protobuf学习
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 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。 ================================================================ 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