How to handle inheritance in Google Protocol Buffers 3.0?
Java equivalent code:
public class Bar {
String name;
}
public class Foo extends Bar {
String id;
}
What would be Proto equivalent code?
message Bar {
string name = 1;
}
message Foo {
string id = 2;
}
Protocol Buffers does not support inheritance. Instead, consider using composition:
message Foo {
Bar bar = 1;
string id = 2;
}
However, that said, there is a trick you can use which is like inheritance -- but which is an ugly hack, so you should only use it with care. If you define your message types like:
message Bar {
string name = 1;
}
message Foo {
string name = 1;
string id = 2;
}
These two types are compatible, because Foo contains a superset of the fields of Bar. This means if you have an encoded message of one type, you can decode it as the other type. If you try to decode a Bar as type Foo, the field id will not be set (and will get its default value). If you decode a Foo as type Bar, the field id will be ignored. (Notice that these are the same rules that apply when adding new fields to a type over time.)
You can possibly use this to implement something like inheritance, by having several types all of which contain a copy of the fields of the "superclass". However, there are a couple big problems with this approach:
Foo to type Bar, you have to serialize and re-parse; you can't just cast. This can be inefficient.
See the Protocol Buffer Basics tutorial:
Don't go looking for facilities similar to class inheritance, though – protocol buffers don't do that.
