # Here, it starts building CXX objects for unneeded boost libs if without EXCLUDE_FROM_ALL

This seems to imply that deps transitivity is resolved with target_link_libraries but not resolved with target_include_directories. Does it look like I am missing a easy solution?

[pdimov]

That is how it works, yes.

I still don't see a better way to accomplish what you want than removing Boost::compute and Boost::serialization from the ublas dependencies; either by patching the CMakeLists.txt file - which you can do because you're adding it anyway - or as a "post-processing step" by getting the INTERFACE_LINK_LIBRARIES property of the Boost::numeric_ublas target, using list(REMOVE_ITEM to remove those two, and setting it back.

[q-ycong-p]

Thank you @pdimov!! Removing these two deps uncovers issue of my own understanding - even though my main project depends on header-only boost, my test has indirect dependency on Boost::serialization via ublas's vector and matrix - now I've disabled those tests for my cross-compilation case, and I am able to get through my cmake build issues related to Boost. Thanks again for your prompt and accurate help!

[q-ycong-p]

Closing the issue since @pdimov has explained this to be the intended behavior with add_subdirectory usage, and suggested working solutions. Ty!