protobuf 序列化库

https://developers.google.cn/protocol-buffers/docs/cpptutorial

简介

首先编写.proto文件定义数据结构，然后用protobuf工具生成对应编程语言的序列化和反序列化代码。

随着数据格式版本的更新，protobuf会自动处理对旧格式的兼容。

定义 .proto 文件

syntax = "proto2"

package tutorial

message Person {
  optional string name = 1;
  optional int32 id = 2;
  optional string email = 3;

  enum PhoneType {
    MOBILE = 0;
    HOME = 1;
    WORK = 2;
  }

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

  repeated PhoneNumber phones = 4;
}

message AddressBook {
  repeated Person people = 1;
}

变量后的=1、=2是为每个元素定义唯一标识（tag）。

repeated表示该字段可出现零次或多次。

optional表示该字段是可选的，假如原来的格式是有这个字段的，后来去掉了，发送方使用新格式的程序发送，接收方使用旧程序解析时没有解析出该字段，则该字段会取默认值，方便兼容旧格式。

required相对于optional表示该字段是必需的，对于以上例子，当解析时没有发现该字段将解析失败。

编译 .proto 文件

protoc -I=$SRC_DIR --cpp_out=$DST_DIR $SRC_DIR/addressbook.proto

该命令将生成C++源文件：addressbook.pb.h、addressbook.pb.cc。

protobuf 将为 .proto 文件中的每个 message 生成一个类。

部分成员的相关函数如下：

// name
  inline bool has_name() const;
  inline void clear_name();
  inline const ::std::string& name() const;
  inline void set_name(const ::std::string& value);
  inline void set_name(const char* value);
  inline ::std::string* mutable_name();

  // id
  inline bool has_id() const;
  inline void clear_id();
  inline int32_t id() const;
  inline void set_id(int32_t value);

  // email
  inline bool has_email() const;
  inline void clear_email();
  inline const ::std::string& email() const;
  inline void set_email(const ::std::string& value);
  inline void set_email(const char* value);
  inline ::std::string* mutable_email();

  // phones
  inline int phones_size() const;
  inline void clear_phones();
  inline const ::google::protobuf::RepeatedPtrField< ::tutorial::Person_PhoneNumber >& phones() const;
  inline ::google::protobuf::RepeatedPtrField< ::tutorial::Person_PhoneNumber >* mutable_phones();
  inline const ::tutorial::Person_PhoneNumber& phones(int index) const;
  inline ::tutorial::Person_PhoneNumber* mutable_phones(int index);
  inline ::tutorial::Person_PhoneNumber* add_phones();

序列化和反序列化

bool SerializeToString(string* output) const;
bool ParseFromString(const string& data);

bool SerializeToOstream(ostream* output);
bool ParseFromIstream(istream* input);

Writing A Message

#include <iostream>
#include <fstream>
#include <string>
#include "addressbook.pb.h"
using namespace std;

// This function fills in a Person message based on user input.
void PromptForAddress(tutorial::Person* person) {
  cout << "Enter person ID number: ";
  int id;
  cin >> id;
  person->set_id(id);
  cin.ignore(256, '\n');

  cout << "Enter name: ";
  getline(cin, *person->mutable_name());

  cout << "Enter email address (blank for none): ";
  string email;
  getline(cin, email);
  if (!email.empty()) {
    person->set_email(email);
  }

  while (true) {
    cout << "Enter a phone number (or leave blank to finish): ";
    string number;
    getline(cin, number);
    if (number.empty()) {
      break;
    }

    tutorial::Person::PhoneNumber* phone_number = person->add_phones();
    phone_number->set_number(number);

    cout << "Is this a mobile, home, or work phone? ";
    string type;
    getline(cin, type);
    if (type == "mobile") {
      phone_number->set_type(tutorial::Person::MOBILE);
    } else if (type == "home") {
      phone_number->set_type(tutorial::Person::HOME);
    } else if (type == "work") {
      phone_number->set_type(tutorial::Person::WORK);
    } else {
      cout << "Unknown phone type.  Using default." << endl;
    }
  }
}

