C++ 关于模板之间的继承， 导致的模板子类的成员看不到（cannot
- WBOY原創
- 2016-06-07 15:37:271325瀏覽
首先， 开门见山， 这个难题的解决办法是用this指针， 或者使用父类配合着scope resolution。 这个问题是我在学习linked list as an ADT , linked list 是含有纯虚函数， 所以是抽象基础类。 然后又linked list 继承出unordered linked list。 注意， 还可以
首先， 开门见山， 这个难题的解决办法是用this指针， 或者使用父类配合着scope resolution。
这个问题是我在学习linked list as an ADT , linked list 是含有纯虚函数， 所以是抽象基础类。 然后又linked list 继承出unordered linked list。 注意， 还可以由linked list 继承出ordered linked list。
言归正传， 出现问题的代码如下：
linkedlist.h 文件如下：
#ifndef H_LinkedListType
#define H_LinkedListType
#include <iostream>
#include <cassert>
using namespace std;
//Definition of the node
template <class type>
struct nodeType
{
Type info;
nodeType<type> *link;
};
template <class type>
class linkedListIterator
{
public:
linkedListIterator();
//Default constructor
//Postcondition: current = NULL;
linkedListIterator(nodeType<type> *ptr);
//Constructor with a parameter.
//Postcondition: current = ptr;
Type operator*();
//Function to overload the dereferencing operator *.
//Postcondition: Returns the info contained in the node.
linkedListIterator<type> operator++();
//Overload the pre-increment operator.
//Postcondition: The iterator is advanced to the next
// node.
bool operator==(const linkedListIterator<type>& right) const;
//Overload the equality operator.
//Postcondition: Returns true if this iterator is equal to
// the iterator specified by right,
// otherwise it returns the value false.
bool operator!=(const linkedListIterator<type>& right) const;
//Overload the not equal to operator.
//Postcondition: Returns true if this iterator is not
// equal to the iterator specified by
// right; otherwise it returns the value
// false.
private:
nodeType<type> *current; //pointer to point to the current
//node in the linked list
};
template <class type>
linkedListIterator<type>::linkedListIterator()
{
current = NULL;
}
template <class type>
linkedListIterator<type>::
linkedListIterator(nodeType<type> *ptr)
{
current = ptr;
}
template <class type>
Type linkedListIterator<type>::operator*()
{
return current->info;
}
template <class type>
linkedListIterator<type> linkedListIterator<type>::operator++()
{
current = current->link;
return *this;
}
template <class type>
bool linkedListIterator<type>::operator==
(const linkedListIterator<type>& right) const
{
return (current == right.current);
}
template <class type>
bool linkedListIterator<type>::operator!=
(const linkedListIterator<type>& right) const
{ return (current != right.current);
}
//***************** class linkedListType ****************
template <class type>
class linkedListType
{
public:
const linkedListType<type>& operator=
(const linkedListType<type>&);
//Overload the assignment operator.
void initializeList();
//Initialize the list to an empty state.
//Postcondition: first = NULL, last = NULL, count = 0;
bool isEmptyList() const;
//Function to determine whether the list is empty.
//Postcondition: Returns true if the list is empty,
// otherwise it returns false.
void print() const;
//Function to output the data contained in each node.
//Postcondition: none
int length() const;
//Function to return the number of nodes in the list.
//Postcondition: The value of count is returned.
void destroyList();
//Function to delete all the nodes from the list.
//Postcondition: first = NULL, last = NULL, count = 0;
Type front() const;
//Function to return the first element of the list.
//Precondition: The list must exist and must not be
// empty.
//Postcondition: If the list is empty, the program
// terminates; otherwise, the first
// element of the list is returned.
Type back() const;
//Function to return the last element of the list.
//Precondition: The list must exist and must not be
// empty.
//Postcondition: If the list is empty, the program
// terminates; otherwise, the last
// element of the list is returned.
virtual bool search(const Type& searchItem) const = 0;
//Function to determine whether searchItem is in the list.
//Postcondition: Returns true if searchItem is in the
// list, otherwise the value false is
// returned.
virtual void insertFirst(const Type& newItem) = 0;
//Function to insert newItem at the beginning of the list.
//Postcondition: first points to the new list, newItem is
// inserted at the beginning of the list,
// last points to the last node in the list,
// and count is incremented by 1.
virtual void insertLast(const Type& newItem) = 0;
//Function to insert newItem at the end of the list.
