问c++单例在父类中创建子类

EN

db.hpp
-------
#ifndef DB_HPP
#define DB_HPP

#include <string>
#include "mysqldb.hpp"

class Db
{
  public:
    static Db& instance()
    {
      // can be added other database implementations
      #ifdef DBMYSQL
        static Db *instance = new MySQLDb();
      #elseif DBORACLE
        //static Db *instance = new OracleDb();
      #endif
      return *instance;
    }

    virtual ~Db() {}
    virtual void Insert(std::string& query) = 0;
  protected:
    Db() {}
};
#endif // DB_HPP

mysqldb.hpp
-----------
#ifndef MYSQLDB_HPP
#define MYSQLDB_HPP

#include "db.hpp"
#include <mysql.h>

class MySQLDb : public Db
{
  public:
    virtual void Insert(std::string& query);

  private:
    MYSQL *MySQLConnection_;
    MySQLDb();
    ~MySQLDb();
};
#endif // MYSQLDB_HPP

g++ -DDBMYSQL  `mysql_config --cflags` `mysql_config --libs` -DBOOST_LOG_DYN_LINK -std=c++11 -c -o mysqldb.o mysqldb.cpp

In file included from mysqldb.hpp:4:0,
             from mysqldb.cpp:1:
db.hpp: In static member function ‘static Db& Db::instance()’:
db.hpp:16:35: error: expected type-specifier before ‘MySQLDb’
     static Db *instance = new MySQLDb();
                               ^
Makefile:39: recipe for target 'mysqldb.o' failed