本文仅对几个常用示例做一个简单介绍, 详细的api文档可以查看对应的头文件中的api描述, 或者查看https://developers.google.com/protocol-buffers/docs/overview

一 动态编译

• 前文介绍了Protobuf的安装及编译方法，不过protobuf不仅提供了使用protoc进行静态编译的方法，也提供了动态编译的方法

• 动态编译 也就是说无须编译.proto生成.pb.cc和.pb.h

• 需要使用protobuf中提供的两个头文件, protobuf中头文件的位置/usr/local/include/google/protobuf

  • google/protobuf/compiler/importer.h 以及 google/protobuf/dynamic_message.h

• 先看代码示例

#include <iostream>
#include <string>
#include <google/protobuf/compiler/importer.h>
#include <google/protobuf/dynamic_message.h>

class MyErrorCollector: public google::protobuf::compiler::MultiFileErrorCollector
{
    virtual void AddError(const std::string & filename, int line, int column, const std::string & message){
        // define import error collector
        printf("%s, %d, %d, %s\n", filename.c_str(), line, column, message.c_str());
    }
};

int main()
{
    google::protobuf::compiler::DiskSourceTree sourceTree; // source tree
    sourceTree.MapPath("", "/home/szw/code/protobuf/tmp2"); // initialize source tree
    MyErrorCollector errorCollector; // dynamic import error collector
    google::protobuf::compiler::Importer importer(&sourceTree, &errorCollector); // importer
    
    importer.Import("test.proto");
    
    // find a message descriptor from message descriptor pool
    const google::protobuf::Descriptor * descriptor = importer.pool()->FindMessageTypeByName("lm.helloworld");
    if (!descriptor){
        perror("Message is not found!");
        exit(-1);
    }

    // create a dynamic message factory
    google::protobuf::DynamicMessageFactory * factory = new google::protobuf::DynamicMessageFactory(importer.pool());
    // create a const message ptr by factory method and message descriptor 
    const  google::protobuf::Message * tmp = factory->GetPrototype(descriptor);
    
    // create a non-const message object by const message ptr
    // define import error collector
    google::protobuf::Message * msg = tmp->New();
    
    return 0;
}

• 头文件<google/protobuf/compiler/importer.h>包含了编译器对象相关
• 头文件<google/protobuf/dynamic_message.h>包含了Message_Descriptor和Factory相关

1. 首先初始化一个DiskSourceTree对象, 指明目标.proto文件的根目录
2. 创建一个MyErrorCollector对象, 用于收集动态编译过程中产生的编译bug, 该对象需要根据proto提供的纯虚基类派生, 并需要重写其中的纯虚函数, 使之不再是一个抽象类
3. 初始化一个Importer对象, 用于动态编译, 将DiskSourceTree和MyErrorCollector的地址传入
4. 将目标.proto动态编译, 并加入编译器池中
5. 可以通过FindMessageTypeName()和消息名, 来获取到目标消息的Message_Descriptor
6. 创建动态工厂, 并利用工厂方法GetPrototype和目标消息的Message_Descriptor获取一个指针const message *
7. 利用消息实例指针的New()方法获取到non-const message ptr

二 类型反射

• 类型反射即是根据字段的名称获取到字段类型的一种机制

• protobuf自身便配备了类型反射机制

• 需要头文件<google/protobuf/descriptor.h>与<google/protobuf/message.h>

• 下面的示例，结合了动态编译以及类型反射机制

• 代码解析：

  1. 首先使用动态编译机制，将test1.proto与test2.proto两个文件动态编译进编译池
  2. 根据消息名获取到目标消息的descriptor
  3. 创建一个动态工厂，并利用消息的descriptor获取到const Message * tmp
  4. 利用const Message * tmp的方法New()获取Message * msg
  5. 根据msg获取反射器Reflection对象
  6. 利用descriptor可以遍历各个字段, 并利用descriptor的cpp_type()方法获取到每个字段的类型, 并借此进行反射填充

代码示例

proto文件

// @file test1.proto
syntax = "proto3";
package lm;
message Player
{
    int32 id = 1;
    string name = 2;
    repeated string hero = 3;
}

