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

[Markus Armbruster <armbru@redhat.com>]

Eric Blake <eblake@redhat.com> writes:

> 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.

Fair enough.