Re: [Qemu-devel] [PATCH v4 03/14] qapi: Introduce default values for struct members

[Eric Blake <eblake@redhat.com>]

On 11/21/19 9:07 AM, Markus Armbruster wrote:
> Max Reitz <mreitz@redhat.com> writes:
> 
>> With this change, it is possible to give default values for struct
>> members, as follows:
>>
>>    What you had to do so far:
>>
>>      # @member: Some description, defaults to 42.
>>      { 'struct': 'Foo',
>>        'data': { '*member': 'int' } }
>>
>>    What you can do now:
>>
>>      { 'struct': 'Foo',
>>        'data': { '*member': { 'type': 'int', 'default': 42 } }
> 
> The '*' is redundant in this form.
> 
> Can anyone think of reasons for keeping it anyway?  Against?

Is there ever a reason to allow an optional member but without a 
'default' value?  Or can we just blindly state that if 'default' is not 
present, that is the same as 'default':0/'default':null?

Or, applying your statement further,

'data': { '*a':'int', '*b':'str' }

is shorthand for:

'data': { 'a': { 'type':'int', 'default':0 },
           'b': { 'type':'str', 'default':null } }

So I could live with permitting '*' only in the shorthand form, and 
declaring that it is incompatible with longhand form because the 
existence of a 'default' key in longhand form is evidence that the 
member is therefore optional.

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
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