// @file test2.proto
syntax = "proto3";
import "test1.proto";
package lm;
message Game{
    repeated Player player = 1;
    string gameName = 2;
};

cpp

// @file test.cpp
#include <iostream>
#include <string>
#include <google/protobuf/compiler/importer.h> // Importer DiskSourceTree MultiFileErrorCollector
#include <google/protobuf/dynamic_message.h> // DynamicMessageFactory 
#include <google/protobuf/descriptor.h> // Desctriptor FiledDescriptor ... all descriptor
#include <google/protobuf/message.h> // Message Reflection MessageFactory


class MyErrorCollector: public google::protobuf::compiler::MultiFileErrorCollector
{
    virtual void AddError(const std::string & filename, int line, int column, const std::string & message){
        // define import error collector
        printf("%s, %d, %d, %s\n", filename.c_str(), line, column, message.c_str());
    }
};

int main()
{
    google::protobuf::compiler::DiskSourceTree sourceTree; // source tree
    sourceTree.MapPath("", "./"); // initialize source tree
    MyErrorCollector errorCollector; // dynamic import error collector
    google::protobuf::compiler::Importer importer(&sourceTree, &errorCollector); // importer
    
    importer.Import("test1.proto");
    importer.Import("test2.proto");
    
    // find a message descriptor from message descriptor pool
    const google::protobuf::Descriptor * descriptor = importer.pool()->FindMessageTypeByName("lm.Game");
    if (!descriptor){
        perror("Message is not found!");
        exit(-1);
    }

    // create a dynamic message factory
    google::protobuf::DynamicMessageFactory * factory = new google::protobuf::DynamicMessageFactory(importer.pool());
    // create a const message ptr by factory method and message descriptor 
    const google::protobuf::Message * tmp = factory->GetPrototype(descriptor);
    
    // create a non-const message ptr object by const message ptr
    // define import error collector
    google::protobuf::Message * msg = tmp->New();
    
    // get a reflection by msg
    const google::protobuf::Reflection * reflection = msg->GetReflection();
    
    // travel the message by message_descriptor
    for (int i=0; i<descriptor->field_count(); ++i){
        const google::protobuf::FieldDescriptor * fieldDescriptor = descriptor->field(i);
        if (fieldDescriptor->is_repeated()){
            switch (fieldDescriptor->cpp_type()){
                case google::protobuf::FieldDescriptor::CPPTYPE_INT32:
                {
                    reflection->AddInt32(msg, fieldDescriptor, 10);
                    break;
                }
                case google::protobuf::FieldDescriptor::CPPTYPE_STRING:
                {
                    reflection->AddString(msg, fieldDescriptor, "abc");
                }
                case google::protobuf::FieldDescriptor::CPPTYPE_MESSAGE:
                {
                    printf("Here is a repeadted message: %s\n", fieldDescriptor->full_name().c_str());
                }
                case google::protobuf::FieldDescriptor::CPPTYPE_BOOL:
                {
                    break;
                }
                case google::protobuf::FieldDescriptor::CPPTYPE_ENUM:
                {
                    break;
                }
                case google::protobuf::FieldDescriptor::CPPTYPE_FLOAT:
                {
                    break;
                }
                case google::protobuf::FieldDescriptor::CPPTYPE_INT64:
                {
                    break;
                }
                case google::protobuf::FieldDescriptor::CPPTYPE_DOUBLE:
                {
                    break;
                }
                case google::protobuf::FieldDescriptor::CPPTYPE_UINT32:
                {
                    break;
                }
                case google::protobuf::FieldDescriptor::CPPTYPE_UINT64:
                {
                    break;
                }
                
            }
        }else{
            switch (fieldDescriptor->cpp_type()){
                case google::protobuf::FieldDescriptor::CPPTYPE_INT32:
                {
                    reflection->SetInt32(msg, fieldDescriptor, 10);
                    break;
                }
                case google::protobuf::FieldDescriptor::CPPTYPE_STRING:
                {
                    reflection->SetString(msg, fieldDescriptor, "abc");
                }
                case google::protobuf::FieldDescriptor::CPPTYPE_MESSAGE:
                {
                    printf("Here is a message: %s\n", fieldDescriptor->full_name().c_str());
                }
                case google::protobuf::FieldDescriptor::CPPTYPE_BOOL:
                {
                    break;
                }
                case google::protobuf::FieldDescriptor::CPPTYPE_ENUM:
                {
                    break;
                }
                case google::protobuf::FieldDescriptor::CPPTYPE_FLOAT:
                {
                    break;
                }
                case google::protobuf::FieldDescriptor::CPPTYPE_INT64:
                {
                    break;
                }
                case google::protobuf::FieldDescriptor::CPPTYPE_DOUBLE:
                {
                    break;
                }
                case google::protobuf::FieldDescriptor::CPPTYPE_UINT32:
                {
                    break;
                }
                case google::protobuf::FieldDescriptor::CPPTYPE_UINT64:
                {
                    break;
                }
            }
        }
    }

    printf("\nmsg\n");
    msg->PrintDebugString();
    return 0;
}