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_Hclass 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

[解决办法]
扩展类：
#ifdef DERIVED_H
#define DERIVED_H
 
这里要改成
#ifndef DERIVED_H
#define DERIVED_H

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