Q:

Protocol buffer3 and json

Protocol buffer v3 claims, that library is json friendly (https://developers.google.com/protocol-buffers/docs/proto3#json), but I cannot find how to achieve get that mapping. Should I add some plugin, or some option into protoc, or call something special instead SerializeTo/ParseFrom?
Is it someone who use that feature?

A:

I'm using Protobuf 3.3.0, which does have a built-in JSON serializer and parser. You can use 2 functions from google/protobuf/util/json_util.h called MessageToJsonString() and JsonStringToMessage() to make your C++ generated Message objects go to and from JSON respectively.
Here's a simple test that uses them:
test-protobuf.proto:
syntax = "proto3";

message SearchRequest {
  string query = 1;
  int32 page_number = 2;
  int32 result_per_page = 3;
}

test-protobuf.cpp:
#include <iostream>
#include <google/protobuf/util/json_util.h>

#include "test-protobuf.pb.h"

int main()
{
  std::string json_string;
  SearchRequest sr, sr2;

  // Populate sr.
  sr.set_query(std::string("Hello!"));
  sr.set_page_number(1);
  sr.set_result_per_page(10);

  // Create a json_string from sr.
  google::protobuf::util::JsonPrintOptions options;
  options.add_whitespace = true;
  options.always_print_primitive_fields = true;
  options.preserve_proto_field_names = true;
  MessageToJsonString(sr, &json_string, options);

  // Print json_string.
  std::cout << json_string << std::endl;

  // Parse the json_string into sr2.
  google::protobuf::util::JsonParseOptions options2;
  JsonStringToMessage(json_string, &sr2, options2);

  // Print the values of sr2.
  std::cout
    << sr2.query() << ", "
    << sr2.page_number() << ", "
    << sr2.result_per_page() << std::endl
  ;

  return 0;
}

You can compile these files (assuming that you have protobuf, a compiler, and CMake installed) by using the following CMakeLists.txt file (tested on Windows).
cmake_minimum_required(VERSION 3.8)

project(test-protobuf)

find_package(Protobuf REQUIRED)

# Use static runtime for MSVC
if(MSVC)
  foreach(flag_var
      CMAKE_CXX_FLAGS CMAKE_CXX_FLAGS_DEBUG CMAKE_CXX_FLAGS_RELEASE
      CMAKE_CXX_FLAGS_MINSIZEREL CMAKE_CXX_FLAGS_RELWITHDEBINFO)
    if(${flag_var} MATCHES "/MD")
      string(REGEX REPLACE "/MD" "/MT" ${flag_var} "${${flag_var}}")
    endif(${flag_var} MATCHES "/MD")
  endforeach(flag_var)
endif(MSVC)

protobuf_generate_cpp(test-protobuf-sources test-protobuf-headers
  "${CMAKE_CURRENT_LIST_DIR}/test-protobuf.proto"
)

list(APPEND test-protobuf-sources
  "${CMAKE_CURRENT_LIST_DIR}/test-protobuf.cpp"
)

add_executable(test-protobuf ${test-protobuf-sources} ${test-protobuf-headers})
target_include_directories(test-protobuf
  PUBLIC
    ${PROTOBUF_INCLUDE_DIRS}
    ${CMAKE_CURRENT_BINARY_DIR}
)
target_link_libraries(test-protobuf
  ${PROTOBUF_LIBRARIES}
)

Assuming that CMakeLists.txt, test-protobuf.proto, and test-protobuf.cpp are in the same directory, here are the commands to compile and run them on Windows with Visual Studio 15 2017 and 64-bit protobuf libraries.
mkdir build
cd build
cmake -G "Visual Studio 15 2017 Win64" ..
cmake --build . --config Release
Release/test-protobuf

You should see the following output:
{
 "query": "Hello!",
 "page_number": 1,
 "result_per_page": 10
}

Hello!, 1, 10

