On Oct 6, 2010, at 9:23 , Igor Gatis wrote:
It would be nice to have mapped fields, e.g. key-value pairs.
I think that map support would probably be useful. I've basically
created my own maps in protocol buffers a couple times, either by
using two repeated fields, or a repeated field of a custom "pair"
type. In these cases, it would have been nice to be able to use the
Protocol Buffer as a map directly, rather than needing to transfer the
data to some other object that actually implements the map. I would be
interested to hear the opinion of the Google maintainers. I'm assuming
that there are probably many applications inside Google that exchange
map-like messages.
This would be a big change, although it wouldn't be an impossible one,
I don't think. I think it could be implemented as "syntactic sugar"
over a repeated Pair message. I think the biggest challenge is that
maps are a "higher level" abstraction than repeated fields, which
leads to many design challenges:
* Are the maps ordered or unordered?
* If ordered, how are keys compared? This needs to be consistent
across programming languages.
* If unordered, how are hash values computed? This could result in a
message being parsed and re-serialized differently, if different
languages compute the hashes differently.
* For both, how are "'unknown" fields handled?
* Do the maps support repeated keys?
* If not, what happens when parsing a message with repeated keys?
Other message protocols contain map-like structures: JSON, Thrift, and
Avro. Avro only supports string keys. JSON only supports primitive
keys. Thrift has a similar note about maps:
http://wiki.apache.org/thrift/ThriftTypes
For maximal compatibility, the key type for map should be a basic
type rather than a struct or container type. There are some
languages which do not support more complex key types in their
native map types. In addition the JSON protocol only supports key
types that are base types.
Evan
--
Evan Jones
http://evanjones.ca/
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