[cmake-developers] Make non-REQUIRED find_package() always disable-able Was: Re: Adding argument "OPTIONAL" to find_package() and add_subdirectory

[Michael Wild]

On 06/07/2011 10:28 AM, Alexander Neundorf wrote:
> Hi,
> 
> I slept over it, I think here's a better idea.
> 
> For every find_package() which is not REQUIRED, some people or packagers may 
> want to explicitely disable each one of them.
> 
> So how about this:
> if there is no REQUIRED keyword in the find_package() call, there is always an 
> option added which can be used to disable this find_package() call.
> These options could be named DISABLE_PACKAGE_<PackageName>.
> 
> This would
> 1) make it possible to disable each of the optional dependencies explicitely
> 2) these options will have the same standard name in all projects
> 3) does not add additional arguments to the find_package() interface
> 
> Options 1) and 2) are things regularly requested from developers used to 
> autotools.
> 
> Alex


I would like to have such a feature. However, there is another use-case
not covered by your proposal which is important for me. In my packages I
include important dependencies and offer the user the choice to either

1. use the system-installed dependency if it is available
2. build the dependency from the code included in the package
3. if it is an optional dependency, don't use it at all

I have some logic which goes along the following lines (for an optional
dependency):

option(DISABLE_PACKAGE_<PackageName> "Don't use <PackageName>" OFF)
if(NOT DISABLE_PACKAGE_<PackageName>)
  find_package(<PackageName>)
  if(<PackageName>_FOUND)
    set(_DEFAULT_USE_SYSTEM_<PackageName> TRUE)
  else()
    set(_DEFAULT_USE_SYSTEM_<PackageName> FALSE)
  endif()
  option(USE_SYSTEM_<PackageName> "Use system-installed <PackageName>"
    ${_DEFAULT_USE_SYSTEM_<PackageName>})
  if(USE_SYSTEM_<PackageName>)
    find_package(<PackageName> REQUIRED)
  else()
    add_subdirectory(externals/<PackageName>)
  endif()
endif()

If it is a hard dependency, just don't offer the
DISABLE_PACKAGE_<PackageName> option, and always set it to OFF.

I think your proposal and my above use-case should probably be best
implemented in macros/functions and distributed either with CMake or
some CMake modules library project.

My 2c.

Michael