vc++6.0 类中定义 static 变量报错

在实验《提高C++性能的编程技术》中第一个源代码是遇到了报错。。。。。

自己补全了点代码：

#pragma once
#include <iostream>
#include <string>
using namespace std;

class Trace{
public:
		Trace(const string &name);
		~Trace();
		void debug(const string &msg);
		static bool traceIsActive;
		static int t;
private:
		string theFunctionName;
};

#include "Trace.h"

Trace::Trace(const string &name):theFunctionName(name)
{
	if(traceIsActive)
	{
		cout<<"Enter function"<<name<<endl;
	}
}

inline void Trace::debug(const string &msg)
{
	if(traceIsActive)
	{
		cout<<msg<<endl;
	}
}

Trace::~Trace()
{
	if(traceIsActive)
	{
		cout<<"Exit function"<<theFunctionName<<endl;
	}
}

#include <windows.h>
#include <iostream>
#include "Trace.h"

using namespace std;

int addOne( int x)
{

	return x+1;
}

int main()
{
	Trace::traceIsActive = false;
	int j=1000000,y;
	SYSTEMTIME t1,t2;
	

	GetSystemTime(&t1);
	for(int i=0 ; i< j ; i++)
	{
		y = addOne(i);
	}
	GetSystemTime(& t2);

	cout<<t2.wSecond*1000+t2.wMilliseconds-t1.wSecond*1000-t1.wMilliseconds<<endl;
	return 0;
}

上边的代码会报如下错误：mian.obj : error LNK2001: unresolved external symbol "public: static bool Trace::traceIsActive" ( ?traceIsActive@Trace@@2_NA)Trace.obj : error LNK2001: unresolved external symbol "public: static bool Trace::traceIsActive" ( ?traceIsActive@Trace@@2_NA)Debug/1_1.exe : fatal error LNK1120: 1 unresolved externals执行 link.exe 时出错.我在百度了好久找的答案是在Trace.cpp文件中加入 bool Trace::traceIsActive ;修改后源文件为：

#include "Trace。好“
bool Trace::traceIsActive ;//新增加的行
Trace::Trace(const string &name):theFunctionName(name)
{
 if(traceIsActive)
 {
  cout<<"Enter function"<<name<<endl;
 }
}

........

虽然可以编译通过了，不过我还想知道具体的原因，先谢过了

 

关于static变量的详细请看我的新帖http://blog.youkuaiyun.com/xt0916020331/article/details/7530195  说的不当之处请指出