//Postcondition: first points to the new list, newItem
// is inserted at the end of the list,
// last points to the last node in the list,
// and count is incremented by 1.
virtual void deleteNode(const Type& deleteItem) = 0;
//Function to delete deleteItem from the list.
//Postcondition: If found, the node containing
// deleteItem is deleted from the list.
// first points to the first node, last
// points to the last node of the updated
// list, and count is decremented by 1.
linkedListIterator<type> begin();
//Function to return an iterator at the begining of the
//linked list.
//Postcondition: Returns an iterator such that current is
// set to first.
linkedListIterator<type> end();
//Function to return an iterator one element past the
//last element of the linked list.
//Postcondition: Returns an iterator such that current is
// set to NULL.
linkedListType();
//default constructor
//Initializes the list to an empty state.
//Postcondition: first = NULL, last = NULL, count = 0;
linkedListType(const linkedListType<type>& otherList);
//copy constructor
~linkedListType();
//destructor
//Deletes all the nodes from the list.
//Postcondition: The list object is destroyed.
protected:
int count; //variable to store the number of
//elements in the list
nodeType<type> *first; //pointer to the first node of the list
nodeType<type> *last; //pointer to the last node of the list
private:
void copyList(const linkedListType<type>& otherList);
//Function to make a copy of otherList.
//Postcondition: A copy of otherList is created and
// assigned to this list.
};
template <class type>
bool linkedListType<type>::isEmptyList() const
{
return(first == NULL);
}
template <class type>
linkedListType<type>::linkedListType() //default constructor
{
first = NULL;
last = NULL;
count = 0;
}
template <class type>
void linkedListType<type>::destroyList()
{
nodeType<type> *temp; //pointer to deallocate the memory
//occupied by the node
while (first != NULL) //while there are nodes in the list
{
temp = first; //set temp to the current node
first = first->link; //advance first to the next node
delete temp; //deallocate the memory occupied by temp
}
last = NULL; //initialize last to NULL; first has already
//been set to NULL by the while loop
count = 0;
}
template <class type>
void linkedListType<type>::initializeList()
{
destroyList(); //if the list has any nodes, delete them
}
template <class type>
void linkedListType<type>::print() const
{
nodeType<type> *current; //pointer to traverse the list
current = first; //set current so that it points to
//the first node
while (current != NULL) //while more data to print
{
cout info link;
}
}//end print
template <class type>
int linkedListType<type>::length() const
{
return count;
} //end length
template <class type>
Type linkedListType<type>::front() const
{
assert(first != NULL);
return first->info; //return the info of the first node
}//end front
template <class type>
Type linkedListType<type>::back() const
{
assert(last != NULL);
return last->info; //return the info of the last node
}//end back
template <class type>
linkedListIterator<type> linkedListType<type>::begin()
{
linkedListIterator<type> temp(first);
return temp;
}
template <class type>
linkedListIterator<type> linkedListType<type>::end()
{
linkedListIterator<type> temp(NULL);
return temp;
}
template <class type>
void linkedListType<type>::copyList
(const linkedListType<type>& otherList)
{
nodeType<type> *newNode; //pointer to create a node
nodeType<type> *current; //pointer to traverse the list
if (first != NULL) //if the list is nonempty, make it empty
destroyList();
if (otherList.first == NULL) //otherList is empty
{
first = NULL;
last = NULL;
count = 0;
}
else
{
current = otherList.first; //current points to the
//list to be copied
count = otherList.count;
//copy the first node
first = new nodeType<type>; //create the node
first->info = current->info; //copy the info
first->link = NULL; //set the link field of
//the node to NULL
last = first; //make last point to the
//first node
current = current->link; //make current point to
//the next node
//copy the remaining list
while (current != NULL)
{
newNode = new nodeType<type>; //create a node
newNode->info = current->info; //copy the info
newNode->link = NULL; //set the link of
//newNode to NULL
last->link = newNode; //attach newNode after last
last = newNode; //make last point to
//the actual last node
current = current->link; //make current point
//to the next node
}//end while
}//end else
}//end copyList
template <class type>
linkedListType<type>::~linkedListType() //destructor
{
destroyList();
}//end destructor
template <class type>
linkedListType<type>::linkedListType
(const linkedListType<type>& otherList)
{
first = NULL;
copyList(otherList);
}//end copy constructor
//overload the assignment operator
template <class type>
const linkedListType<type>& linkedListType<type>::operator=
(const linkedListType<type>& otherList)
{
if (this != &otherList) //avoid self-copy
{
copyList(otherList);
}//end else
return *this;
}
#endif
</type></type></type></class></type></type></class></type></class></type></type></type></type></type></type></class></type></type></type></class></type></type></type></class></type></class></type></class></type></class></type></type></class></type></class></type></type></class></type></class></type></class></type></type></type></type></type></type></type></type></class></type></type></class></type></type></class></type></type></class></type></class></type></type></class></type></class></type></type></type></type></type></class></type></class></cassert></iostream>
unorderedLinkedList.h文件如下：
#ifndef H_UnorderedLinkedList
#define H_UnorderedLinkedList
#include "linkedList.h"
using namespace std;
template <class type>
class unorderedLinkedList: public linkedListType<type>
{
public:
bool search(const Type& searchItem) const;
//Function to determine whether searchItem is in the list.
