[CMake] Apply FIND_PACKAGE_HANDLE_STANDARD_ARGS() on COMPONENTS

[Michael Wild]

On 22. Apr, 2010, at 13:50 , S Roderick wrote:

> On Apr 22, 2010, at 06:21 , Michael Hertling wrote:
> 
>> 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.
> 
> That seems to make sense. YY isn't listed as required, therefore it is optional. Having XXX_FOUND=TRUE seems reasonable if CMake found some portion of XXX, regardless of whether it did, or did not, find YY. If we required YY, then we should have added REQUIRED.
> 
> Now the fact that COMPONENTS and REQUIRED are mutually exclusive is not right IMHO. I'd love to hear Kitware's take on why it was done this way.
> Stephen

I haven't tried it out, but as I read the synopsis of find_package, they are not mutually exclusive. Rather, COMPONENTS is implied if any components are listed after REQUIRED:

# optionally finds component YY of package XX
find_package(XXX COMPONENTS YY)

# requires that component YY of package XX is found
find_package(XXX REQUIRED YY)


Michael