How to check if find_package found the package (boost) - cmake

Question

I want to not add boost.cxx if cmake find_package found no boost installed. Does find_package return something that I can wrap in condition to compile boost.cxx or not. Here is my current cmake file:
add_executable (complex complex.cxx lexer.cxx boost.cxx ../../src/lili.cxx ../../src/lilu.cxx)
# Make sure the compiler can find all include files
include_directories (../../src)
include_directories (.)
# Make sure the linker can find all needed libraries
# rt: clock_gettime()
target_link_libraries(complex rt)
# Install example application
install (TARGETS complex
RUNTIME DESTINATION bin)
IF(UNIX)
find_package(Boost COMPONENTS system filesystem REQUIRED)
## Compiler flags
if(CMAKE_COMPILER_IS_GNUCXX)
set(CMAKE_CXX_FLAGS "-O2")
set(CMAKE_EXE_LINKER_FLAGS "-lsqlite3 -lrt -lpthread")
endif()
target_link_libraries(complex
${Boost_FILESYSTEM_LIBRARY}
${Boost_SYSTEM_LIBRARY}
#${PROTOBUF_LIBRARY}
)
ENDIF(UNIX)

Answer 1

The FindXXX scripts are supposed to set a variable <Packagename>_FOUND to TRUEif the package was found. So in your case, it will set Boost_FOUND if boost was found.
When compiling your Boost.cxx, I assume that you will need Boost headers as well, so you should adjust your include directories as well.*
look for Boost before creating your executable. Furhtermore, you need to set your include directories before adding the executable.
IF(UNIX)
find_package(Boost COMPONENTS system filesystem REQUIRED)
# IF( Boost_FOUND ) # checking this variable isnt even necessary, since you added
# REQUIRED to your call to FIND_PACKAGE
SET( BOOST_SRC_FILES boost.cxx )
INCLUDE_DIRECTORIES( ${Boost_INCLUDE_DIRS} ) # you could move this down as well
# as ${Boost_INCLUDE_DIRS} will be
# empty if Boost was not found
# ENDIF()
ENDIF()
add_executable (complex complex.cxx lexer.cxx ${BOOST_SRC_FILES} ../../src/lili.cxx ../../src/lilu.cxx)
# Make sure the compiler can find all include files
include_directories (../../src)
include_directories (.)
# INCLUDE_DIRECTORIES( ${Boost_INCLUDE_DIRS} ) # alternative location to
# add include dirs, see above
# Make sure the linker can find all needed libraries
# rt: clock_gettime()
target_link_libraries(complex rt)
# Install example application
install (TARGETS complex
RUNTIME DESTINATION bin)
IF(UNIX)
## Compiler flags
if(CMAKE_COMPILER_IS_GNUCXX)
set(CMAKE_CXX_FLAGS "-O2")
set(CMAKE_EXE_LINKER_FLAGS "-lsqlite3 -lrt -lpthread")
endif()
target_link_libraries(complex
${Boost_FILESYSTEM_LIBRARY}
${Boost_SYSTEM_LIBRARY}
#${PROTOBUF_LIBRARY}
)
ENDIF(UNIX)
Afternote: Since you use the REQUIRED flag when looking for Boost (since you only need it on Unix platform) it is even sufficient to use the optional-source-files-in-a-variable trick.
(*) Thanks to your question, I just found out that it doesn't matter whether include_directories(...) is called before or after creating the target with ADD_EXECUTABLE or ADD_LIBRARY since the directories are added to all targets in the same project.

Answer 2

Yes, if the find_package(Boost COMPONENTS system filesystem REQUIRED) succeeds, Boost_FOUND will be true.
Also, there will be component-specific versions, so Boost_date_time_FOUND, Boost_filesystem_FOUND, etc.
For further info, run
cmake --help-module FindBoost

Answer 3

Yes, it sets variable Boost_FOUND. Example from FindBoost.cmake:
== Using actual libraries from within Boost: ==
#
# set(Boost_USE_STATIC_LIBS ON)
# set(Boost_USE_MULTITHREADED ON)
# set(Boost_USE_STATIC_RUNTIME OFF)
# find_package( Boost 1.36.0 COMPONENTS date_time filesystem system ... )
#
# if(Boost_FOUND)
# include_directories(${Boost_INCLUDE_DIRS})
# add_executable(foo foo.cc)
# target_link_libraries(foo ${Boost_LIBRARIES})
# endif()