Roger Leigh wrote:
> On 21/12/2015 15:07, Stephen Kelly wrote:
>> Roger Leigh wrote:
>>> I can understand why REQUIRED and related
>>> arguments are omitted--that is why find_dependency exists--but I'd quite
>>> like to be able to specify COMPONENTS where needed e.g. with FindBoost,
>>> and this is not currently possible.
>>
>> I don't remember whether this was discussed when designing
>> find_dependency. Perhaps COMPONENTS could be added to the macro now.
>
> Currently, for my own needs, COMPONENTS is the single missing piece. If
> it would be possible to add this, it would certainly be very useful.
>
> I'd be happy to provide a patch for COMPONENTS if you like.
I think the reason that the macro does not support COMPONENTS is that I
don't understand how COMPONENTS is supposed to work. For example:
find_package(Qt5 COMPONENTS Widgets)
# Is Qt5_FOUND set to true or false?
find_package(Qt5 COMPONENTS Quick)
# ... what about now?
What does _FOUND ever mean if the find_package is called multiple
times with different components? Does it mean anything?
Should CMake disallow multiple use of find_package with the same package and
with different components?
If user code has
find_package(Qt5 COMPONENTS Widgets)
find_package(FooBar REQUIRED)
and FooBarConfig.cmake has
find_dependency(Qt5 COMPONENTS Quick)
then the if(Qt5_FOUND) in the macro will mean it will not try to find the
Quick component.
So, there may be a need to establish a convention such as
__FOUND
and check that for each component in the macro.
You would also need to think through everything and find out whether that is
really a full solution. I haven't thought more deeply about it than the
above.
Note that the above example with Qt 5 can not currently happen in practice
because the find_dependency macro does not support COMPONENTS, so config
files must instead use
find_dependency(Qt5Quick)
if they want to use the macro currently. This is a good thing.
So, I would like to know if
* adding COMPONENTS support to the find_dependency macro is the right thing
to do
* or if the right thing to do is to move away from support for COMPONENTS
generally because of its messy semantics
* or if find_package should change to not support multiple calls for the
same package.
I really don't know the answer - I think someone needs to do deep thinking
about it.
I would prefer the macro not be changed before that deep thinking about
COMPONENTS generally is done.
Thanks,
Steve.
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