union

Overview ¶

The onlyone plugin generates code for the onlyone extension. All fields must be nullable and only one of the fields may be set, like a union. Two methods are generated

GetValue() interface{}

and

SetValue(v interface{}) (set bool)

These provide easier interaction with a onlyone.

The onlyone extension is not called union as this causes compile errors in the C++ generated code. There can only be one ;)

It is enabled by the following extensions:

• onlyone
• onlyone_all

The onlyone plugin also generates a test given it is enabled using one of the following extensions:

• testgen
• testgen_all

Lets look at:

github.com/gogo/protobuf/test/example/example.proto

Btw all the output can be seen at:

github.com/gogo/protobuf/test/example/*

The following message:

  message U {
	  option (gogoproto.onlyone) = true;
	  optional A A = 1;
	  optional B B = 2;
  }

given to the onlyone plugin, will generate code which looks a lot like this:

func (this *U) GetValue() interface{} {
	if this.A != nil {
		return this.A
	}
	if this.B != nil {
		return this.B
	}
	return nil
}

func (this *U) SetValue(value interface{}) bool {
	switch vt := value.(type) {
	case *A:
		this.A = vt
	case *B:
		this.B = vt
	default:
		return false
	}
	return true
}

and the following test code:

  func TestUUnion(t *testing.T) {
	popr := math_rand.New(math_rand.NewSource(time.Now().UnixNano()))
	p := NewPopulatedU(popr)
	v := p.GetValue()
	msg := &U{}
	if !msg.SetValue(v) {
		t.Fatalf("Union: Could not set Value")
	}
	if !p.Equal(msg) {
		t.Fatalf("%#v !Union Equal %#v", msg, p)
	}
  }