动态生成 protobuf Message 和 return 指向它的指针

Question

首先，我对 C++ 不是很有经验，所以也许我正在监督这里的一些事情。我正在尝试使用以下代码从 .proto 文件动态生成 protobuf 消息：

int init_msg(const std::string & filename, protobuf::Arena* arena, protobuf::Message** new_msg){
  using namespace google::protobuf;
  using namespace google::protobuf::compiler;

  DiskSourceTree source_tree;
  source_tree.MapPath("file", filename);

  MuFiErCo error_mist;
  Importer imp(&source_tree, &error_mist);

  printf("Lade Datei:%s \n", filename.c_str());

  const FileDescriptor* f_desc = imp.Import("file");

  const Descriptor* desc = f_desc->FindMessageTypeByName("TestNachricht");

  const Message* new_msg_proto = dmf.GetPrototype(desc);

  *new_msg = new_msg_proto->New(arena);

  //Debug
  cout << (*new_msg)->GetTypeName() << endl;

  return 0;
}

int main(int argc, char* argv[]){

  protobuf::Arena arena;

  protobuf::Message *adr2, *adr1;

  init_msg("schema-1.proto", &arena, &adr1);
  init_msg("schema-1.proto", &arena, &adr2);

  printf("MSG_Pointer: %p, %p\n", adr1, adr2);

  cout << adr1->GetTypeName() << endl;

  arena.Reset();

  return 0;
}

我想如果我使用Arena，新消息在功能范围之外也可用。但是，如果我尝试访问消息，总会出现段错误。我想这是一个简单的错误，但我不知道如何解决这个问题。

这是输出：

Lade Datei:schema-1.proto 
packet.TestNachricht
Lade Datei:schema-1.proto 
packet.TestNachricht
MSG_Pointer: 0x1b293b0, 0x1b287f0
Speicherzugriffsfehler (Speicherabzug geschrieben)

Answer 1

The problem, I think, is that FileDescriptor et al are destroyed when init_msg returns, leaving the newly created message with no way to interrogate its .proto definition. You'd need to move Importer instance to main and keep it alive. This has nothing to do with arenas. – Igor Tandetnik

这就是解决方案。

这是一些工作示例代码

#include <string>
#include <stdio.h>
#include <stdlib.h>
#include <iostream>
#include <memory>
#include <google/protobuf/descriptor.h>
#include <google/protobuf/message.h>
#include <google/protobuf/compiler/importer.h>
#include <google/protobuf/dynamic_message.h>
#include <google/protobuf/arena.h>


using namespace std;
using namespace google::protobuf;


class MuFiErCo : public compiler::MultiFileErrorCollector
{
public:
    void AddError(const string & filename, int line, int column, const string & message){
        printf("Err: %s\n", message.c_str());
    }
    void AddWarning(const string & filename, int line, int column, const string & message){
        printf("Warn: %s\n", message.c_str());
    }

};


compiler::Importer* init_proto_dir(Arena* arena, const std::string &root_dir){
    using namespace compiler;

    static DiskSourceTree source_tree;
    source_tree.MapPath("", root_dir);

    static MuFiErCo error_mist;
    static Importer* imp = Arena::Create<Importer>(arena, &source_tree, &error_mist);

    return imp;
}


void init_proto_def(compiler::Importer* imp, const std::string &proto_file){
    using namespace compiler;

    imp->Import(proto_file);

    return;
}


Message* init_msg(compiler::Importer* imp, Arena* arena, const std::string &msg_name){

    const DescriptorPool* pool = imp->pool();

    static DynamicMessageFactory dmf;

    const Descriptor* desc = pool->FindMessageTypeByName(msg_name);

    const Message* msg_proto = dmf.GetPrototype(desc);

    return msg_proto->New(arena);
}


int set_value(Message* msg, const char* value_name, unsigned long int value){
    const Message::Reflection* reflec = msg->GetReflection();
    const Descriptor* desc = msg->GetDescriptor();

    const FieldDescriptor* fdesc = desc->FindFieldByName(value_name);

    reflec->SetUInt64(msg, fdesc, value);

    return 0;

}


int main(int argc, char* argv[]){

    Arena arena;

    compiler::Importer* imp = init_proto_dir(&arena, "");
    init_proto_def(imp, "schema-1.proto");

    Message* msg = init_msg(imp, &arena, "packet.TestNachricht");

    set_value(msg, "zahl", 23434);

    cout << msg->DebugString() << endl;

    return 0;
}

动态生成 protobuf Message 和 return 指向它的指针

Dynamically generate protobuf Message and return a pointer to it

c++

protocol-buffers