模板类c ++

Question

i try to design a template for my university project. i wrote the follwing code:

#ifndef _LinkedList_H_
#define _LinkedList_H_
#include "Link.h"
#include <ostream>
template <class L>//error one
class LinkedList
{
private:
 Link<L> *pm_head;
 Link<L> * pm_tail;
 int m_numOfElements;
 Link<L>* FindLink(L * dataToFind);
public:
 LinkedList();
 ~LinkedList();
 int GetNumOfElements(){return m_numOfElements;}
 bool Add( L * data);
 L *FindData(L * data);

template <class L> friend ostream & operator<<(ostream& os,const LinkedList<L> listToprint);//error two
   L* GetDataOnTop();
   bool RemoveFromHead();
   L* Remove(L * toRemove);

this templete uses the link class templete

#ifndef _Link_H_
#define _Link_H_
template <class T>//error 3
class Link
{
private:
 T* m_data;
 Link* m_next;
 Link* m_prev;
public:
 Link(T* data);
 ~Link(void);
 bool Link::operator ==(const Link& other)const;

 /*getters*/
 Link* GetNext()const {return m_next;}
 Link* GetPrev()const {return m_prev;}
 T* GetData()const {return m_data;}
 //setters
 void SetNext(Link* next) {m_next = next;}
 void SetPrev(Link* prev) {m_prev = prev;}
 void SetData(T* data) {m_data = data;}

};

error one: shadows template parm `class L'
error two:declaration of `class L'
error three: shadows template parm `class T'

i dont understand what is the problem. i can really use your help thank you :)

Answer 1

These error messages really belong together:

a.cc:41: error: declaration of ‘class L’
a.cc:26: error:  shadows template parm ‘class L’

This means that in line 41, you introduce a template parameter L; in my copy, this refers to

template <class L> friend ostream & operator<<(ostream& os,
               const LinkedList<L> listToprint);//error two

And that declaration shadows the template parameter in line 26:

template <class L>//error one
class LinkedList

You need to rename the template parameter in the friend declaration.

Edit : The relevant language specification is 14.6.1/7

A template-parameter shall not be redeclared within its scope (including nested scopes). A template-parameter shall not have the same name as the template name.

When you refer to L in const LinkedList<L> listToprint , it's not clear whether you mean the L of the friend or the L of the class. So write

template <class L1> friend ostream & operator<<(ostream& os,
    const LinkedList<L1> listToprint);

Answer 2

Just remove the

 template <class L>

from the friend member function declaration.

You also need to replace uses of ostream with std::ostream unless you have a using namespace std somewhere in your code.

Otherwise, the code looks fine.