Lesson 20 · Proto3 default values - Build a Protobuf Decoder

Proto3 default values

In proto3 an absent scalar field is not missing, it takes a well-defined default, and the decoder must fill those in. Today you make a field that never appeared decode to its type's zero value.

The goal

Fill absent scalar fields with their proto3 default values.

Start here - the target

TO DO

Scenario: An absent field decodes to its default

Givena descriptor with field 1 "name" string, field 2 "age" int32, and field 3 "active" bool

Anda message containing only field 1 with the string "Bob" (bytes 0x0A, 0x03, 0x42, 0x6F, 0x62)

Whenthe message is decoded

Thenname is "Bob", age is 0, and active is false

Andthe defaults are the type zero values: 0 for numbers, false for bool, the empty string for string

Background

Proto3 dropped the notion of an explicitly set field for scalars: on the wire, a scalar equal to its default is simply not written at all. So the decoder cannot treat “absent” as “unknown” - it must supply the default value, which for every proto3 scalar is the type’s zero: 0 for the integer and floating types, false for bool, the empty string for string, empty bytes for bytes, and the zero enum value. A message that carries only name still has an age and an active; they are 0 and false.

This is why proto3 encoders omit default-valued fields to save space, and why two messages can be equal even though one wrote a field and the other did not. Fill defaults after placing the fields that actually appeared, so a present value is never overwritten. Getting this right is what makes the round-trip in a couple of lessons behave: encode drops defaults, decode puts them back, and the structured value is unchanged.

Make it work

// after filling fields that appeared, add defaults for the rest
func applyDefaults(d Descriptor, out map[string]any) {
  for _, fd := range d {
    if _, present := out[fd.Name]; present { continue }
    // out[fd.Name] = zero value for fd.Type: 0, false, "", etc.
  }
}

CheckpointDONE

Absent fields decode to their proto3 defaults. Commit and stop here.