[CMake] Best practice for configuration-dependent defaults?

[Petr Kmoch]

Hi Sam,

it seems to me that your user-facing option is not actually Boolean, but
tri-state: On vs. Off vs. Use_default. So I would represent it accordingly:
present the user with a string variable (with suitable STRINGS property
https://cmake.org/cmake/help/latest/prop_cache/STRINGS.html), and then
control my internal CMake logic like this:

if(FOO STREQUAL "USE_DEFAULT")
  # ...
elseif(FOO)
  # user turned FOO on
else()
  # user turned FOO off
endif()

Petr

On Fri, 17 Aug 2018 at 19:18, Sam Edwards <cfsworks at gmail.com> wrote:

> Hi all!
>
> I have a project with some options that have different defaults depending
> on the configuration used to build the project. For example, support for a
> certain (easy to support, but relatively uncommon) file format should be on
> by default, except when building in the MinSizeRel configuration. Or
> inclusion of a certain optional troubleshooting feature should be on by
> default only when building for Debug, and should default to off in all
> other configurations.
>
> For single-configuration generators, this is pretty easy: I just look at
> the CMAKE_BUILD_TYPE variable and switch the option() defaults depending on
> the selected build type, then generate my config.h once.
>
> I'm trying to support multi-configuration now. My current plan is to
> generate one config.h per build configuration (e.g. include/Debug/config.h,
> include/MinSizeRel/config.h, ...) so that the options which the user hasn't
> explicitly set can have different per-configuration values depending on
> their per-configuration defaults.
>
> However, where I'm getting stuck is in changing the default for an option
> and having that default take precedence when the user hasn't overridden the
> option explicitly. I can't just do something like:
> option(FOO "This is foo" ON)
> message("FOO is ${FOO}")
> option(FOO "This is foo" OFF)
> message("FOO is ${FOO}")
>
> ...because the first option(FOO ...) sets it to ON when it sees it isn't
> in cache and isn't selected by the user, so the second option(FOO ...)
> thinks it's already been set explicitly. Unless there's some way of
> distinguishing "ON because it's the default" from "ON because the user
> explicitly requested it" while having everything still show up correctly in
> the GUI, this won't work.
>
> Is this really the best practice for what I'm trying to do, or is there a
> better "CMake way" to do this? How do you folks solve this problem in your
> own projects?
>
> Thanks,
> Sam
> --
>
> Powered by www.kitware.com
>
> Please keep messages on-topic and check the CMake FAQ at:
> http://www.cmake.org/Wiki/CMake_FAQ
>
> Kitware offers various services to support the CMake community. For more
> information on each offering, please visit:
>
> CMake Support: http://cmake.org/cmake/help/support.html
> CMake Consulting: http://cmake.org/cmake/help/consulting.html
> CMake Training Courses: http://cmake.org/cmake/help/training.html
>
> Visit other Kitware open-source projects at
> http://www.kitware.com/opensource/opensource.html
>
> Follow this link to subscribe/unsubscribe:
> https://cmake.org/mailman/listinfo/cmake
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <https://cmake.org/pipermail/cmake/attachments/20180824/83902f0a/attachment.html>