Prevents the looking for boost if built without it for dependents

[AlexisTM]

The library is still using Boost for the dependents and will fail to build if the dependents do not have BOOST installed.

By adding the following to the configured file, we will fix the usage with either looking or not for Boost. (@MSGPACK_USE_BOOST@ will be translated to ON or OFF at configure time)

IF (@MSGPACK_USE_BOOST@)
    find_dependency(Boost REQUIRED)
ENDIF ()

[codecov-commenter]

Codecov Report

Merging #1032 (0b2c6d9) into cpp_master (cef8a6f) will not change coverage.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           cpp_master    #1032   +/-   ##
===========================================
  Coverage       85.60%   85.60%           
===========================================
  Files              79       79           
  Lines            5023     5023           
===========================================
  Hits             4300     4300           
  Misses            723      723           

[redboltz]

Thank you for sending and updating the PR.
I tried to search using the words cmake @ @VARIABLE@ @OPTION@ cmake.in etc on google and cmake.org.
But I couldn't find the document.

Could you tell me the link of cmake manual about @MSGPACK_USE_BOOST@ notation?

[AlexisTM]

Sure! CMake is wonderfully confusing! (And I am far from an expert)

When calling configure_file(...), in this case the equivalent CONFIGURE_PACKAGE_CONFIG_FILE(...), we will configure the msgpack-config.cmake.in file which will then be installed in INSTALL_PREFIX/lib/cmake/msgpackc-cxx/msgpack-config.cmake

The file configuration step will do a search and replace for @VARIABLE@ by their value. In this case, we are setting MSGPACK_USE_BOOST to ON or OFF in the parent thus will be replaced by that value.

This config file is executed by cmake on find_package(msgpack), thus successfully transferring the information "You do not need Boost" to dependents.

After this PR, the config file becomes the following if we built msgpack with MSGPACK_USE_BOOST=OFF

install/lib/cmake/msgpackc-cxx$ cat msgpack-config.cmake

####### Expanded from @PACKAGE_INIT@ by configure_package_config_file() #######
####### Any changes to this file will be overwritten by the next CMake run ####
####### The input file was msgpack-config.cmake.in                            ########

get_filename_component(PACKAGE_PREFIX_DIR "${CMAKE_CURRENT_LIST_DIR}/../../../" ABSOLUTE)

macro(set_and_check _var _file)
  set(${_var} "${_file}")
  if(NOT EXISTS "${_file}")
    message(FATAL_ERROR "File or directory ${_file} referenced by variable ${_var} does not exist !")
  endif()
endmacro()

macro(check_required_components _NAME)
  foreach(comp ${${_NAME}_FIND_COMPONENTS})
    if(NOT ${_NAME}_${comp}_FOUND)
      if(${_NAME}_FIND_REQUIRED_${comp})
        set(${_NAME}_FOUND FALSE)
      endif()
    endif()
  endforeach()
endmacro()

####################################################################################

include(CMakeFindDependencyMacro)

IF (OFF)
    find_dependency(Boost REQUIRED)
ENDIF ()

include("${CMAKE_CURRENT_LIST_DIR}/msgpackc-cxx-targets.cmake")

check_required_components(msgpackc-cxx)

[redboltz]

Merged. Thank you!

[AlexisTM]

You are welcome!