//Postcondition: Returns true if searchItem is in the
// list, otherwise the value false is
// returned.
void insertFirst(const Type& newItem);
//Function to insert newItem at the beginning of the list.
//Postcondition: first points to the new list, newItem is
// inserted at the beginning of the list,
// last points to the last node in the
// list, and count is incremented by 1.
void insertLast(const Type& newItem);
//Function to insert newItem at the end of the list.
//Postcondition: first points to the new list, newItem
// is inserted at the end of the list,
// last points to the last node in the
// list, and count is incremented by 1.
void deleteNode(const Type& deleteItem);
//Function to delete deleteItem from the list.
//Postcondition: If found, the node containing
// deleteItem is deleted from the list.
// first points to the first node, last
// points to the last node of the updated
// list, and count is decremented by 1.
};
template <class type>
bool unorderedLinkedList<type>::
search(const Type& searchItem) const
{
nodeType<type> *current; //pointer to traverse the list
bool found = false;
current = first; //set current to point to the first
//node in the list
while (current != NULL && !found) //search the list
if (current->info == searchItem) //searchItem is found
found = true;
else
current = current->link; //make current point to
//the next node
return found;
}//end search
template <class type>
void unorderedLinkedList<type>::insertFirst(const Type& newItem)
{
nodeType<type> *newNode; //pointer to create the new node
newNode = new nodeType<type>; //create the new node
newNode->info = newItem; //store the new item in the node
newNode->link = this -> first; //insert newNode before first
first = newNode; //make first point to the
//actual first node
count++; //increment count
if (last == NULL) //if the list was empty, newNode is also
//the last node in the list
last = newNode;
}//end insertFirst
template <class type>
void unorderedLinkedList<type>::insertLast(const Type& newItem)
{
nodeType<type> *newNode; //pointer to create the new node
newNode = new nodeType<type>; //create the new node
newNode->info = newItem; //store the new item in the node
newNode->link = NULL; //set the link field of newNode
//to NULL
if ( first == NULL) //if the list is empty, newNode is
//both the first and last node
{
first = newNode;
last = newNode;
count++; //increment count
}
else //the list is not empty, insert newNode after last
{
last->link = newNode; //insert newNode after last
last = newNode; //make last point to the actual
//last node in the list
count++; //increment count
}
}//end insertLast
template <class type>
void unorderedLinkedList<type>::deleteNode(const Type& deleteItem)
{
nodeType<type> *current; //pointer to traverse the list
nodeType<type> *trailCurrent; //pointer just before current
bool found;
if (first == NULL) //Case 1; the list is empty.
