Protocol Buffer学习教程之类库应用(四)
此教程是通过一个简单的示例，给C++开发者介绍一下如何使用protocol buffers编程，主要包括以下几部分：
定义一个.proto文件
如何使用protocol buffer编译器生成C++类文件
如何使用Protocol buffer api读写消息
这并不是分全面的protocol buffer的C++编程手册，更详尽的资料请参见Protocol Buffer Language Guide(https://developers.google.com/protocol-buffers/docs/proto), the C++ API Reference (https://developers.google.com/protocol-buffers/docs/reference/cpp/index.html), the C++ Generated Code Guide (https://developers.google.com/protocol-buffers/docs/reference/cpp-generated), 和the Encoding Reference (https://developers.google.com/protocol-buffers/docs/encoding)
定义一个.proto文件
创建一个文本文件，重命名为“addressbook.proto”，注意扩展名为“.proto”。
syntax = "proto2";
package tutorial;
message Person {
required string name = 1;
required int32 id = 2;
optional string email = 3;
enum PhoneType {
MOBILE = 0;
HOME = 1;
WORK = 2;
}
message PhoneNumber {
required string number = 1;
optional PhoneType type = 2 [default = HOME];
}
repeated PhoneNumber phones = 4;
}
message AddressBook {
repeated Person people = 1;
}
生成类文件
然后通过前面第三部份介绍生成的proto.exe编译器，生成C++类文件，在命令行中，cd到proto.exe根目录，执行以下命令：
protoc -I=$SRC_DIR --cpp_out=$DST_DIR $SRC_DIR/addressbook.proto
然后将生成两个文件addressbook.pb.h和addressbook.pb.cc。打开头文件，能看到以下内容：
// 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();
方法分析
可以看到，每个段都有读、写方法，读方法就是段名，没有加前后缀，而写方法以set_前缀开头，可能有多个对应不同的设置方式，同时还有清空clear_前缀的方法，has_前缀的方法是用于判断某个段是否存在，即赋过值。而mutable_前缀的方法，是直接给返回对象的指针。
对于repeated字段，会发现有更多的接口，repeated字段相当于队列数据，里面存储的是同一类型的数据。
_size后缀的方法，用于统计队列里的元素数量。
同时提供直接通过index索引来读、写元素的方法，段名加元素索引的方法进行读写。写通过索引，返回对应元素的指针，然后通过指针对元素进行修改操作。
还有一个add_前缀方法，这个方法相当于给当前队列增加一个元素空间，它返回新增的元素空间的指针。可以通过此指针对它进行赋值操作。
标准方法
bool IsInitialized() const;检验是否必须赋值的段都赋值了。
string DebugString() const;返回一个对于消息可读的描述，对于调试非常有用。
void CopyFrom(const Person& from);从提供的消息对象复制一个副本。
void Clear();清空消息中的所有段到空的状态。
序列化与反序列化
bool SerializeToString(string* output) const;序列化消息并以字节流的方式存储到string类型的output中，它是二进制的，并不是text文本，使用string类只是为了方便。
bool ParseFromString(const string& data);对提供的data数据进行序列化成消息对象。
bool SerializeToOstream(ostream* output) const;反序列化成C++的流
bool ParseFromIstream(istream* input);把流序列化成消息对象。
填充消息
#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,
'
');
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;
}
读消息
#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;
}
关于完整的API接口说明，请查看：https://developers.google.com/protocol-buffers/docs/reference/cpp/google.protobuf.message#MessageFactory