libboost-headers CMake 3.30 compatibility

[mgovers]

Comment:

Is there a way to get libboost-headers (or equivalent; i.e. without any binaries) as well as a BoostConfig.cmake file?

We develop tooling with a header-only core and use conda-forge/libboost-headers to install the installed Boost headers as a dependency (for Windows). Up until recently, that has worked just fine. Recently, however, CMake has decided to phase out FindBoost.cmake. That means that we are unfortunately forced to either:

• explicitly set the associated CMake Policy CMP0167 to OLD
  • This policy is deprecated and might be removed entirely in the future
  • We went with this solution for compatibility (see also disable cmake policy that removes FindBoost.cmake PowerGridModel/power-grid-model#873)
• or migrate to libboost-devel
  • This is much larger and contains many unnecessary binaries.

It would be great if we could have a libboost-headers-devel (or similar), that's compatible with both the most recent CMake policy and the smaller dependency size.

I am willing to contribute myself if needed.

[h-vetinari]

The problem is that we cannot split a single file (like BoostConfig.cmake) across several packages. If we put it in libboost-headers-devel, then CMake will contain that it cannot find the binaries. If we package a header-only version in such an output, then the BoostConfig.cmake in libboost-devel will clobber that file (with undetermined rules which file wins).

So unfortunately, I don't think there's a realistic alternative to using the full libboost-devel if you want to have CMake metadata (or you cross your fingers that CMake will not remove the old policy for a while yet).

[mgovers]

Wouldn't the COMPONENTS and OPTIONAL_COMPONENTS allow for such a solution?

In particular, Boost::headers is a required COMPONENT. Other components are not required, unless ALL is specified, or the user requests specific COMPONENTS in the find_package command.

I.e. find_package(Boost CONFIG COMPONENTS Boost::headers) may pass regardless of whether the currently investigated installation of Boost is header-only or not. On the other hand, find_package(Boost CONFIG COMPONENTS ALL) (or plainly find_package(Boost CONFIG) by default) will fail on a header-only installation, and instead try to look for the next one that does in fact result in an installation with all libs installed.

[for reference] Citing the BoostConfig.cmake from libboost-devel version 1.87.0:

# Find boost_headers

boost_find_component(headers 1 0)

if(NOT boost_headers_FOUND)

  set(Boost_FOUND 0)
  set(Boost_NOT_FOUND_MESSAGE "A required dependency, boost_headers, has not been found.")

  return()

endif()

# Compatibility variables

set(Boost_MAJOR_VERSION ${Boost_VERSION_MAJOR})
set(Boost_MINOR_VERSION ${Boost_VERSION_MINOR})
set(Boost_SUBMINOR_VERSION ${Boost_VERSION_PATCH})

set(Boost_VERSION_STRING ${Boost_VERSION})
set(Boost_VERSION_MACRO ${Boost_VERSION_MAJOR}0${Boost_VERSION_MINOR}0${Boost_VERSION_PATCH})

get_target_property(Boost_INCLUDE_DIRS Boost::headers INTERFACE_INCLUDE_DIRECTORIES)
set(Boost_LIBRARIES "")

# Find components

if("ALL" IN_LIST Boost_FIND_COMPONENTS)

  # Make sure "ALL" is the only requested component.
  list(LENGTH Boost_FIND_COMPONENTS __boost_find_components_count)
  if(NOT ${__boost_find_components_count} EQUAL 1)
    message(AUTHOR_WARNING "ALL cannot be combined with named components; the named components will be ignored.")
  endif()

  unset(__boost_find_components_count)

  set(Boost_ALL_TARGETS Boost::headers)

  boost_find_all_components()

else()

  foreach(__boost_comp IN LISTS Boost_FIND_COMPONENTS)

    boost_find_component(${__boost_comp} ${Boost_FIND_REQUIRED_${__boost_comp}} 0)

  endforeach()

endif()

[h-vetinari]

find_package(Boost CONFIG) [...] will fail on a header-only installation

That's the main issue, because a lot of packages are not specifying explicit components.

[mgovers]

find_package(Boost CONFIG) [...] will fail on a header-only installation

That's the main issue, because a lot of packages are not specifying explicit components.

I understand. What about the following?

• Making ALL components the default (on find_package(Boost CONFIG))
• Only on find_package(Boost CONFIG COMPONENTS headers) be OK with the lesser

I get that it requires a change to the CMakeLists but maybe this is a sustainable solution.

Alternatively, maybe it's possible to do something along the lines of

set(Boost_HEADER_ONLY TRUE)  # or maybe even as a CACHE entry
find_package(Boost CONFIG)