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
#include
#include
#include
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
- 头文件
Message_Descriptor和Factory相关
- 首先初始化一个
DiskSourceTree对象, 指明目标.proto文件的根目录 - 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 - 初始化一个
Importer对象, 用于动态编译, 将DiskSourceTree和MyErrorCollector的地址传入 - 将目标
.proto动态编译, 并加入编译器池中 - 可以通过
FindMessageTypeName()和消息名, 来获取到目标消息的Message_Descriptor - 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 * - 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
下面的示例，结合了动态编译以及类型反射机制
-
代码解析：
- 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
- 根据消息名获取到目标消息的descriptor
- 创建一个动态工厂，并利用消息的descriptor获取到
const Message * tmp - 利用
const Message * tmp的方法New()获取Message * msg - 根据msg获取反射器
Reflection对象 - 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
#include
#include // Importer DiskSourceTree MultiFileErrorCollector
#include // DynamicMessageFactory
#include // Desctriptor FiledDescriptor ... all descriptor
#include // 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; ifield_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;
}