Oneof kind vs Enum in protobuf

Question

What's the difference between using an Enum and a oneof kind in protobuf3? As far as I can tell, an Enum restricts the field to be one of a predefined set of values, but so does the oneof kind.

Klenk

Answer 1

Enums are named numbers. You define the names in the enum definition and assign them a value. An enum should always have the value zero it it's set.

enum State {
  A = 0;
  B = 1;
  C = 2;
}

Next you can use this enum in any of your message

message Update {
  State currentState = 1;
  State previousState = 2;
}

A oneof is something very different. It allows you to send different types but only allocate limited memory for them. This as you can only set one of those types at a time. This is similar to an union in C/C++ or a std::variant as of C++17.

Take this example in which we have a message, a integer and double defined in our oneof.

// The message in our oneof
message someMsg {
  // Multiple fields
}

// The message holding our oneof
message msgWithOneof {

  oneof theOneof {
    someMsg msg     = 1;
    int32   counter = 2;
    double  value   = 3;
  }
  // Feel free to add more fields her of before the oneof
}

You are only able to set msg, counter or value at one time. If you set another this will clear the other field.

Assuming an C/C++ implementation the largest field will determine the amount of memory allocated. Say someMsg is the largest, setting a integer or double will not be a problem as they fit in the same space. If you not use a oneof the total memory allocated would be the sum of sizeof(someMsg) + sizeof(int32) + sizeof(double).

There is some overhead to keep track which field has been set. In the google C++ implementation this is a bit in the presence variable. This is similar to fields which are marked optional.

Dissociate

Minor addition to "different types", it also allows you to represent the same type, but with different semantic meaning - for example, msgWithOneof.theOneof could also have an additional double valueDelta = 4; and someMsg criticialMessage = 5; (but at most one of the values in theOneof can have a value at any one time) – Binder

@MarcGravell Excellent remark. It is indeed very well possible to repeat the same type multiple times in a oneof yet allocate memory only once. – Dissociate