what is the problem with under?

[lovec]

#include<iostream.h>

class First{
public:
First(){cout<<"First Constructor"<<endl;};
virtual ~First();
virtual void GetID(){ cout<<_GetID();};
private:
virtual int _GetID() const = 0 ;
};

class Sec: public First {
public:
Sec(int id=2): ID(id) {cout<<"Second Constructor"<<endl;};
~Sec();
private:
int ID;
int _GetID() const { return ID ; };
};

void f(First& fir)
{
fir.GetID();
}
int main()
{
Sec sec;
f(sec);
return 1;
}

the result of compiling is:
att1% g++ VPriv.C
/var/tmp/cca0013C1.o: In function `main':
/var/tmp/cca0013C1.o(.text+0x58): undefined reference to `Sec::Sec(int)'
/var/tmp/cca0013C1.o(.text+0x80): undefined reference to `Sec::~Sec(void)'

and the version of g++ is 2.7

这个例子是我看了 孟岩 的与大虾对话中的“Virtually Yours -- Template Method模式”，临时写出来的，可能领悟的不透，大虾多多执教，小菜鸟感激涕零！

[hz129]

virtual ~First(); --> virtual ~First() {}

~Sec(); --> ~Sec() {}

[lovec]

ha,犯这样低级错误，真是失败啊，多谢hz129(孤星古雨)　，好的把分都给你了！