[Development] Allowed C++11 features [was: Re: Re: MSVC2012 in CI]

[Sean Harmer]

On Monday 21 March 2016 12:23:44 Poenitz Andre wrote:
> Marc Mutz <marc.mutz at kdab.com> wrote:
> > On Monday 21 March 2016 12:07:24 Poenitz Andre wrote:
> > > Marc Mutz <marc.mutz at kdab.com> wrote:
> > > > I said then and I repeat it now that imho template aliases are not
> > > > interesting for library development. In particular, I don't think
> > > > something like
> > > > 
> > > > > > > template<typename T>
> > > > > > > using QNodeCreatedChangePtr =
> > > > > > > QSharedPointer<QNodeCreatedChange<T>>;
> > > > 
> > > > should be part of an API of a library. We have auto to avoid having to
> > > > type this type. The typedefs just hide the (essential) fact that this
> > > > is
> > > > a shared pointer type.
> > > 
> > > Do I understand correctly that you argue that the fact that the actual
> > > type is a shared pointer is so important that it shall not be hidden by
> > > a template alias whereas it would be completely fine to hide everything
> > > related to type by using auto?
> > 
> > Yes, you understood me correctly.
> > 
> > And to answer your implied objection, too:
> > 
> > No, there's no contradiction, because auto is for use in implementations
> > and I was talking explicitly about APIs (which implies API documentation,
> > too). It's one thing to write
> > 
> >     auto o = factory->create();
> > 
> > and quite another to have
> > 
> >    QNodeCreatedChangePtr<Foo> create() [virtual]
> > 
> > (to make up a plausible, but nonsensical example) in the docs.
> > 
> > Thanks,
> > Marc
> 
> I will argue that if user code looks ugly by default, or needs one specific,
> controversial coding style when using an API, the API is bad.
> 
> A possible question here is why there's a need to have the QSharedPointer
> show up in the API at all.
> 
> Is it meant to be a vehicle to provide value semantics? If so, why isn't
> the sharing hidden in the implementation of the object?
> 
> Is it meant to hide unclear ownership and object lifetime? If so, shouldn't
> ownership and lifetime better be clear?

It's to support one-to-many delivery of an event-like object to backends 
potentially running on multiple threads concurrently. The type is not a 
QObject so can't be managed by the QObject parent-child relationship. Using a 
naked pointer goes against modern C++ recommendations and would mean having to 
put in place code to delete the objects after all consumers are done with it.

A QSharedPointer to a QNodeCreatedChange which is a templated type works well 
here and I'm using the create() static to avoid having two allocations instead 
of one per event.

If I can't use the template alias syntax, so be it, I can use the more verbose 
alternative.

Cheers,

Sean
--
Dr Sean Harmer | sean.harmer at kdab.com | Managing Director UK
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