Protobuf binary file learning and parsing

0. brief introduction

protobuf Also called protocol buffer yes google A data exchange format of , It is independent of language , Platform independent .google Provides the implementation of multiple languages ：java、c#、c++、go and python, Each implementation contains the compiler and library files of the corresponding language .

Because it's a binary format , Than using xml 、json Data exchange is much faster . It can be used for data communication between distributed applications or data exchange in heterogeneous environment . As a binary data transmission format with excellent efficiency and compatibility , Can be used for such as network transmission 、 The configuration file 、 Data storage and many other fields .

For more details, please see ：https://developers.google.com/protocol-buffers/docs/overview

1. Basic grammar

1 Field restrictions

required: Characters that must be assigned
optional: Optional fields , have access to [default = xxx] Configure defaults
repeated: Repeatable variable length field , It's like an array

2 tag

Each field has a unique tag
tag 1-15 Is byte encoding ,16-2047 Use 2 Byte encoding , therefore 1-15 For frequently used fields

3 type

System default ：
string The default is an empty string ;
bool The default is false;
The default value is 0;
enum The default is the first element

4 Parsing and serialization

Every message Both contain the following methods , For parsing and serializing , Note that the target is in byte form , Non text .
bool SerializeToString(string* output) const: take message Serialization into binary is stored in output in , Note that it is binary , It's not text ; just string As a container .
bool ParseFromString(const string& data): From the given binary value, it can be resolved to message
bool SerializeToOstream(ostream* output) const: Serialize to ostream in
bool ParseFromIstream(istream* input): from istream Resolve in message

3. Explanation examples

establish .proto file

syntax = "proto3";// Specify version information , No error will be reported 

message Person //message Keyword , The function is to define a message type 
{
    
    string name = 1;    // full name 
    int32 id = 2;       //id
    string email = 3;   // mail 
}

message AddressBook
{
    
    repeated Person people = 1;
}

Field API

Generally, we often use protoc To automatically generate :

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

among protoc Tools to address ：protoc

The field modifier is repeated Function generated by the field of , It's a little different , Such as people Field , Then the compiler will generate the following code ：

int people_size() const;
void clear_people();
const ::Person& people(int index) const;
::Person* mutable_people(int index);
::Person* add_people();
::google::protobuf::RepeatedPtrField< ::Person >* mutable_people();
const ::google::protobuf::RepeatedPtrField< ::Person >& people() const;

The test program

void set_addressbook()
{
    
    AddressBook obj;

    Person *p1 = obj.add_people(); // Add a new Person
    p1->set_name("mike");
    p1->set_id(1);
    p1->set_email("[email protected]");

    Person *p2 = obj.add_people(); // Add a new Person
    p2->set_name("jiang");
    p2->set_id(2);
    p2->set_email("[email protected]");

    Person *p3 = obj.add_people(); // Add a new Person
    p3->set_name("abc");
    p3->set_id(3);
    p3->set_email("[email protected]");

    fstream output("pb.xxx", ios::out | ios::trunc | ios::binary);

    bool flag = obj.SerializeToOstream(&output);// serialize 
    if (!flag)
    {
    
        cerr << "Failed to write file." << endl;
        return;
    }

    output.close();// Close file 
}

void get_addressbook()
{
    
    AddressBook obj;
    fstream input("./pb.xxx", ios::in | ios::binary);
    obj.ParseFromIstream(&input);  // Deserialization 
    input.close(); // Close file 

    for (int i = 0; i < obj.people_size(); i++)
    {
    
        const Person& person = obj.people(i);// Take the first place i individual people
        cout << " The first " << i + 1 << " Messages \n";
        cout << "name = " << person.name() << endl;
        cout << "id = " << person.id() << endl;
        cout << "email = " << person.email() << endl << endl;
    }
}

Running results ：

… Please refer to Ancient Moon House