// Main function:  Reads the entire address book from a file,
//   adds one person based on user input, then writes it back out to the same
//   file.
int main(int argc, char* argv[]) {
  // Verify that the version of the library that we linked against is
  // compatible with the version of the headers we compiled against.
  GOOGLE_PROTOBUF_VERIFY_VERSION;

  if (argc != 2) {
    cerr << "Usage:  " << argv[0] << " ADDRESS_BOOK_FILE" << endl;
    return -1;
  }

  tutorial::AddressBook address_book;

  {
    // Read the existing address book.
    fstream input(argv[1], ios::in | ios::binary);
    if (!input) {
      cout << argv[1] << ": File not found.  Creating a new file." << endl;
    } else if (!address_book.ParseFromIstream(&input)) {
      cerr << "Failed to parse address book." << endl;
      return -1;
    }
  }

  // Add an address.
  PromptForAddress(address_book.add_people());

  {
    // Write the new address book back to disk.
    fstream output(argv[1], ios::out | ios::trunc | ios::binary);
    if (!address_book.SerializeToOstream(&output)) {
      cerr << "Failed to write address book." << endl;
      return -1;
    }
  }

  // Optional:  Delete all global objects allocated by libprotobuf.
  google::protobuf::ShutdownProtobufLibrary();

  return 0;
}

Reading A Message

#include <iostream>
#include <fstream>
#include <string>
#include "addressbook.pb.h"
using namespace std;

// Iterates though all people in the AddressBook and prints info about them.
void ListPeople(const tutorial::AddressBook& address_book) {
  for (int i = 0; i < address_book.people_size(); i++) {
    const tutorial::Person& person = address_book.people(i);

    cout << "Person ID: " << person.id() << endl;
    cout << "  Name: " << person.name() << endl;
    if (person.has_email()) {
      cout << "  E-mail address: " << person.email() << endl;
    }

    for (int j = 0; j < person.phones_size(); j++) {
      const tutorial::Person::PhoneNumber& phone_number = person.phones(j);

      switch (phone_number.type()) {
        case tutorial::Person::MOBILE:
          cout << "  Mobile phone #: ";
          break;
        case tutorial::Person::HOME:
          cout << "  Home phone #: ";
          break;
        case tutorial::Person::WORK:
          cout << "  Work phone #: ";
          break;
      }
      cout << phone_number.number() << endl;
    }
  }
}

// Main function:  Reads the entire address book from a file and prints all
//   the information inside.
int main(int argc, char* argv[]) {
  // Verify that the version of the library that we linked against is
  // compatible with the version of the headers we compiled against.
  GOOGLE_PROTOBUF_VERIFY_VERSION;

  if (argc != 2) {
    cerr << "Usage:  " << argv[0] << " ADDRESS_BOOK_FILE" << endl;
    return -1;
  }

  tutorial::AddressBook address_book;

  {
    // Read the existing address book.
    fstream input(argv[1], ios::in | ios::binary);
    if (!address_book.ParseFromIstream(&input)) {
      cerr << "Failed to parse address book." << endl;
      return -1;
    }
  }

  ListPeople(address_book);

  // Optional:  Delete all global objects allocated by libprotobuf.
  google::protobuf::ShutdownProtobufLibrary();

  return 0;
}

扩展 .proto 数据格式

兼容旧数据格式的原则：

• 不能修改已有字段的唯一标识符（tag）
• 不能添加或删除任何required字段
• 可以删除optional或repeated字段
• 可以添加新的optional或repeated字段，但tag编号必须是全新的未被使用过的

与 json 互相转换

#include <google/protobuf/util/json_util.h>
// namespace google::protobuf::util
util::Status MessageToJsonString(const Message &message, std::string *output);
util::Status JsonStringToMessage(StringPiece input, Message *message);