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

[Markus Armbruster <armbru@redhat.com>]

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?

> On the C side, this change would remove Foo.has_member, because
> Foo.member is always valid now.  The input visitor deals with setting
> it.  (Naturally, this means that such defaults are useful only for input
> parameters.)
>
> At least three things are left unimplemented:
>
> First, support for alternate data types.  This is because supporting
> them would mean having to allocate the object in the input visitor, and
> then potentially going through multiple levels of nested types.  In any
> case, it would have been difficult and I do not think there is need for
> such support at this point.
>
> Second, support for null.  The most important reason for this is that
> introspection already uses "'default': null" for "no default, but this
> field is optional".  The second reason is that without support for
> alternate data types, there is not really a point in supporting null.
>
> Third, full support for default lists.  This has a similar reason to the
> lack of support for alternate data types: Allocating a default list is
> not trivial -- unless the list is empty, which is exactly what we have
> support for.
>
> Signed-off-by: Max Reitz <mreitz@redhat.com>