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 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
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