Protobuf 与 Json 数据格式转换

以下为接下来要使用的 example.proto 文件内容

syntax = "proto3";

package example;

message Hello {
  string name = 1;
  int32 times = 2;
}

在查找 Node.js 下 Protobuf 使用相关资料的过程中，发现 Node.js 可以直接载入 proto 文件，然后直接获取 Protobuf 编解码对象进行 Protobuf 的编解码，如下所示：

var ProtoBuf = require("protobufjs");
var dgram = require('dgram');
var server = dgram.createSocket('udp4');

var builder = ProtoBuf.loadProtoFile("./cover.helloworld.proto"),
    Cover = builder.build("cover"),
    HelloCoverReq = Cover.helloworld.helloCoverReq;
    HelloCoverRsp = Cover.helloworld.helloCoverRsp;

引用自 在NodeJS中玩转Protocol Buffer - 腾讯Web前端 IMWeb 团队社区 | blog | 团队博客

这就是动态语言的优势，而且 Javascript 原生支持 Json，用来演示再合适不过。

以下为完整的 Protobuf 与 Json 动态互转示例代码：

"use strict";

const ProtoBuf = require("protobufjs");
const util = require('util');
const assert = require('assert');

ProtoBuf.load("./example.proto").then((root) => {
    const Hello = root.lookup('example.Hello');
    const source = {
        name: "world",
        times: 100
    };
    const hello = Hello.create(source);
    const buffer = Hello.encode(hello).finish();
    const destination = Hello.decode(buffer);
    assert(JSON.stringify(source) == JSON.stringify(destination));

    console.log({
        source: JSON.stringify(source),
        destination: JSON.stringify(destination)
    });
});

运行结果

$ node /home/tangxinfa/Examples/learn_node/protobuf_json.js
{ source: '{"name":"world","times":100}',
  destination: '{"name":"world","times":100}' }

C++ 是静态类型的语言，肯定不如 Node.js 那么方便，主要用到 Protobuf 的反射（Reflection）机制，具体实现见：

• HaustWang/pb2json: protobuf message与json互转，使用C++11特性
• https://github.com/shafreeck/pb2json
• shramov/json2pb: Python/C++ implementation of JSON to/from Protobuf convertor

HaustWang/pb2json 实现了静态和动态 Protobuf 与 Json 互转，这里以它写一个完整的示例

• 拉取 HaustWang/pb2json 代码

  git clone https://github.com/HaustWang/pb2json.git
  

• protobuf_json.cpp

  #include "pb2json/byReflection/pb2json.h"
  #include <google/protobuf/compiler/importer.h>
  #include <google/protobuf/dynamic_message.h>
  #include <iostream>
  #include <string>
  #include <cassert>
  using std::string;
  
  class ErrorCollector: public google::protobuf::compiler::MultiFileErrorCollector {
  public:
    ErrorCollector()
      :warnings_(0), errors_(0)
    {}
    virtual ~ErrorCollector() {}
    virtual void AddError(const string& filename, int line, int column,
                          const string& message) {
      ++errors_;
      std::cerr << filename << ":" << line << ":" << column << ": " << message << std::endl;
    }
  
    virtual void AddWarning(const string& filename, int line, int column,
                            const string& message) {
      ++warnings_;
      std::cerr << filename << ":" << line << ":" << column << ": " << message << std::endl;
    }
  
    uint errors() const { return errors_; }
    uint warnings() const { return warnings_; }
  
  private:
    uint errors_;
    uint warnings_;
  };
  
  
  int main(int argc, char *argv[]) {
    // parse proto file with protobuf compiler module,
    // also can generate self-described protobuf message with protoc,
    // then we can get file descriptor without protobuf compiler,
    // see https://developers.google.com/protocol-buffers/docs/techniques#self-description
    google::protobuf::compiler::DiskSourceTree sourceTree;
    sourceTree.MapPath("", ".");
    ErrorCollector collector;
    google::protobuf::compiler::Importer importer(&sourceTree, &collector);
    const char* proto_file = "example.proto";
    const google::protobuf::FileDescriptor* fileDescriptor = importer.Import(proto_file);
    if (fileDescriptor == NULL) {
      std::cerr << "proto file " << proto_file << " import failed" << std::endl;
      return EXIT_FAILURE;
    }
  
    const google::protobuf::Descriptor* descriptor = fileDescriptor->FindMessageTypeByName("Hello");
    assert(descriptor);
  
    google::protobuf::DynamicMessageFactory factory;
    const google::protobuf::Message* prototype = factory.GetPrototype(descriptor);
    assert(prototype);
    google::protobuf::Message* message = prototype->New();
    assert(message);
  
    string source = "{\"name\":\"world\",\"times\":100}";
    bool ok = Pb2Json::Json2Message(nlohmann::json::parse(source), *message);
    assert(ok);
  
    nlohmann::json destinationJson;
    Pb2Json::Message2Json(*message, destinationJson);
    string destination = destinationJson.dump();
  
    std::cout << "source: " << source << "\tdestination: " << destination << std::endl;
    assert(source == destination);
  
    return EXIT_SUCCESS;
  }
  

  • 编译

    g++ -O2 protobuf_json.cpp ./pb2json/byReflection/pb2json.cpp -lm -o protobuf_json -lprotobuf
    

  • 运行

    $ ./protobuf_json
    source: {"name":"world","times":100}    destination: {"name":"world","times":100}
    

相关参考

• c++使用Protobuf Message转Json字符串（Json库使用Json cpp） - - CSDN博客
• Protobuf与Json互转 - - CSDN博客
• C++ Protobuf to/from JSON conversion - Stack Overflow
• c++ - How to dynamically build a new protobuf from a set of already defined descriptors? - Stack Overflow
• Google Protocol Buffer 的使用和原理
• C++ API  |  Protocol Buffers  |  Google Developers