Re: [Qemu-devel] [PATCH v7 03/15] qapi: Require all branches of flat union enum to be covered

[Eric Blake <eblake@redhat.com>]

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On 06/14/2016 07:46 AM, Eric Blake wrote:
> On 06/14/2016 07:24 AM, Markus Armbruster wrote:
>> Eric Blake <eblake@redhat.com> writes:
>>
>>> We were previously enforcing that all flat union branches were
>>> found in the corresponding enum, but not that all enum values
>>> were covered by branches.  The resulting generated code would
>>> abort() if the user passes the uncovered enum value.
>>>
>>> Signed-off-by: Eric Blake <eblake@redhat.com>
>>
>> I'd let the cases not mentioned default to the empty type (ample
>> precedence in other languages), but I can live with making the user ask
>> for the empty type explicitly.  But we should then make that less
>> cumbersome than now: you have to define an empty struct type, and use
>> that.  Examples of such hoop-jumping: CpuInfoOther, Abort,
>> NetdevNoneOptions.
> 
> Later in the series, I do just that, so that we can write 'other':{}
> instead of 'other':'CpuInfoOther'.
> 
> What I did not do (but maybe should) is make that short syntax possible
> on simple unions (so that we could do 'abort':{} rather than
> 'abort':'Abort') - and merely distinguish that simple unions cannot
> stick anything within the {}, thus leaving non-empty anonymous branches
> only for discriminated unions.  Can do that as a followup or if this
> series needs a respin.

In fact, simple unions can't omit branches (the enum is generated from
the branch names that are explicitly mentioned); so for symmetry, an
explicit empty branch in a flat union is nicer than an omitted branch.

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


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