cout info == deleteItem) //Case 2
{
current = first;
first = first->link;
count--;
if (first == NULL) //the list has only one node
last = NULL;
delete current;
}
else //search the list for the node with the given info
{
found = false;
trailCurrent = first; //set trailCurrent to point
//to the first node
current = first->link; //set current to point to
//the second node
while (current != NULL && !found)
{
if (current->info != deleteItem)
{
trailCurrent = current;
current = current-> link;
}
else
found = true;
}//end while
if (found) //Case 3; if found, delete the node
{
trailCurrent->link = current->link;
this -> count--;
if (last == current) //node to be deleted
//was the last node
last = trailCurrent; //update the value
//of last
delete current; //delete the node from the list
}
else
cout <br>
主程序如下main.cpp：
<pre class="brush:php;toolbar:false">//This program tests various operation of a linked list
//34 62 21 90 66 53 88 24 10 -999
#include <iostream>
#include "unorderedLinkedList.h"
using namespace std;
int main()
{
unorderedLinkedList<int> list1, list2; //Line 1
int num; //Line 2
cout > num; //Line 4
while (num != -999) //Line 5
{
list1.insertLast(num); //Line 6
cin >> num; //Line 7
}
cout > num; //Line 19
cout it; //Line 27
for (it = list1.begin(); it != list1.end();
++it) //Line 28
cout <br>
编译结果error message 如下：
<p><img src="/inc/test.jsp?url=http%3A%2F%2Fimg.blog.csdn.net%2F20140730194616575%3Fwatermark%2F2%2Ftext%2FaHR0cDovL2Jsb2cuY3Nkbi5uZXQvYTEzMDczNw%3D%3D%2Ffont%2F5a6L5L2T%2Ffontsize%2F400%2Ffill%2FI0JBQkFCMA%3D%3D%2Fdissolve%2F70%2Fgravity%2FSouthEast&refer=http%3A%2F%2Fblog.csdn.net%2Fa130737%2Farticle%2Fdetails%2F38305293" alt="C++ 关于模板之间的继承， 导致的模板子类的成员看不到（cannot" ><br>
</p>
<p><br>
</p>
<p>错误分析： 按照标准看， 上述的错误似乎有点问题。 因为first， last， count 都是父类的成员变量。 存取类型是protected。 我们的子类unorderedLinkedList 类是公开方式（public）继承linkedList。按说子类即unorderedLinkedList 的成员函数（虚函数）当然可以access 父类的protected 成员变量。 但是在这里出错了。 为什么呢？</p>
<p><br>
</p>
<p>原因是我们这里是模板类之间的继承。 如果是正常的普通类之间的继承， 结果一定是没有问题的。 当然access会通过， 编译会okay。 但是这里是模本类之间的继承。</p>
<p>在这里， 我们需要使用this 指针， 也可以使用scope resolution解决模板类之间的继承时变量访问的问题。 下面我们修改unorderedLinkedList.h 如下：</p>
<p>将模板成员函数定义count， first， last 分别用this -> count, this -> first, this -> last（第二中解决办法是换为linkedListType<type>::first, linkedListType<type>::count,linkedListType<type>::last）。 不光如此， 当在子类中调用模板父类的成员函数的时候， 也要使用this 指针或者用scope resotion 解决这个问题。</type></type></type></p>
<p><br>
</p>
<pre class="brush:php;toolbar:false">#ifndef H_UnorderedLinkedList
#define H_UnorderedLinkedList
#include "linkedList.h"
using namespace std;
template <class type>
class unorderedLinkedList: public linkedListType<type>
{
public:
bool search(const Type& searchItem) const;
//Function to determine whether searchItem is in the list.
//Postcondition: Returns true if searchItem is in the
// list, otherwise the value false is
// returned.
void insertFirst(const Type& newItem);
//Function to insert newItem at the beginning of the list.
//Postcondition: first points to the new list, newItem is
// inserted at the beginning of the list,
// last points to the last node in the
// list, and count is incremented by 1.
void insertLast(const Type& newItem);
//Function to insert newItem at the end of the list.
//Postcondition: first points to the new list, newItem
// is inserted at the end of the list,
// last points to the last node in the
// list, and count is incremented by 1.
void deleteNode(const Type& deleteItem);
//Function to delete deleteItem from the list.
//Postcondition: If found, the node containing
// deleteItem is deleted from the list.
// first points to the first node, last
// points to the last node of the updated
// list, and count is decremented by 1.
