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authorKevin Wolf <kwolf@redhat.com>2013-07-08 16:14:21 +0200
committerKevin Wolf <kwolf@redhat.com>2013-07-26 21:10:11 +0200
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qapi: Anonymous unions
The discriminator for anonymous unions is the data type. This allows to have a union type that allows both of these: { 'file': 'my_existing_block_device_id' } { 'file': { 'filename': '/tmp/mydisk.qcow2', 'read-only': true } } Unions like this are specified in the schema with an empty dict as discriminator. For this example you could take: { 'union': 'BlockRef', 'discriminator': {}, 'data': { 'definition': 'BlockOptions', 'reference': 'str' } } { 'type': 'ExampleObject', 'data: { 'file': 'BlockRef' } } Signed-off-by: Kevin Wolf <kwolf@redhat.com> Reviewed-by: Eric Blake <eblake@redhat.com>
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@@ -125,6 +125,31 @@ Resulting in this JSON object:
"lazy-refcounts": true }
+A special type of unions are anonymous unions. They don't form a dictionary in
+the wire format but allow the direct use of different types in their place. As
+they aren't structured, they don't have any explicit discriminator but use
+the (QObject) data type of their value as an implicit discriminator. This means
+that they are restricted to using only one discriminator value per QObject
+type. For example, you cannot have two different complex types in an anonymous
+union, or two different integer types.
+
+Anonymous unions are declared using an empty dictionary as their discriminator.
+The discriminator values never appear on the wire, they are only used in the
+generated C code. Anonymous unions cannot have a base type.
+
+ { 'union': 'BlockRef',
+ 'discriminator': {},
+ 'data': { 'definition': 'BlockdevOptions',
+ 'reference': 'str' } }
+
+This example allows using both of the following example objects:
+
+ { "file": "my_existing_block_device_id" }
+ { "file": { "driver": "file",
+ "readonly": false,
+ 'filename': "/tmp/mydisk.qcow2" } }
+
+
=== Commands ===
Commands are defined by using a list containing three members. The first