一个类模板程序：error LNK2019: 无法解析的外部符号 "class std::basic_ostream
本人现在正在看《C++ Primer（第三版）》的类模板一章，在实现书中的代码时，出现了这样的错误。

下面是一个简单的利用了类模板的实现队列简单操作的代码，总共包括两个文件（Queue.h和Queue.cpp）：
Queue.h：

C/C++ code

#ifndef QUEUE_H#define QUEUE_H#include <iostream>#include <cstdlib>using namespace std;template <class T> class QueueItem;//类模板Queue的定义template <class Type>class Queue{public:    Queue():front(0),back(0){}    ~Queue();    Type remove();    void add(const Type&);    bool is_empty() const{        return front == 0;    }    friend ostream& operator<<(ostream&,const Queue<Type> &);private:    QueueItem<Type> *front;    QueueItem<Type> *back;};//类模板QueueItem的定义template <class Type> class QueueItem{public:    QueueItem(const Type&t):item(t),next(0){}    friend class Queue<Type>;    friend ostream& operator<<(ostream&,const QueueItem<Type>&);private:    Type item;    QueueItem *next;};template <class  Type>Queue<Type>::~Queue(){    while(!is_empty())        remove();}template <class Type>void Queue<Type>::add(const Type &val){    QueueItem<Type> *pt = new QueueItem<Type>(val);    if (is_empty())        front = back = pt;    else    {        back ->next = pt;        back = pt;    }}template <class Type>Type Queue<Type>::remove(){    if (is_empty())    {        cerr << "remove() on empty queue!"<<endl;        exit(-1);    }    QueueItem<Type> *pt = front;    front = front ->next;    Type retval = pt ->item;    delete pt;    return retval;}template <class Type>ostream& operator<<(ostream &os,const Queue<Type> &q){    os << "<";    QueueItem<Type> *p;    for (p = q .front;p;p = p ->next)    {        os << *p << " ";    }    os << ">";    return os;}template <class Type>ostream& operator<<(ostream &os,const QueueItem<Type> &qi){    os << qi.item;    return os;}#endif

Queue.cpp文件：

C/C++ code

#include "Queue.h"#include <iostream>using namespace std;int main(){    //Queue<int> *p_qi = new Queue<int>;    Queue<int> p_qi;    cout << p_qi <<endl;    for (int ival = 0;ival < 10;++ival)    {        p_qi .add(ival);    }    cout << p_qi <<endl;    int err_cnt = 0;    for (int ival = 0;ival < 10;++ival)    {        int qval = p_qi .remove();        if (qval != ival)            ++err_cnt;    }    cout << p_qi <<endl;    if (!err_cnt)        cout << "!!Queue executed OK!"<<endl;    else        cout << "??Queue errors:"<<err_cnt << endl;    return 0;}

在VS2008中运行出现以下错误：
1>------ 已启动生成: 项目: Queue, 配置: Debug Win32 ------
1>正在链接...
1>Queue.obj : error LNK2019: 无法解析的外部符号 "class std::basic_ostream<char,struct std::char_traits<char> > & __cdecl operator<<(class std::basic_ostream<char,struct std::char_traits<char> > &,class Queue<int> const &)" (??6@YAAAV?$basic_ostream@DU?$char_traits@D@std@@@std@@AAV01@ABV?$Queue@H@@@Z)，该符号在函数 _main 中被引用
1>G:\程序练习\CPPPrimer\Queue\Debug\Queue.exe : fatal error LNK1120: 1 个无法解析的外部命令
1>生成日志保存在“file://g:\程序练习\CPPPrimer\Queue\Queue\Debug\BuildLog.htm”
1>Queue - 2 个错误，0 个警告
========== 生成: 成功 0 个，失败 1 个，最新 0 个，跳过 0 个 ==========

请问这是问什么啊？求指导，感激不尽！

------解决方案--------------------

template <class Type>
ostream& operator<<(ostream &os,const Queue<Type> &q)
{
os << "<";
QueueItem<Type> *p;
for (p = q .front;p;p = p ->next)
{
os << *p << " ";
}
os << ">";

return os;
}
cannot access private member declared in class 'QueueItem<int>'

template <class Type>
class QueueItem{
public:
QueueItem(const Type&t):item(t),next(0){}

friend class Queue<Type>;

friend ostream& operator<<(ostream&,const QueueItem<Type>&);
private: // 这里改为 public 或 提供方法 访问next
Type item;
QueueItem *next;
};


[解决办法]

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引用:

template <class Type>
ostream& operator<<(ostream &os,const Queue<Type> &q)
{
os << "<";
QueueItem<Type> *p;
for (p = q .front;p;p = p ->next)……

[解决办法]

C/C++ code

#include <iostream>#include <cstdlib>using namespace std;template <class T> class QueueItem;//类模板Queue的定义template <class Type>class Queue{public:    Queue():front(0),back(0){}    ~Queue();    Type remove();    void add(const Type&);    bool is_empty() const{        return front == 0;    }    // 不这么写函数实现的模板不会实例化     template <class Type>    friend ostream& operator<<(ostream&,const Queue<Type> &);    //或者写 效果一样 下同    //friend ostream& operator<<<>(ostream&,const Queue<Type> &);private:    QueueItem<Type> *front;    QueueItem<Type> *back;};//类模板QueueItem的定义template <class Type> class QueueItem{public:    QueueItem(const Type&t):item(t),next(0){}    friend class Queue<Type>;    // 少了一个friend    template <class Type>    friend ostream& operator<<(ostream&,const Queue<Type> &);    template <class Type>    friend ostream& operator<<(ostream&,const QueueItem<Type>&);private:    Type item;    QueueItem *next;};template <class  Type>Queue<Type>::~Queue(){    while(!is_empty())        remove();}template <class Type>void Queue<Type>::add(const Type &val){    QueueItem<Type> *pt = new QueueItem<Type>(val);    if (is_empty())        front = back = pt;    else    {        back ->next = pt;        back = pt;    }}template <class Type>Type Queue<Type>::remove(){    if (is_empty())    {        cerr << "remove() on empty queue!"<<endl;        exit(-1);    }    QueueItem<Type> *pt = front;    front = front ->next;    Type retval = pt ->item;    delete pt;    return retval;}template <class Type>ostream& operator<<(ostream &os,const Queue<Type> &q){    os << "<";    QueueItem<Type> *p;    for (p = q .front;p;p = p ->next)    {        os << *p << " ";    }    os << ">";    return os;}template <class Type>ostream& operator<<(ostream &os,const QueueItem<Type> &qi){    os << qi.item;    return os;}#include <iostream>using namespace std;int main(){    //Queue<int> *p_qi = new Queue<int>;    Queue<int> p_qi;    cout << p_qi <<endl;    for (int ival = 0;ival < 10;++ival)    {        p_qi .add(ival);    }    cout << p_qi <<endl;    int err_cnt = 0;    for (int ival = 0;ival < 10;++ival)    {        int qval = p_qi .remove();        if (qval != ival)            ++err_cnt;    }    cout << p_qi <<endl;    if (!err_cnt)        cout << "!!Queue executed OK!"<<endl;    else        cout << "??Queue errors:"<<err_cnt << endl;    return 0;} 
 
[解决办法]
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引用:

引用:

引用:

template <class Type>
ostream& operator<<(ostream &os,const Queue<Type> &a……