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Re: [Qemu-devel] [PATCH v7 03/15] qapi: Require all branches of flat uni

From: Markus Armbruster
Subject: Re: [Qemu-devel] [PATCH v7 03/15] qapi: Require all branches of flat union enum to be covered
Date: Tue, 28 Jun 2016 09:57:15 +0200
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/24.5 (gnu/linux)

Eric Blake <address@hidden> writes:

> On 06/14/2016 07:46 AM, Eric Blake wrote:
>> On 06/14/2016 07:24 AM, Markus Armbruster wrote:
>>> Eric Blake <address@hidden> 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 <address@hidden>
>>> 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.

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