[CMake] Apply FIND_PACKAGE_HANDLE_STANDARD_ARGS() on COMPONENTS

[Michael Hertling]

On 04/21/2010 09:29 PM, S Roderick wrote:
> On Apr 21, 2010, at 15:13 , Alexander Neundorf wrote:
> 
>> On Tuesday 20 April 2010, Michael Hertling wrote:
>>> Dear CMake community, dear CMake developers,
>>>
>> ...
>>> There's another aspect related to this I'd like to comment on: During
>>> the abovementioned considerations on the bug tracker and the mailing
>>> list, the question has arisen if it's reasonable to set XXX_FOUND to
>>> FALSE if any of the requested components aren't found. As for myself,
>>> I'd say: No, it isn't. Let's have a look at the following scenario:
>>>
>>> Package XXX normally provides components YY1 and YY2, but for some
>>> reason, only YY1 is installed. Moreover, XXX provides a config file
>>> XXXConfig.cmake. Now, a project's CMake script requests both YY1/2 by
>>> FIND_PACKAGE(XXX COMPONENTS YY1 YY2). As Brad King has pointed out in
>>> <http://www.mail-archive.com/cmake@cmake.org/msg15952.html>, finding a
>>> config file results in XXX_FOUND to be set to TRUE automatically. Thus,
>>> the absence of YY2 does not mean the absence of XXX as a whole in any
>>> case, and, notwithstanding, the requesting CMake script should have a
>>> chance to proceed even if YY2 isn't available, i.e. the following seems
>>> reasonable: XXX_YY1_FOUND=TRUE, XXX_YY2_FOUND=FALSE *but* XXX_FOUND=TRUE.
>>
>> I think I don't agree here.
>> If I say
>> find_package(XXX COMPONENTS YY1 YY2 REQUIRED)
>> I think it makes a lot of sense to interpret this as "search package XXX, and 
>> I need YY1 and YY2 from it".
>> What other reason would I have to give YY1 and YY2 there otherwise ?
>> If it still succeeds if they are not found, why should I list them then ?
> 
> 
> +1
> 
> find_package(XXX COMPONENTS YY1 YY2 REQUIRED)
> 
> means to me, I _require_ both YY1 and YY2 from XXX. Any other YYx I don't care about.

Yes, of course, the REQUIRED option is definitive; it's
COMPONENTS without REQUIRED which raises the questions.

> The first version above translates to me to be
> 
> find_package(XXX COMPONENTS YY1 REQUIRED)
> 
> I only _require_ YY1.

As I have replied recently to AN, I sometimes dislike FIND_PACKAGE() to
look for unrequested components, so I need to request them even if they
are optional, but this is just my personal preference and, also, not my
concern here.

My concern is that FIND_PACKAGE(XXX COMPONENTS YY) will always return
XXX_FOUND=TRUE if it's driven by XXXConfig.cmake, regardless if YY is
found or not, while the same command could return XXX_FOUND=FALSE if
driven by FindXXX.cmake since the latter has XXX_FOUND in its hands.
Thus, IMHO, it should be reconsidered how XXX_FOUND is interpreted
w.r.t. components because I would expect both variants - XXXConfig
and FindXXX - to behave the same.

Best regards,

Michael