error C2065: '　' : undeclared identifier

场景：error C2065: undeclared identifier解决方案

error C2065: undeclared identifier
初学C++，麻烦各位大神帮帮忙，很简单的程序：
基类：

C/C++ code

#ifndef BASE_H
#define BASE_H
class Base
{
public:
    size_t size() const { return n; }
    Base(size_t n1 = 0) : n(n1){}
protected:
    size_t n;
};
#endif

扩展类：
#ifdef DERIVED_H
#define DERIVED_H
#include "Base.h"
class Derived : private Base
{
public:
    using Base::size;
    Derived(size_t n1 = 0):Base(n1),n(n1){}
protected:
    using Base::n;
};
#endif

测试类：
#include <iostream>
#include "Derived.h"
using namespace std;
int main(){
    int i = 3;
    Derived d(i);
    cout << d.size() << endl;
    return 0;
}

调试的时候出现如下错误：

--------------------Configuration: 7 - Win32 Debug--------------------
Compiling...
7.cpp
C:\Documents and Settings\user\桌面\C++\7.cpp(6) : error C2065: 'Derived' : undeclared identifier
C:\Documents and Settings\user\桌面\C++\7.cpp(6) : error C2146: syntax error : missing ';' before identifier 'd'
C:\Documents and Settings\user\桌面\C++\7.cpp(6) : error C2065: 'd' : undeclared identifier
C:\Documents and Settings\user\桌面\C++\7.cpp(7) : error C2228: left of '.size' must have class/struct/union type
执行 cl.exe 时出错.

7.obj - 1 error(s), 0 warning(s)



麻烦问一下错在哪，该怎么改呢，谢谢

------解决方案--------------------
扩展类：
#ifdef DERIVED_H
#define DERIVED_H
 
这里要改成
#ifndef DERIVED_H
#define DERIVED_H