[CMake] Way to use precompilation directive on cmake and conditionnal linking

[Philip Lowman]

On Tue, Mar 24, 2009 at 12:11 PM, Nadir SOUALEM <nadir.soualem at irisa.fr>wrote:

> Tyler wrote:
> Not so bad, but it would be very nice to do some things such adding
> preprocessing on command line calls:
> cmake -DPREPROCCESSING_VAR
>

You could certainly file a feature request that did something like this but
I think it would probably prove too confusing for people.

You could implement it with a macro provided you are comfortable with some
standard way of converting CMake variables into preprocessor definitions.
Any variables that start with DEF_foo would result in
add_definitions(-Dfoo).  The list of variables that exist in the system is
available.

Ultimately though build systems usually deal with a fixed number of
preprocessor definitions and usually they just add build options for them

OPTION(USE_FOO "Use foo" ON)
if(USE_FOO)
   add_definitions(-DHAVE_FOO)
   find_package(FOO)
   if(NOT FOO_FOUND)
      MESSAGE(FATAL_ERROR "You enabled USE_FOO but FOO was not found")
   endif()
endif()

Obviously if you just want to add a preprocessor definition the code is far
simpler

OPTION(ENABLE_BAR "Enable special bar feature" OFF)
if(ENABLE_BAR)
   ADD_DEFINITIONS(-DBAR_ENABLED)
endif()

This has the benefit of documenting your build options as you go along
somewhere.

-- 
Philip Lowman
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