Override option in CMake subproject

问题:

I'm trying to reuse the CMakeLists.txt of a third-party project whose source I don't want to change (expat, to be exact). I've added the project as a subproject of the top level using add_subdirectory.

This works but now I would like to set the value of some of the subproject's options in the top level CMakeLists.txt. How do I do this?

回答1:

See the similar question with a good answer.

Answer in short:

SET(SOME_EXPAT_OPTION OFF CACHE BOOL "Use some expat option")

回答2:

If the sub-project uses option (not set) for its configuration settings, then you can specify values using option before adding the subdirectory:

option(LIB_OPTION1 "" OFF)
option(LIB_OPTION2 "" ON)
add_subdirectory(${CMAKE_SOURCE_DIRECTORY}/lib)

回答3:

You can define the options with the desired settings (ON or OFF) before calling ADD_SUBDIRECTORY. This will then take precedence over the OPTION commands in expat's CMakeLists.txt since the last parameter to OPTION is only a default value (which is neglected if that settings already exists).

回答4:

The SET-command has the 'PARENT_SCOPE' option:

If PARENT_SCOPE is present, the variable will be set in the scope above the current
scope. Each new directory or function creates a new scope. This command will set the 
value of a variable into the parent directory or calling function (whichever is 
applicable to the case at hand). PARENT_SCOPE cannot be combined with CACHE.

(see: http://www.cmake.org/cmake/help/v2.8.10/cmake.html#command:set )