求助 关于模板类中的重载运算符的友元函数

[lyac2006]

[root@localhost genDLLst]# g++ -o genDLLst2 genDLLst2.cpp
genDLLst2.cpp:41: warning: friend declaration `std::ostream& operator<<(std::ostream&, const DoublyLinkedList<T>&)' declares a non-template function
genDLLst2.cpp:41: warning: (if this is not what you intended, make sure the function template has already been declared and add <> after the function name here) -Wno-non-template-friend disables this warning

源代码如下:
#include<iostream>
using namespace std;
template<typename T>
class DLLNode {
public:
DLLNode() {
next = prev = 0;
}
DLLNode(const T& el, DLLNode *n = 0, DLLNode *p = 0) {
info = el; next = n; prev = p;
}
T info;
DLLNode *next, *prev;
};
template<typename T>
class DoublyLinkedList {
public:
DoublyLinkedList() {
head = tail = 0;
}
void addToDLLTail(const T&);
T deleteFromDLLTail();
~DoublyLinkedList() {
clear();
}
bool isEmpty() const {
return head == 0;
}
void clear();
void setToNull() {
head = tail = 0;
}
void addInMiddle(const T&);
void addToDLLHead(const T&);
T deleteFromDLLHead();
T& firstEl();
T* find(const T&) const;
void run();
protected:
DLLNode<T> *head, *tail;
friend ostream& operator<<(ostream&, const DoublyLinkedList<T>&);
};
template<typename T>
ostream& operator<<(ostream& out, const DoublyLinkedList<T>& dll) {
DLLNode<T> *tmp;
for (tmp = dll.head; tmp != 0; tmp = tmp->next)
out << tmp->info << ' ';
return out;
}
int main()
{
return 0;
}

[飞哥]

不必理会

[lyac2006]

请问 为什么一定要就地实现?

[taodm]

googel herb sutter的 Befriending Templates

[taodm]

friend ostream& operator<<(ostream&, const DoublyLinkedList<T>&)
{
DLLNode<T> *tmp;
for (tmp = dll.head; tmp != 0; tmp = tmp->next)
out << tmp->info << ' ';
return out;
}
friend声明时就地实现。

[pjy717]

我在VC2003下编译运行没有问题哦

[Thinkingking]

你需要在class DoublyLinkedList{.....}前面
申明class DoublyLinkedList;

[loongee]

我在DEV C++ 下编译通过，不过有两个警告。
41 H:\C++\test.cpp [Warning] friend declaration `std::ostream& operator<<(std::ostream&, const DoublyLinkedList<T>&)' declares a non-template function
41 H:\C++\test.cpp [Warning] (if this is not what you intended, make sure the function template has already been declared and add <> after the function name here) -Wno-non-template-friend disables this warning

[loongee]

按taodm的方法将友元函数就地实现，警告消失。