On 04/22/2010 02:04 PM, Michael Wild wrote: > > 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:
Yes, this applies better, I think. Possibly, COMPONENTS arises from a syntactical need: You can't say, e.g., FIND_PACKAGE(Qt4 QtCore QtXml), but you must have a keyword to introduce the list of components; else, FIND_PACKAGE() gets confused. Thus, if you don't want to say REQUIRED you have to say something different. > # 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) As far as I've observed, you can use options like QUIET or REQUIRED to mark the end of the components' list; indeed, the following syntax is widely-used: FIND_PACKAGE(Qt4 COMPONENTS QtCore QtGui QtXml REQUIRED), although this can hardly be seen from FIND_PACKAGES()'s specification, IMHO, and hopefully, no one will ever have a package with a component named "QUIET", "REQUIRED" or "NO_POLICY_SCOPE"... ;) Best regards, Michael _______________________________________________ Powered by www.kitware.com Visit other Kitware open-source projects at http://www.kitware.com/opensource/opensource.html Please keep messages on-topic and check the CMake FAQ at: http://www.cmake.org/Wiki/CMake_FAQ Follow this link to subscribe/unsubscribe: http://www.cmake.org/mailman/listinfo/cmake
