[libvirt] [GSoC] Design ideas for implementing cleanup attribute

[Pavel Hrdina]

On Fri, May 25, 2018 at 12:06:50PM +0200, Andrea Bolognani wrote:
> On Fri, 2018-05-25 at 10:04 +0200, Pavel Hrdina wrote:
> > On Fri, May 25, 2018 at 09:13:51AM +0200, Andrea Bolognani wrote:
> > > However, I realize it might not be possible to register free
> > > functions for a native type without having to introduce something
> > > like
> > > 
> > >   typedef char * virString;
> > > 
> > > thus causing massive churn. How does GLib deal with that?
> > 
> > If you would look into GLib documentation you would see that this
> > design basically copies the one in GLib:
> 
> Sorry, I should have looked up the documentation and implementation
> before asking silly questions. Guess the morning coffee hadn't quite
> kicked in yet :/
> 
> >     GLib                libvirt
> > 
> >     g_autofree          VIR_AUTOFREE
> >     g_autoptr           VIR_AUTOPTR
> >     g_auto              VIR_AUTOCLEAR
> 
> For what it's worth, I think VIR_AUTOCLEAR is a much better name
> than g_auto :)
> 
> > In GLib you are using them like this:
> > 
> > g_autofree char *string = NULL;
> > g_autoptr(virDomain) dom = NULL;
> > g_auto(virDomain) dom = { 0 };
> > 
> > So yes it would require to introduce a lot of typedefs for basic types
> > and that is not worth it.
> 
> I'm not sure we would need so many typedefs, but there would
> certainly be a lot of churn involved.
> 
> Personally, I'm not so sure it wouldn't be worth the effort,
> but it's definitely something that we can experiment with it at
> a later time instead of holding up what's already a pretty
> significant improvement.
> 
> > In libvirt we would have:
> > 
> > VIR_AUTOFREE char *string = NULL;
> > VIR_AUTOPTR(virDomainPtr) dom = NULL;
> > VIR_AUTOCLEAR(virDomain) dom = { 0 };
> > 
> > If you notice the difference, in libvirt we can use virDomainPtr
> > directly because we have these typedefs, in GLib macro
> > G_DEFINE_AUTOPTR_CLEANUP_FUNC creates similar typedef.
> 
> While I'm not a fan of our *Ptr typedefs in general, I guess this
> time I'm glad we have them because VIR_AUTOPTR() doesn't hide the
> fact that what you're declaring is a pointer; that is, the macro
> argument is also exactly the type of the variable.

So let's make a summary of how it could look like:

VIR_AUTOFREE(char *) string = NULL;
VIR_AUTOPTR(virDomainPtr) vm = NULL;
VIR_AUTOCLEAR(virDomain) dom = { 0 };

VIR_DEFINE_AUTOFREE_FUNC(virDomainPtr, virDomainFree);
VIR_DEFINE_AUTOCLEAR_FUNC(virDomain, virDomainClear);

Note: don't take the types and function names as something that actually
exists and be used like that, it's just an example to show how it would
work :).

Pavel
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