protobuf C++入门

1、在.proto文件中定义消息格式

2、使用protobuf编译器

3、使用c++ api来读写消息

0、为何使用protobuf?

1、原始内存数据结构，可以以二进制方式sent/saved.这种方式需要相同的内存布局和字节序。

2、以ad-hoc方式将数据项编码成一个简单字符串—-比如，将4个int类型编码成”12:3:-23:67”。这种方式简灵活。适用于简单数据。

3、将数据序列化为XML。这种方式很流行，因为xml可读性好，编码解码方便，性能也好。仅仅XML dom树比较复杂。

protobuf可以很好的解决上述问题。你编写一个.proto文件来描述数据结构。protobuf编译器使用它创建一个类，使用二进制方式自动编码/解码该数据结构。生成的类提供getter/setter方法。

最重要的是，protobuf支持在此基础上进行格式扩展。

示例

1、定义协议格式

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 phone = 4;
}

message AddressBook {
repeated Person person = 1;
}

该结构与c++或java很像.

.proto文件以包声明开始，防止名字冲突。

简单类型：bool, int32, float, double, string.

其它类型：如上述的Person, PhoneNumber

类型可以嵌套。

“=1”， “=2”标识唯一“tag”.tag数1-15需要至少一个字节。

required: 必须设置它的值

optional: 可以设置，也可以不设置它的值

repeated: 可以认为是动态分配的数组

google工程师认为使用required威害更大， 他们更喜欢使用optional, repeated.

2、编译你的协议

运行protoc 来生成c++文件：

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

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

生成的文件为：

addressbook.pb.h,

addressbook.pb.cc

3、protobuf API

生成的文件中有如下方法：

// 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();

// phone
inline int phone_size() const;
inline void clear_phone();
inline const ::google::protobuf::RepeatedPtrField< ::tutorial::Person_PhoneNumber >& phone() const;
inline ::google::protobuf::RepeatedPtrField< ::tutorial::Person_PhoneNumber >* mutable_phone();
inline const ::tutorial::Person_PhoneNumber& phone(int index) const;
inline ::tutorial::Person_PhoneNumber* mutable_phone(int index);
inline ::tutorial::Person_PhoneNumber* add_phone();

4、枚举与嵌套类

生成的代码包含一个PhoneType枚举。Person::PhoneType, Person:MOBILE, Person::HOME, Person:WORK.

编译器生成的嵌套类称为Person::PhoneNumber. 实际生成类为Person_PhoneNumber.

5、标准方法

bool IsInitialized() const:             //   确认required字段是否被设置

string DebugString() const:             //   返回消息的可读表示，用于调试

void CopyFrom(const Person& from):      //   使用给定消息值copy

void Clear():                           //   清除所有元素为空状态

6、解析与序列化

bool SerializeToString(string* output) const:        //序列化消息，将存储字节的以string方式输出。注意字节是二进制，而非文本；

bool ParseFromString(const string& data):           // 解析给定的string

bool SerializeToOstream(ostream* output) const:     // 写消息给定的c++  ostream中

bool ParseFromIstream(istream* input):             //  从给定的c++ istream中解析出消息

7、protobuf和 oo设计

不要继承生成类并在此基础上添加相应的行为

8、写消息

示例：它从一个文件中读取AddressBook，基于io添加一个新的Person，并将新的AddressBook写回文件。

#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_phone();
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_person());

{
// 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;
}

注意使用GOOGLE_PROTOBUF_VERIFY_VERSION宏。每一个.pb.cc文件在启动时都将自动调用该宏。

注意在程序结尾处调用ShutdownProtobufLibrary()。

9、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.person_size(); i++) {
const tutorial::Person& person = address_book.person(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.phone_size(); j++) {
const tutorial::Person::PhoneNumber& phone_number = person.phone(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;
}

10、扩展protobuf

如果希望向后兼容，必须遵循：

a、不必更改tag数

b、不必添加或删除任何required字段

c、可以删除optional或repeated字段

d、可以添加新的optional或repeated字段，但你必须使用新的tag数。

11、优化

c++的protobuf库，已经极大地优化了。合理使用可以改善性能。

a、如果可能，复用message对象。

b、关于多线程的内存分配器

12、高级用法

protobuf的消息类的一个关键特性是，反射(reflection)。可以使用xml或json来实现。参考。

13、 ProtoBuf 常用序列化/反序列化API

（1）. C数组的序列化和反序列化API

//C数组的序列化和序列化API
bool ParseFromArray(const void* data, int size);
bool SerializeToArray(void* data, int size) const;
//使用
void set_people()
{
wp.set_name("sealyao");
wp.set_id(123456);
wp.set_email("sealyaog@gmail.com");
wp.SerializeToArray(parray,256);
}

void get_people()
{
rap.ParseFromArray(parray,256);
cout << "Get People from Array:" << endl;
cout << "\t Name : " <<rap.name() << endl;
cout << "\t Id : " << rap.id() << endl;
cout << "\t email : " << rap.email() << endl;
}

（2）. C++ String的序列化和反序列化API

//C++string序列化和序列化API
bool SerializeToString(string* output) const;
bool ParseFromString(const string& data);
//使用：
void set_people()
{
wp.set_name("sealyao");
wp.set_id(123456);
wp.set_email("sealyaog@gmail.com");
wp.SerializeToString(&pstring);
}

void get_people()
{
rsp.ParseFromString(pstring);
cout << "Get People from String:" << endl;
cout << "\t Name : " <<rsp.name() << endl;
cout << "\t Id : " << rsp.id() << endl;
cout << "\t email : " << rsp.email() << endl;
}

（3）. 文件描述符序列化和反序列化API

//文件描述符的序列化和序列化API
bool SerializeToFileDescriptor(int file_descriptor) const;
bool ParseFromFileDescriptor(int file_descriptor);

//使用:
void set_people()
{
fd = open(path,O_CREAT|O_TRUNC|O_RDWR,0644);
if(fd <= 0){
perror("open");
exit(0);
}
wp.set_name("sealyaog");
wp.set_id(123456);
wp.set_email("sealyaog@gmail.com");
wp.SerializeToFileDescriptor(fd);
close(fd);
}

void get_people()
{
fd = open(path,O_RDONLY);
if(fd <= 0){
perror("open");
exit(0);
}
rp.ParseFromFileDescriptor(fd);
std::cout << "Get People from FD:" << endl;
std::cout << "\t Name : " <<rp.name() << endl;
std::cout << "\t Id : " << rp.id() << endl;
std::cout << "\t email : " << rp.email() << endl;
close(fd);
}

（4）. C++ stream 序列化和反序列化API

//C++ stream 序列化/反序列化API
bool SerializeToOstream(ostream* output) const;
bool ParseFromIstream(istream* input);

//使用：
void set_people()
{
fstream fs(path,ios::out|ios::trunc|ios::binary);
wp.set_name("sealyaog");
wp.set_id(123456);
wp.set_email("sealyaog@gmail.com");
wp.SerializeToOstream(&fs);
fs.close();
fs.clear();
}

void get_people()
{
fstream fs(path,ios::in|ios::binary);
rp.ParseFromIstream(&fs);
std::cout << "\t Name : " <<rp.name() << endl;
std::cout << "\t Id : " << rp.id() << endl;
std::cout << "\t email : " << rp.email() << endl;
fs.close();
fs.clear();
}

===========================分割线=========================

常见问题：

1、undefined reference to `pthread_once’

使用-lpthread:

2、error while loading shared libraries: libprotobuf.so.7: cannot open shared object file: No such file or directory

使用-Wl,-Bstatic -lprotobuf -Wl,-Bdynamic -lpthread