标签:dex 文本 tar reads 读写 方便 int 改善 oat
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。
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 Personperson = 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 voidclear_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);
inline bool has_email() const;
inline voidclear_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 intphone_size() const;
inline voidclear_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;
// Thisfunction 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 tofinish): ";
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 << "Unknownphone type. Using default." << endl;
}
}
}
// Mainfunction: 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[]) {
// Verifythat 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 << "Failedto parse address book." << endl;
return -1;
}
}
// Add anaddress.
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 << "Failedto 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;
}
}
}
// Mainfunction: Reads the entire address book from a file and prints all
// the information inside.
int main(int argc, char* argv[]) {
// Verifythat 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 << "Failedto 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
标签:dex 文本 tar reads 读写 方便 int 改善 oat
原文地址:http://www.cnblogs.com/lidabo/p/6223178.html