[cmake-developers] Adding argument "OPTIONAL" to find_package() and add_subdirectory

[Eric Noulard]

2011/6/4 Alexander Neundorf <neundorf at kde.org>:
> Hi,
>
> again from the KDE sprint...
>
> 1) We have a macro
> macro_optional_find_package().
> The purpose is to be able to build without a specific package even if that
> package is installed and would actually be found by the find_package() call.
>
> Basically this is a wrapper around find_package(), but additionally it adds an
> option WITH_Foo, which is enabled by default.
> If disabled, the actual find_package() is not executed, and the variables
> FOO_FOUND/INCLUDES/LIBRARIES are reset to empty, so that even if the package
> hasn't been searched this time, results from previous runs are discarded.
>
> What do you think about adding this as a built-in feature to find_package(),
> i.e. add a argument OPTIONAL to find_package(), then probably also a
> "COMMENT".

I'm not sure about this feature and since we already have REQUIRED why
would be the meaning of OPTIONAL?

May be some syntax like

find_package(Foo IF <VarName>) would be clearer.

find_package(Foo IF  BUILD_WITH_Foo)

would create option BUILD_WITH_Foo
(default value would the value of BUILD_WITH_Foo_Default var or ON if
it is not defined)
and the search of the package will be done iff the option is ON.

> 2) we have something similar for add_subdirectory(),
> macro_optional_add_subdirectory().
> The purpose is to disable parts of a big project by skipping the
> add_subdirectory.
>
> This is a wrapper around add_subdirectory(), but adds an option BUILD_FOO,
> which is enabled by default.
> If disabled, the actual add_subdirectory() is not executed.
> Additionally there is an additional global option
> DISABLE_ALL_OPTIONAL_SUBDIRECTORIES, which is disabled by default.
> If enabled, all these optional subdirectories are disabled (but can be enabled
> again one by one).
>
> What do you think about adding the keyword OPTIONAL to add_subdirectory ?

Same remark may IF keyword with the option name as argument would be better.

> Both have been proven useful, the one for find_package() especially for
> packagers.

Agreed, the feature is nice I use similar thing with hand-crafted
option in some project.
-- 
Erk
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