那我們將friend bool operator >(Node node1,Node node2)修改為friend bool operator >(Node* node1,Node* node2)，也就是傳遞的是Node的指標行不行呢?

答案是不可以，因為根據c++primer中過載操作符中講的“程式設計師只能為類型別或列舉型別的運算元定義過載操作符，在把操作符宣告為類的成員時，至少有一個類或列舉型別的引數按照值或者引用的方式傳遞”，也就是說friend bool operator >(Node* node1,Node* node2)形參中都是指標型別的是不可以的。我們只能再建一個類，用其中的過載（）操作符作為優先佇列的比較函式。

就得到了下面正確的程式碼：

/************************************************************************/
/*	File Name: Huffman.cpp
*		@Function: Lossless Compression
		@Author: Sophia Zhang
		@Create Time: 2012-9-26 10:40
		@Last Modify: 2012-9-26 12:10
*/
/************************************************************************/

#include"iostream"
#include "queue"
#include "map"
#include "string"
#include "iterator"
#include "vector"
#include "algorithm"
using namespace std;

#define NChar 8	//suppose use 8 bits to describe all symbols
#define Nsymbols 1<<NChar	//can describe 256 symbols totally (include a-z, A-Z)
typedef vector<bool> Huff_code;//8 bit code of one char
map<char,Huff_code> Huff_Dic;	//huffman coding dictionary

/************************************************************************/
/* Tree Class elements:
*2 child trees
*character and frequency of current node
*/
/************************************************************************/
class HTree
{
public :
	HTree* left;
	HTree* right;
	char ch;
	int weight;

	HTree(){left = right = NULL; weight=0;ch ='\0';}
	HTree(HTree* l,HTree* r,int w,char c){left = l;	right = r;	weight=w;	ch=c;}
	~HTree(){delete left; delete right;}
	bool Isleaf(){return !left && !right; }
};

/************************************************************************/
/* prepare for pointer sorting*/
/*because we cannot use overloading in class HTree directly*/
/************************************************************************/
class Compare_tree
{
public:
	bool operator () (HTree* t1, HTree* t2)
	{
		return t1->weight> t2->weight;
	}
};

/************************************************************************/
/* use priority queue to build huffman tree*/
/************************************************************************/
HTree* BuildTree(int *frequency)
{
	priority_queue<HTree*,vector<HTree*>,Compare_tree> QTree;

	//1st level add characters
	for (int i=0;i<Nsymbols;i++)
	{
		if(frequency[i])
			QTree.push(new HTree(NULL,NULL,frequency[i],(char)i));			
	}

	//build
	while (QTree.size()>1)
	{
		HTree* lc  = QTree.top();
		QTree.pop();
		HTree* rc = QTree.top();
		QTree.pop();

		HTree* parent = new HTree(lc,rc,lc->weight+rc->weight,(char)256);
		QTree.push(parent);
	}
	//return tree root
	return QTree.top();
}

/************************************************************************/
/* Give Huffman Coding to the Huffman Tree*/
/************************************************************************/
void Huffman_Coding(HTree* root, Huff_code& curcode)
{
	if(root->Isleaf())
	{
		Huff_Dic[root->ch] = curcode;
		return;
	}
	Huff_code lcode = curcode;
	Huff_code rcode = curcode;
	lcode.push_back(false);
	rcode.push_back(true);

	Huffman_Coding(root->left,lcode);
	Huffman_Coding(root->right,rcode);
}

int main()
{
	int freq[Nsymbols] = {0};
	char *str = "this is the string need to be compressed";

	//statistic character frequency
	while (*str!='\0')
		freq[*str++]++;

	//build tree
	HTree* r = BuildTree(freq);
	Huff_code nullcode;
	nullcode.clear();
	Huffman_Coding(r,nullcode);

	for(map<char,Huff_code>::iterator it = Huff_Dic.begin(); it != Huff_Dic.end(); it++)
	{
		cout<<(*it).first<<'\t';
		std::copy(it->second.begin(),it->second.end(),std::ostream_iterator<bool>(cout));
		cout<<endl;
	}
}