在模板类中声明模板friend类中的编译器错误 [英] Compiler error in declaring template friend class within a template class

问题描述

我一直试图实现我自己的链表类教学目的。

我在迭代器声明中指定了List类作为朋友，似乎正在编译。

这些是我使用的3个类的接口：

strong> Node.h：

  #define null（Node< T& b $ b template< class T> 
 class Node {
 public：
 T content; 
 Node< T> * next; 
 Node< T> * prev; 
 
 Node（const T& _content）：
 content（_content），
 next（null），
 prev（null）
 {} 
}; 
  

Iterator.h：

  #includeNode.h
 
 template< class T> 
 class Iterator {
 private：
 Node< T> * current; 
 
迭代器（Node< T> *）; 
 
 public：
 bool isDone（）const; 
 
 bool hasNext（）const; 
 bool hasPrevious（）const; 
 void stepForward（）; 
 void stepBackwards（）; 
 
 T& currentElement（）const; 
 
 friend class List< T> ;; 
}; 
  

List.h

  #include< stdexcept> 
 #includeIterator.h
 
 template< class T> 
 class List {
 private：
 Node< T> * head; 
 Node< T> * tail; 
 unsigned int items; 
 
 public：
 List（）; 
 
 List（const List< T>&）; 
 List& operator =（const List< T&;）; 
 
〜List（）; 
 
 bool isEmpty（）const {
 return items == 0; 
} 
 unsigned int length（）const {
 return items; 
} 
 void clear（）; 
 
 void add（const T&）; 
 T remove（const T&）throw（std :: length_error& std :: invalid_argument&）; 
 
迭代器< T> createStartIterator（）const throw（std :: length_error&）; 
迭代器< T> createEndIterator（）const throw（std :: length_error&）; 
}; 
  

这是我一直试图运行的测试程序：

trial.cpp

  using namespace std; 
 #include< iostream> 
 #includeList / List.cc
 
 int main（）
 {
 List< int> myList; 
 
 for（int i = 1; i <= 10; i ++）{
 myList.add（i）; 
} 
 
 for（Iterator< int> it = myList.createStartIterator（）;！it.isDone（）; it.stepForward（））{
 cout< it.currentElement（）<< endl; 
} 
 
 return 0; 
} 
  

当我尝试编译它时，编译器会给我以下错误： / p>

Iterator.h：26：错误：'列表'不是模板

Iterator.h：Iterator实例化：

trial.cpp：18：

Iterator.h：12：错误：'struct List'所需的模板参数

List.cc：在成员函数'Iterator List :: createStartIterator（）const [with T = int]'：

strong> trial.cpp：18：从此处实例化

Iterator.h：14：error：'Iterator :: Iterator [with T = int]'is private

List.cc:120：error：在此上下文中

似乎它不能识别朋友声明。
我在哪里错了？

解决方案

尝试添加前瞻性声明

 模板< class T>类List;  Iterator.h 开头的
  

< - 这可能是您需要允许 Iterator 类中的 friend 声明工作。

I have been trying to implement my own linked list class for didactic purposes.

I specified the "List" class as friend inside the Iterator declaration, but it doesn't seem to compile.

These are the interfaces of the 3 classes I've used:

Node.h:

#define null (Node<T> *) 0

template <class T>
class Node {
 public:
    T content;
    Node<T>* next;
    Node<T>* prev;

    Node (const T& _content) :
        content(_content),
        next(null),
        prev(null)
    {}
};

Iterator.h:

#include "Node.h"

template <class T>
class Iterator {
 private:
    Node<T>* current;

    Iterator (Node<T> *);

 public:
    bool isDone () const;

    bool hasNext () const;
    bool hasPrevious () const;
    void stepForward ();
    void stepBackwards ();

    T& currentElement () const;

    friend class List<T>;
};

List.h

#include <stdexcept>
#include "Iterator.h"

template <class T>
class List {
 private:
    Node<T>* head;
    Node<T>* tail;
    unsigned int items;

 public:
    List ();

    List (const List<T>&);
    List& operator = (const List<T>&);

    ~List ();

    bool isEmpty () const {
        return items == 0;
    }
    unsigned int length () const {
        return items;
    } 
    void clear ();

    void add (const T&);
    T remove (const T&) throw (std::length_error&, std::invalid_argument&);

    Iterator<T> createStartIterator () const throw (std::length_error&);
    Iterator<T> createEndIterator () const throw (std::length_error&);
};

And this is the test program I've been trying to run:

trial.cpp

using namespace std;
#include <iostream>
#include "List/List.cc"

int main ()
{
 List<int> myList;

 for (int i = 1; i <= 10; i++) {
  myList.add(i);
 }

 for (Iterator<int> it = myList.createStartIterator(); !it.isDone(); it.stepForward()) {
  cout << it.currentElement() << endl;
 }

 return 0;
}

When I try to compile it, the compiler gives me the following errors:

Iterator.h:26: error: ‘List’ is not a template

Iterator.h: In instantiation of ‘Iterator’:

trial.cpp:18: instantiated from here

Iterator.h:12: error: template argument required for ‘struct List’

List.cc: In member function ‘Iterator List::createStartIterator() const [with T = int]’:

trial.cpp:18: instantiated from here

Iterator.h:14: error: ‘Iterator::Iterator(Node*) [with T = int]’ is private

List.cc:120: error: within this context

Seems like it is not recognizing the friend declaration. Where did I go wrong?

解决方案

try adding a forward declaration

template <class T> class List;

at the start of Iterator.h -- that might be what you need to allow the friend declaration inside the Iterator class to work.

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