Exporting Boost components

[jcarpent]

A small remark to mention that at the time being, the Boost components are not exported at all by a project. We need to take on the fact that some of them are only BUILD dependencies, and should not be exported then.

[gergondet]

You can/should use add_project_dependency with projects dependencies and find_package with build time dependencies

So for example:

add_project_dependency(Boost REQUIRED COMPONENTS thread)
target_link_libraries(myLib PUBLIC Boost::thread)
# find_package(MyPackage) will also find_package(Boost REQUIRED COMPONENTS thread)

find_package(Boost REQUIRED COMPONENTS test)
target_link_libraries(myTest PUBLIC Boost::test myLib) # still links to Boost::thread

Note: if you only need Boost headers (i.e. the Boost::boost target) you can omit the COMPONENTS if you require cmake > 3.5 (notably, Ubuntu 16.04 has CMake 3.5)

Note 2: I'm not sure how well this works with boost-python as I don't use it

[nim65s]

Do you mean cmake >= 3.5 ?
If so, I think we can require CMake 3.5.

Debian 8 (oldoldstable) has CMake 3.0, so it's broken anyway (ref #338), and Debian 9 (oldstable, 2017) has 3.7. Fedora 27 (2017) has 3.11.

[gergondet]

Nope, I really mean cmake > 3.5

I'm basing this on a comment I made in a CMakeLists.txt which I'm pretty sure I added after a build failure on Xenial (I was still building on Trusty at the time but with CMake 3.5 too)

Note: this can also be worked-around by using a more recent FindBoost.cmake

[nim65s]

Ok, thanks

[jcarpent]

So, currently in the module, we have a specific macro to handle Boost components. It would be nice to add the features of Optional and Required and Build components. In general, Boost.Python is not exported to link the target to an external target.
@nim65s Would have time to handle it? Do you see any good solution?

[nim65s]

I guess this is just a matter of adding parameters to the macro, and using

add_project_dependency(Boost REQUIRED COMPONENTS ${BOOST_REQUIRED_COMPONENTS} REQUIRED)
add_project_dependency(Boost REQUIRED COMPONENTS ${BOOST_OPTIONAL_COMPONENTS})
find_package(Boost REQUIRED COMPONENTS ${BOOST_REQUIRED_BUILD_COMPONENTS} REQUIRED)
find_package(Boost REQUIRED COMPONENTS ${BOOST_OPTIONAL_BUILD_COMPONENTS})

I can add that, but can you provide a way to test that I'm doing what you need ?

[jcarpent]

I think Pinocchio is a good solution, or even hpp-fcl.

[gergondet]

add_project_dependency(Boost REQUIRED COMPONENTS ${BOOST_REQUIRED_COMPONENTS} REQUIRED)
add_project_dependency(Boost REQUIRED COMPONENTS ${BOOST_OPTIONAL_COMPONENTS})
find_package(Boost REQUIRED COMPONENTS ${BOOST_REQUIRED_BUILD_COMPONENTS} REQUIRED)
find_package(Boost REQUIRED COMPONENTS ${BOOST_OPTIONAL_BUILD_COMPONENTS})

My two cents here:

• you don't have to handle the find_package case (anyway this a CMake function)
• my personnal approach to optional dependency is:

# Check for it
find_package(OptLib QUIET)
if(OptLib_FOUND)
  # Make it required in dependent project
  add_project_dependency(OptLib REQUIRED)
endif()

[olivier-stasse]

My two cents on this: we could just NOT append the package not found when calling find_package in add_project_dependency.
This would avoid cluttering our CMakeLists.txt files.

[jmirabel]

Shouldn't SEARCH_FOR_BOOST use add_project_dependency instead of find_package ?
This would make the boost components automatically exported.