# Enums, oneof, maps and Well-Known Types — gRPC

Source: https://www.skillbyai.com/en/grpc/p-types

> Modelling richer data.

## Beyond scalars

**Enums** must have a zero value, conventionally `*_UNSPECIFIED = 0`, which is the default when the field is unset. `repeated` makes a list, `map<string, string>` a map, and **oneof** says exactly one of several fields is set. **Well-known types** from `google/protobuf` cover common needs: `Timestamp`, `Duration`, `FieldMask` (partial updates), `Empty`, `Struct` and wrapper types. Import other proto files to share messages.

## Enums, oneof and well-known types

Protobuf.

```protobuf
import "google/protobuf/timestamp.proto";

enum OrderStatus {
  ORDER_STATUS_UNSPECIFIED = 0;
  ORDER_STATUS_PENDING = 1;
  ORDER_STATUS_PAID = 2;
  ORDER_STATUS_SHIPPED = 3;
}

message Payment {
  oneof method {
    Card card = 1;
    string upi_id = 2;
    string wallet_token = 3;
  }
  google.protobuf.Timestamp paid_at = 4;
  map<string, string> labels = 5;
}

message Card {
  string last4 = 1;
  string network = 2;
}
```

## Prefix enum values with the enum name

Enum values share a namespace within a package, so ORDER_STATUS_PAID avoids clashes with PAYMENT_STATUS_PAID.

**Quiz:** Why should an enum start with an UNSPECIFIED value of 0?

- [ ] Protobuf forbids other numbers
- [x] Zero is the default for unset fields, so it should mean "not set" rather than a real state
- [ ] It makes messages smaller
- [ ] It sorts enums alphabetically

*Answer:* Zero is the default for unset fields, so it should mean "not set" rather than a real state. Defaults should be safe.