How to serialise protobuf enum values for plain-text / prototxt storage?

Question

I can successfully serialise other fields to ASCII using google::protobuf::TextFormat::Print, however my enums don't show up. What am I doing wrong?

Code as follows:

main.cpp

#include <iostream>
#include <filesystem>
#include <fcntl.h>
#include <unistd.h>

#include <google/protobuf/io/coded_stream.h>
#include <google/protobuf/io/zero_copy_stream_impl.h>
#include <google/protobuf/text_format.h>

#include "example.pb.h"


bool pb_SaveToFile( const std::filesystem::path      File,
                      const google::protobuf::Message &Message )
{
    int fd = open ( File.string ( ).c_str ( ), O_WRONLY | O_CREAT | O_TRUNC, 0666 );

    if ( fd == - 1 )
    {
        return false;
    }

    google::protobuf::io::FileOutputStream * output =
        new google::protobuf::io::FileOutputStream ( fd );

    bool                                     success =
        google::protobuf::TextFormat::Print ( Message, output );

    delete output;

    close ( fd );

    return success;
}



int main(int argc, char *argv[])
{

    std::filesystem::path fullPath = "saved.prototxt";

    pbtest::PbTestMessage tm;


    // Timestamp
    auto ts = tm.mutable_timestamptest();

    ts->set_seconds(time(NULL));
    ts->set_nanos(0);


    // Enum
    tm.set_enumtest(::pbtest::enumTestValues::ENUM_ZERO);

    // String
    tm.set_stringtest("string test!");


    // Save it
    pb_SaveToFile(fullPath, tm);


    std::cout << "Done!" << std::endl;
    return 0;
}

example.proto

syntax = "proto3";

import "google/protobuf/timestamp.proto";

package pbtest;


enum enumTestValues {
    ENUM_ZERO = 0;
    ENUM_ONE = 1;
  }


message PbTestMessage {

    google.protobuf.Timestamp timestampTest = 1;

    enumTestValues enumTest = 2;

    string stringTest = 3;
}

CMakeLists.txt

cmake_minimum_required(VERSION 2.8.9)

project (protobuf-example)

find_package(Protobuf REQUIRED)

include_directories(${Protobuf_INCLUDE_DIRS})
include_directories(${CMAKE_CURRENT_BINARY_DIR})
protobuf_generate_cpp(PROTO_SRCS PROTO_HDRS example.proto)

add_executable(protobuf-example main.cpp ${PROTO_SRCS} ${PROTO_HDRS})
target_link_libraries(protobuf-example ${Protobuf_LIBRARIES})

target_compile_features(protobuf-example PRIVATE cxx_std_17)

If I run this, the output saved to saved.prototxt is as follows:

timestampTest {
  seconds: 1673886921
}
stringTest: "string test!"

How do I get my enum to save to my file?

Does it work if you use `ENUM_ONE` instead? In proto3, the wire format doesn't distinguish between "field is absent" and "field has default value"; default values are stripped from representation. — Igor Tandetnik, Jan 16 '23 at 17:59
This works @IgorTandetnik - thanks! ```timestampTest { seconds: 1673892775 } enumTest: ENUM_ONE stringTest: "string test!"``` — Brian, Jan 16 '23 at 18:14

score 0 · Answer 1 · answered Jan 18 '23 at 10:02

As answered by @Igor Tandetnik in the comments, this is actually a feature. If you try to print the default value of the enum it gets omitted.

Changing my code to:

tm.set_enumtest(::pbtest::enumTestValues::ENUM_ONE);

results in the following output:

timestampTest {
  seconds: 1673892775
}
enumTest: ENUM_ONE
stringTest: "string test!"

How to serialise protobuf enum values for plain-text / prototxt storage?

1 Answers1