};
template <class type>
bool unorderedLinkedList<type>::
search(const Type& searchItem) const
{
nodeType<type> *current; //pointer to traverse the list
bool found = false;
current = this -> first; //set current to point to the first
//node in the list
while (current != NULL && !found) //search the list
if (current->info == searchItem) //searchItem is found
found = true;
else
current = current->link; //make current point to
//the next node
return found;
}//end search
template <class type>
void unorderedLinkedList<type>::insertFirst(const Type& newItem)
{
nodeType<type> *newNode; //pointer to create the new node
newNode = new nodeType<type>; //create the new node
newNode->info = newItem; //store the new item in the node
newNode->link = this -> first; //insert newNode before first
this -> first = newNode; //make first point to the
//actual first node
this -> count++; //increment count
if (this -> last == NULL) //if the list was empty, newNode is also
//the last node in the list
this -> last = newNode;
}//end insertFirst
template <class type>
void unorderedLinkedList<type>::insertLast(const Type& newItem)
{
nodeType<type> *newNode; //pointer to create the new node
newNode = new nodeType<type>; //create the new node
newNode->info = newItem; //store the new item in the node
newNode->link = NULL; //set the link field of newNode
//to NULL
if ( this -> first == NULL) //if the list is empty, newNode is
//both the first and last node
{
this -> first = newNode;
this -> last = newNode;
this -> count++; //increment count
}
else //the list is not empty, insert newNode after last
{
this -> last->link = newNode; //insert newNode after last
this -> last = newNode; //make last point to the actual
//last node in the list
this -> count++; //increment count
}
}//end insertLast
template <class type>
void unorderedLinkedList<type>::deleteNode(const Type& deleteItem)
{
nodeType<type> *current; //pointer to traverse the list
nodeType<type> *trailCurrent; //pointer just before current
bool found;
if (this -> first == NULL) //Case 1; the list is empty.
cout first->info == deleteItem) //Case 2
{
current = this -> first;
this -> first = this -> first->link;
this -> count--;
if (this -> first == NULL) //the list has only one node
this -> last = NULL;
delete current;
}
else //search the list for the node with the given info
{
found = false;
trailCurrent = this -> first; //set trailCurrent to point
//to the first node
current = this -> first->link; //set current to point to
//the second node
while (current != NULL && !found)
{
if (current->info != deleteItem)
{
trailCurrent = current;
current = current-> link;
}
else
found = true;
}//end while
if (found) //Case 3; if found, delete the node
{
trailCurrent->link = current->link;
this -> count--;
if (this -> last == current) //node to be deleted
//was the last node
this -> last = trailCurrent; //update the value
//of last
delete current; //delete the node from the list
}
else
cout <br>
编译通过， 运行结果如下：
<p><img src="/inc/test.jsp?url=http%3A%2F%2Fimg.blog.csdn.net%2F20140730200300296%3Fwatermark%2F2%2Ftext%2FaHR0cDovL2Jsb2cuY3Nkbi5uZXQvYTEzMDczNw%3D%3D%2Ffont%2F5a6L5L2T%2Ffontsize%2F400%2Ffill%2FI0JBQkFCMA%3D%3D%2Fdissolve%2F70%2Fgravity%2FSouthEast&refer=http%3A%2F%2Fblog.csdn.net%2Fa130737%2Farticle%2Fdetails%2F38305293" alt="C++ 关于模板之间的继承， 导致的模板子类的成员看不到（cannot" ><br>
</p>
<p><br>
</p>
<p><br>
</p>
<p>关于这个模板继承， 子类使用父类的成员是使用this， 或者scope resolution， 解释如下：</p>
<p>//To make the code valid either use this->f(), or Base::f(). Using the -fpermissive flag will also<br>
//let the compiler accept the code, by marking all function calls for which no declaration is visible<br>
//at the time of definition of the template for later lookup at instantiation time, as if it were a<br>
//dependent call. We do not recommend using -fpermissive to work around invalid code, and it will also<br>
//only catch cases where functions in base classes are called, not where variables in base classes are<br>
//used (as in the example above).<br>
//<br>
//Note that some compilers (including G++ versions prior to 3.4) get these examples wrong and accept above<br>
//code without an error. Those compilers do not implement two-stage name lookup correctly.<br>
//<br>
</p>
<p><br>
</p>
<p>接下来是我在stack overflow 网站上得到的解答：</p>
<p><img src="/inc/test.jsp?url=http%3A%2F%2Fimg.blog.csdn.net%2F20140730201910930%3Fwatermark%2F2%2Ftext%2FaHR0cDovL2Jsb2cuY3Nkbi5uZXQvYTEzMDczNw%3D%3D%2Ffont%2F5a6L5L2T%2Ffontsize%2F400%2Ffill%2FI0JBQkFCMA%3D%3D%2Fdissolve%2F70%2Fgravity%2FSouthEast&refer=http%3A%2F%2Fblog.csdn.net%2Fa130737%2Farticle%2Fdetails%2F38305293" alt="C++ 关于模板之间的继承， 导致的模板子类的成员看不到（cannot" ><br>
</p>
</type></type></type></class></type></type></type></class></type></type></type></class></type></type></class></type></class>陳述：
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