Read value of enum extension in protocol buffer

Question

I just asked this question and have resolved to write an extension for enum values in my protocol buffer. However I am having an extremely difficult time actually reading the values back, even with this simple .proto file:

package test;

import "google/protobuf/descriptor.proto";

extend google.protobuf.EnumValueOptions {
  optional string abbr = 54321;
}

message Person {
    required string name = 1;
    required int32 id = 2;
    optional string email = 3;
    enum PhoneType { MOBILE = 0 [(abbr)="Mobile ph"]; HOME = 1 [(abbr)="HomePhone"]; WORK = 2 [(abbr)="Work number"]; }

    message PhoneNumber {
        required string number = 1;
        optional PhoneType type = 2 [default = HOME];
    }

    repeated PhoneNumber phone = 4;
}

message AddressBook {
    repeated Person person = 1;
}

I've been trying these and other variants:

test::Person::PhoneNumber::descriptor()->options().GetExtension(test::abbr);
test::Person::PhoneNumber::descriptor().GetExtension(test::abbr);
test::Person::descriptor()->options().GetExtension(test::abbr);

const google::protobuf::Descriptor* message = test::Person::PhoneNumber::descriptor();
const google::protobuf::Descriptor* desc = phone2.descriptor();
desc->options().GetExtension(test::abbr); //D.N.E.

google::protobuf::MessageOptions opts = message->options();
opts.GetExtension(test::abbr);

const google::protobuf::EnumDescriptor* enm = message->FindEnumTypeByName("PhoneNumber"); // null, not found
google::protobuf::EnumValueOptions opts2 = enm->value(1)->options();
opts2.GetExtension(test::abbr);

test::Person::PhoneNumber::descriptor()->options().GetExtension(test::abbr);

None of the above work - either the method does not exist at all, or there is no matching call to that function signature. I've been going through the documentation for hours to no avail. I know it should be possible, but the only examples are of writing the .proto files, not reading back from them -- How do I do this? A short example would be immensely appreciated. Thanks in advance.

score 5 · Accepted Answer · answered Jul 17 '12 at 00:58

It's a wee bit convoluted, but you need to get the EnumValueDescriptor for the phone type, and then call options().GetExtension(test::abbr) on this.

For example:

test::Person person;
person.set_name("Caol Ila");
person.set_id(1);

test::Person::PhoneNumber *phone = person.add_phone();
phone->set_number("01496 840207");
phone->set_type(test::Person::MOBILE);

phone = person.add_phone();
phone->set_number("01496 840207");
phone->set_type(test::Person::HOME);

phone = person.add_phone();
phone->set_number("01496 840207");
phone->set_type(test::Person::WORK);

phone = person.add_phone();
phone->set_number("01496 840207");

const google::protobuf::EnumDescriptor* enum_desc =
    test::Person::PhoneType_descriptor();
std::string value;

for (int phone_index = 0; phone_index < person.phone_size(); ++phone_index) {
  if (person.phone(phone_index).has_type()) {
    int phone_type = person.phone(phone_index).type();
    value = enum_desc->value(phone_type)->options().GetExtension(test::abbr);
  }
}

Answer is misleading a little bit: to get EnumValueDescriptor from EnumDescriptor one need enum value index and not enum value itself. So `phone_type` must be `phone_type_enum_value_index` and be obtained another way. It doesnt matter when enum values start at zero and increase monotonically without gaps, but there's no guarantee they do. — Dmitry Teslenko, Feb 12 '16 at 08:56
I've found out that there's `FindValueByNumber` alternative to `value` that returns EnumValueDescriptor by enum value and not by index. Last line would be `value = enum_desc->FindValueByNumber(phone_type)->options().GetExtension(test::abbr);` — Dmitry Teslenko, Feb 12 '16 at 09:14

Read value of enum extension in protocol buffer

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