Fwd: KF5 CMake usage question

Shaheed Haque srhaque at theiet.org
Sat May 20 18:41:20 BST 2017

Actually, there is one thing about "target CMake"-based KF5 that I
don't quite understand: is there a way to get to the C++ compile flags
needed from CMake? That is, the modern equivalent of Foo_COMPILE_FLAGS
but for target Foo? Even if the general answer is "no", I'm interested
in at least the CMake variables/properties/commands needed to get to
"-fPIC" and "-std=gnu++14".

I'm aware of the target properties
POSITION_INDEPENDENT_CODE and CXX_STANDARD but none of these seem to
be set on targets I have tried.

Perhaps these are only set if somehow the compiler name etc. is specified?

Thanks, Shaheed

On 18 May 2017 at 18:04, Shaheed Haque <srhaque at theiet.org> wrote:
> Hi,
> On 18 May 2017 at 12:51, Andreas Hartmetz <ahartmetz at gmail.com> wrote:
>> On Samstag, 13. Mai 2017 23:48:33 CEST Shaheed Haque wrote:
>>> Hi,
>>> On 13 May 2017 at 22:04, Sven Brauch <mail at svenbrauch.de> wrote:
>>> > Hi,
>>> >
>>> > On 05/13/2017 06:06 PM, Shaheed Haque wrote:
>>> >> The printed output shows that the variable KF5KIO_INCLUDE_DIRS is
>>> >> not
>>> >> set. In poking around, I see references to a (new-to-me)
>>> >> target-based
>>> >
>>> >> system, and using that like this:
>>> > The question is, why do you need to do that?
>> The idea with the target based system aka "Modern CMake" is that you say
>> you want to compile against a library, and that is all you have to do. A
>> library requires you to add an include path for its own headers, include
>> paths for headers of one of its dependencies, and link against a bunch
>> of libraries? All covered by target properties.
>> If for some reason (e.g. handover to an external tool) you need those
>> properties, you can still query them. Under enforced names nonetheless,
> Ack. The problem from the point of view of an automated tool which starts
> with a component called Foo arises ONLY because the target(s) of Foo are
> called FooFoo and FooBar. CMake does not - AFAICS - allow one to query Foo
> and somehow find FooFoo and FooBar inorder to look up FooFoo_INCLUDE_DIRS
> etc.
>>> I'm continuing to experiment with trying to build Python bindings for
>>> KF5. As part of that, the SIP tooling creates C++ wrapper code which
>>> must be compiled for each framework, and for that I need to know the
>>> header file directories. So far, I have simply been hardcoding the
>>> needed paths on my own system, but I now want to move to using
>>> standard CMake-based logic instead.
>>> (In some sense, this might be seen as similar to the stuff that was
>>> added to ECM, but I'm trying for a significantly more automated
>>> approach).
>>> Also, I am trying to feel my way towards a Pythonic build system for
>>> the KF5 bindings (as, roughly speaking, PyQt seems to be doing): in
>>> other words I'm interested in using CMake as a stepping stone, not the
>>> actual build system.
>> I would recommend against that unless you really need to have heavy
>> logic in the build system. A build system's main job is to "solve" a
>> dependency tree - that is the difference between a build system and a
>> script that runs the compiler. The dependency tree enables cheap
>> incremental builds and correct parallel builds. Maybe not that important
>> for bindings, admittedly.
>> Your advantage from using Python must be larger than the overhead from
>> doing your own dependency resolution plus the overhead from the CMake-
>> Python interfacing plus the build-related facilities that CMake has and
>> Python doesn't. Or were you considering scons?
>> PyQt may have chosen Python because qmake sucks, and it needs Python
>> anyway, so it avoids any extra dependencies. I know from experience that
>> you really want to avoid extra dependencies in commercial products.
> /me nods vigourosly.
> I'm not (yet) familair with all the intricacies of the Python build system
> (or CMake for that matter!), but I do see that PyQt has to work quite hard
> to keep its build system working as a Python user might expect. Further, the
> system I am seeking to build has to support more than KF5 (or even KDE). So,
> roughly speaking, the split I am going for is:
> - Keep all platform and system independent code in Python
> - Isolate all platform and system independent logic in CMake
> As I say, I am feeling my way a bit here, but this seems like a
> philosophically justifiable separation. Oh, and to solve the problem of
> finding the targets, I resorted to parsing the CMake files (!!). I can live
> with that hack precisely because by having the split, users of this code who
> are not using it against KF5 will need to replace this CMake part with their
> own anyway.
> (At this point, abstracting CMake away entirely is a minor detail).
> Thanks for the helpful remarks.
> Shaheed
>>> Thus, I'm after the moral equivalents of:
>>> Thanks, Shaheed
>>> > The usual way is to simply call
>>> >
>>> > target_link_libraries(mybinary KF5::KIOCore)
>>> >
>>> > and include paths etc. will be set up for your target automatically.
>>> >
>>> > Best,
>>> > Sven
>> Cheers,
>> Andreas M9

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