return &structValue{
		Fields:   []*Field{},
		fieldMap: map[string]*Field{},
	}
}

type structValue struct {
	Fields   []*Field
	fieldMap map[string]*Field
}

// AddField appends a MapField to the MapValue and indexes the field by name.
func (sv *structValue) AddField(field *Field) {
	sv.Fields = append(sv.Fields, field)
	sv.fieldMap[field.Name] = field
}