可以protobuffer自动填充新创建对象的默认值

Question

I have a problem using google protobuf, I want to create an object with default attribute value:

message Obj {
    required string id = 1[default=123];
}

When serializing the above object:

string s;
Obj obj;
obj.SerializeToString(&s);

it complains: can't serialize becault it's missing required field id

The doc says the "default" value is used for parsing obj from string, not for serialize to string.

Is it possible to create object with default value?

Answer 1

No, you can't set the default value for the in-memory objects.

Defaults are only applied at deserialization time. required makes the reader error if the field is missing. So the combination of required and default is pointless.

If you want to set an in memory default, make a factory function and use that.

Answer 2

You can't serialize the default value, but you can make it an optional field. That way, When you deserialize the stream, you'll get an object where the field is missing and trying to get the value will give you the default value.