Maps and Oneof

Maps are syntactic sugar

map<K, V> m = N; is NOT a new wire type. It compiles to:

message MapFieldEntry {
    optional K key   = 1;
    optional V value = 2;
}
repeated MapFieldEntry m = N;

That means on the wire a map is just a repeated field of length-delimited sub-messages. A parser that doesn't recognize the field skips it like any other LEN field. Implementations agree that:

• Keys can be any integral or string type (not float, bytes, or message).
• Wire order is unspecified — never rely on iteration order.
• Duplicate keys: the last occurrence wins.
• Maps cannot be repeated, oneof, or have field presence.

Oneof

oneof declares a union — at most one field in the set can be set at a time. Setting one automatically clears any other. On the wire it's just the fields encoded normally; the parser tracks "which field was last seen" for that oneof.

message Update {
    oneof payload {
        string text     = 1;
        bytes  binary   = 2;
        int32  counter  = 3;
    }
}

Key properties:

• Fields in a oneof share field presence semantics even in proto3.
• Setting one clears all others on the sender; on the receiver, the last-encoded field wins.
• Removing a field from a oneof is dangerous — you may inherit junk state from old messages.