如何修复仅在使用名称空间时发生的错误C2664

我得到这个错误:

main.cpp(10) : error C2664: 'lr::codec::codec(protocol_decoder *)' 
: cannot convert parameter 1 from 'proto::protocol_decoder *' to 'protocol_decoder *'

如果我删除proto命名空间的使用，那么这个错误就会消失。如何解决这个问题，并且仍然保留proto命名空间的使用?

代码如下:

main.cpp:

#include "protocol_decoder_a.hpp"
#include "codec.hpp"
int main() {
   //factory function create protocol decoder
   proto::protocol_decoder* pro = new proto::protocol_decoder_a;
   lr::codec cdc(pro);
   return 0;
}

codec.hpp:

#ifndef __CODEC_HPP__
#define __CODEC_HPP__
#include <map>
#include <string>
class protocol_decoder;
//log replay namespace
namespace lr {

typedef bool (*c_f)(const char* id, unsigned char* rq, size_t rq_length, unsigned char*& response, size_t& resp_len);
// generic codec interface will use specific 
class codec {
   public:
      codec(protocol_decoder* decoder);
      ~codec() {}
      bool get_response(const char* id, unsigned char* rq, size_t rq_length, unsigned char*& response, size_t& resp_len);
      const char* get_monitored_dn(const char* id, unsigned char* rq, size_t rq_length);
      void load_msgs_from_disk();
     protocol_decoder* decoder_;
};
}  //namespace lr

codec.cpp:

#include "codec.hpp"
using namespace lr;
codec::codec(protocol_decoder* decoder) : decoder_(decoder) {
   load_msgs_from_disk();
}
void codec::load_msgs_from_disk() {
   //use specific protocol decoder here
}
bool codec::get_response(const char* id, unsigned char* rq, size_t rq_length, unsigned char*& response, size_t& resp_len) {
   return true;
}
const char* codec::get_monitored_dn(const char* id, unsigned char* rq, size_t rq_length) {
   return 0;
}

protocol_decoder.hpp:

#ifndef __PROTOCOL_DECODER_HPP__
#define __PROTOCOL_DECODER_HPP__
namespace proto {
enum id_type { UNKNOWN_ID, INT_ID, STRING_ID };
struct msg_id {
   msg_id() : type(UNKNOWN_ID) {}
   id_type type;
   union {
      const char* s_id;
      size_t i_id;
   };
};

class protocol_decoder {
public:
    virtual const char* get_monitored_dn(unsigned char* msg, size_t msg_len) = 0;
    virtual bool get_response(unsigned char* rq, size_t rq_len, unsigned char* response, size_t resp_len) = 0;
    virtual bool get_msg_id(unsigned char* rq, size_t rq_len, msg_id id) = 0;
};
} //namespace proto
#endif //__PROTOCOL_DECODER_HPP__

protocol_decoder_a.hpp:

#ifndef __PROTOCOL_DECODER_A_HPP__
#define __PROTOCOL_DECODER_A_HPP__
#include "protocol_decoder.hpp"
namespace proto {
class protocol_decoder_a : public proto::protocol_decoder {
public:
   virtual const char* get_monitored_dn(unsigned char* msg, size_t msg_len);
   virtual bool get_response(unsigned char* rq, size_t rq_len, unsigned char* response, size_t resp_len);
   virtual bool get_msg_id(unsigned char* rq, size_t rq_len, proto::msg_id id);
};
} //namespace proto
#endif //__PROTOCOL_DECODER_A_HPP__

protocol_decoder_a.cpp:

#include "protocol_decoder_a.hpp"
using namespace proto;

const char* protocol_decoder_a::get_monitored_dn(unsigned char* msg, size_t msg_len) {
  //specific stuff here
   return 0;
}
bool protocol_decoder_a::get_response(unsigned char* rq, size_t rq_len, unsigned char* response, size_t resp_len) {
   return true;
}
bool protocol_decoder_a::get_msg_id(unsigned char* rq, size_t rq_len, proto::msg_id id) {
    return true;
}