Re: [Qemu-devel] [PATCH RFC v2 07/47] qapi: Generate a nicer struct for flat unions

[Eric Blake]

On 07/24/2015 06:01 AM, Markus Armbruster wrote:

>> It might be worth tweaking the generator to output a C comment either
>> here (at the start of the larger struct)...
>>
>>>         char *string;
>>>         EnumOne enum1;
>>
>> ...or here (at the end of the base struct) mentioning that
>> UserDefFlatUnion can be cast into its base struct UserDefUnionBase, to
>> make it easier when reading the generated code to trace back to that
>> "inheritance" relationship.  Right now, there is nothing in the
>> generated UserDefFlatUnion that points you back to the qapi relationship
>> of a base class.  But it's not a show-stopper if you don't like my
>> suggestion.
> 
> I do like it.  Perhaps something like
> 
>     struct UserDefFlatUnion
>     {
>         /* Members inherited from UserDefUnionBase: */
>         char *string;
>         EnumOne enum1;
>         /* Own members: */
>         ...
>     };

What about nested types?  Whatever we pick has to look good when
indirect bases are involved. Suppose we have Base -> Mid -> Derived as a
struct inheritance.  If we generate a comment at both start and end of
visiting a base class (and visit base classes recursively), we'd have
something like:

struct Derived
{
    /* Members inherited from Mid */
    /* Members inherited from Base */
    int one;
    /* End members inherited from Base */
    int two;
    /* End members inherited from Mid */
    /* own members */
    int three;
    ...
};

Don't know if that helps you think about it more, but now I'm just
painting a bikeshed that just adds the tail comment.  Here's the
resulting RFC:

http://thread.gmane.org/gmane.comp.emulators.qemu/353204

and the resulting generated structs:

 struct BlockdevOptionsQcow2 {
-    BlockdevOptionsGenericCOWFormat *base;
+    BlockdevRef *file;
+    /* End fields inherited from BlockdevOptionsGenericFormat. */
+    bool has_backing;
+    BlockdevRef *backing;
+    /* End fields inherited from BlockdevOptionsGenericCOWFormat. */
     bool has_lazy_refcounts;
     bool lazy_refcounts;
...

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org

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