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

[Eric Noulard]

2011/6/6 Alexander Neundorf <neundorf at kde.org>:
> On Sunday, June 05, 2011 11:50:50 PM Eric Noulard wrote:
>> 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?
>
> Using OPTIONAL and REQUIRED together should be considered an error.
> OPTIONAL means you can disable the searching so that it will also not be
> "found" even if it is actually installed.

Yes I think get your point.
I was just thinking that not specifying REQUIRED already means OPTIONAL
whereas specifying OPTIONAL means "could be disabled".

>> 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.
>
> Maybe.
> I thought OPTIONAL would be good since it hints at "OPTION", and it is an
> option which is created for disabling it.
> Could also be "OPTION" instead of "OPTIONAL".

I'd prefer OPTION over OPTIONAL I think it's less confusing with REQUIRED.

My idea with IF was a little broader than OPTION
you could feed IF argument with an existing var
(not necessarily an option) and the package search would be
done if the var evaluate to true/on/yes....
That way one can encode several ways of enable/disabling a
find_package subset.

If no var is defined then find_package may create an option using
this name with a default value set to ON.
However, may be the IF idea is simply overkill.

-- 
Erk
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