[Development] CMake integration and system headers

[Mikołaj Siedlarek]

On 07 May 2014, at 10:59, Kurt Pattyn <pattyn.kurt at gmail.com> wrote:
> On 06 May 2014, at 11:40, Mikołaj Siedlarek <msiedlarek at nctz.net> wrote:
> 
>> Hi,
>> 
>> I’m on a quest to enable hack-free all-warnings-enabled building of my project using, among others,
>> Qt and CMake. This requires that compiler treat Qt headers included in my code as “system headers”
>> and not warn me about constructs used in them.
>> 
>> I already contributed -iframework (Apple frameworks counterpart to -isystem) flag support to CMake
>> (https://github.com/Kitware/CMake/pull/100), so with 3.0 version it should correctly handle properly
>> marked include directories on all systems.
>> 
>> The only problem is current CMake integration in Qt uses INTERFACE_INCLUDE_DIRECTORIES
>> target property, when to be provide a system include directory it should use
>> INTERFACE_SYSTEM_INCLUDE_DIRECTORIES. On compilers that do not support -isystem flag
>> CMake falls back to -I, so compatibility would not be a problem.
>> 
>> The problem is current CMake integration in Qt requires version 2.8.3, but
>> INTERFACE_SYSTEM_INCLUDE_DIRECTORIES is only available since 2.8.12. Before I propose
>> a patch I’d like to consult a maintainer of this system, or at least someone more familiar with
>> Qt release process, whether it’s best to add some CMake version checks or version requirement
>> can be bumped with next release.
>> 
>> Please direct me to someone I can discuss this with, or tell me directly that my idea is bad.
> It is a very good idea to have the Qt files treated as system files. I’ve gone to great lengths to
> filter out the warnings from Qt in our CMake files. It would be nice if this was included out-of-the-box.
> One problem that is not fixed by the system flag, is the auto-generated files (moc files).

I thought about that and I believe it could be fixed in CMake’s AUTOMOC functionality. I’ll look into it.

> For those I have submitted a patch that should appear in Qt 5.3.1.
> Personally I would not force version 2.8.12 as AFAIK that version is not available out-of-the-box on Debian
> for instance.

Yes, it seems stable Debian has just been released with CMake 2.8.9, so it’s gonna be here for a while.
A version check then and fallback to INTERFACE_INCLUDE_DIRECTORIES.