google protobuf (c++) 学习

probuf是goole推出的微型RPC框架，这里记录下安装测试。
参考文章：
Google Protocol Buffers浅析（一）

一、Ubuntu 18.04安装 C++ Protocol

首先参考官方README文档进行安装：
前提

 sudo apt-get install autoconf automake libtool curl make g++ unzip

当然可以在如下网站进行源码下载离线安装。
https://github.com/protocolbuffers/protobuf/releases/latest
这里通过git直接在终端进行安装：

$ git clone https://github.com/protocolbuffers/protobuf.git
$ cd protobuf
$ git submodule update --init --recursive
$ ./autogen.sh

$ ./configure
$ make
$ make check
$ sudo make install
$ sudo ldconfig # refresh shared library cache.

默认安装在/usr/local下面，可以进行查看库文件。

pkg-config --cflags protobuf         # print compiler flags
pkg-config --libs protobuf           # print linker flags
pkg-config --cflags --libs protobuf  # print both

查看安装版本：

 protoc --version

二、官方examples代码编译测试

参考官方examples

通常，编写一个protocol buffers应用需要经历如下三步：

 1、定义消息格式文件，最好以proto作为后缀名

 2、使用Google提供的protocol buffers编译器来生成代码文件，一般为.h和.cc文件，主要是对消息格式以特定的语言方式描述

 3、使用protocol buffers库提供的API来编写应用程序  

首先编写addressbook.proto文件。

// See README.txt for information and build instructions.
//
// Note: START and END tags are used in comments to define sections used in
// tutorials.  They are not part of the syntax for Protocol Buffers.
//
// To get an in-depth walkthrough of this file and the related examples, see:
// https://developers.google.com/protocol-buffers/docs/tutorials

// [START declaration]
syntax = "proto3";
package tutorial;

import "google/protobuf/timestamp.proto";
// [END declaration]

// [START java_declaration]
option java_package = "com.example.tutorial";
option java_outer_classname = "AddressBookProtos";
// [END java_declaration]

// [START csharp_declaration]
option csharp_namespace = "Google.Protobuf.Examples.AddressBook";
// [END csharp_declaration]

// [START messages]
message Person {
  string name = 1;
  int32 id = 2;  // Unique ID number for this person.
  string email = 3;

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

  message PhoneNumber {
    string number = 1;
    PhoneType type = 2;
  }

  repeated PhoneNumber phones = 4;

  google.protobuf.Timestamp last_updated = 5;
}

// Our address book file is just one of these.
message AddressBook {
  repeated Person people = 1;
}
// [END messages]

之后对其进行编译，产生对应的xxx.pb.cc和xxx.pb.h文件。

protoc -I=./ --cpp_out=./ addressbook.proto

最后,

1. 编写add_person.cc文件，该应用程序实现控制台输入person的相关信息并写入到文本文件：

// See README.txt for information and build instructions.

#include <ctime>
#include <fstream>
#include <google/protobuf/util/time_util.h>
#include <iostream>
#include <string>

#include "addressbook.pb.h"

using namespace std;

using google::protobuf::util::TimeUtil;

// 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;
    }
  }
  *person->mutable_last_updated() = TimeUtil::SecondsToTimestamp(time(NULL));
}

// 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;
}

2. 编写list_people.cc文件，该应用程序实现将add_person.cc写入的文本文件进行可视化读取。

// See README.txt for information and build instructions.

#include <fstream>
#include <google/protobuf/util/time_util.h>
#include <iostream>
#include <string>

#include "addressbook.pb.h"

using namespace std;

using google::protobuf::util::TimeUtil;

// 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.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;
        default:
          cout << "  Unknown phone #: ";
          break;
      }
      cout << phone_number.number() << endl;
    }
    if (person.has_last_updated()) {
      cout << "  Updated: " << TimeUtil::ToString(person.last_updated()) << 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;
}

分别进行编译，产生对应的可执行文件

c++ add_person.cc addressbook.pb.cc -o add_person_cpp `pkg-config --cflags --libs protobuf`
c++ list_people.cc addressbook.pb.cc -o list_people_cpp `pkg-config --cflags --libs  protobuf`

分别运行

./add_person_cpp address_book_file
./list_people_cpp address_book_file

之后对写入的文件进行读取

三、protobuf文件生成分析

参考：

Language Guide (proto3)
C++ Generated Code

一般每一个message对应生成一个类，枚举型对应在高级语言中仍是一个枚举型，值从0开始。
对于一个类型为string的变量，主要产生如下方法：

// string number = 1;
  void clear_number();
  const std::string& number() const;
  void set_number(const std::string& value);
  void set_number(std::string&& value);
  void set_number(const char* value);
  void set_number(const char* value, size_t size);
  std::string* mutable_number();
  std::string* release_number();
  void set_allocated_number(std::string* number);

具体分析参考引文的博主文章：

可以看出，对于每个字段会生成一个has函数(has_number)、clear清除函数(clear_number)、set函数(set_number)、get函数(number和mutable_number)。这儿解释下get函数中的两个函数的区别，对于原型为const std::string &number() const的get函数而言，返回的是常量字段，不能对其值进行修改。但是在有一些情况下，对字段进行修改是必要的，所以提供了一个mutable版的get函数，通过获取字段变量的指针，从而达到改变其值的目的。

而对于字段修饰符为repeated的字段生成的函数，则稍微有一些不同，如phone字段，则编译器会为其产生如下的代码：

// repeated .tutorial.Person.PhoneNumber phones = 4;
  int phones_size() const;
  void clear_phones();
  ::tutorial::Person_PhoneNumber* mutable_phones(int index);
  ::PROTOBUF_NAMESPACE_ID::RepeatedPtrField< ::tutorial::Person_PhoneNumber >*
      mutable_phones();
  const ::tutorial::Person_PhoneNumber& phones(int index) const;
  ::tutorial::Person_PhoneNumber* add_phones();
  const ::PROTOBUF_NAMESPACE_ID::RepeatedPtrField< ::tutorial::Person_PhoneNumber >&
      phones() const;

可以看出，set函数变成了add函数，这个其实很好理解。上面也说过，repeated修饰的字段在高级语言中的实现可能是个数组或动态数组，所以当然通过添加的方式来加入新的字段值。而且get函数也变化很大，这个也不用多说了。

另外，google protobuf提供了常用于调试输出的函数

virtual void CopyFrom(const Message & from); //Make this message int a copy of the given message.
virtual void MergeFrom(const Message & from); //Merge the fields from the given message into this message.