C++ 关于模板之间的继承， 导致的模板子类的成员看不到（cannot access）模板父类的成员函数的解决办法。
2014-07-30 19:20
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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：
unorderedLinkedList.h文件如下：
主程序如下main.cpp：
编译结果error message 如下：
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？
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。
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：
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。
编译通过， 运行结果如下：
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：
//To make the code valid either use this->f(), or Base::f(). Using the -fpermissive flag will also
//let the compiler accept the code, by marking all function calls for which no declaration is visible
//at the time of definition of the template for later lookup at instantiation time, as if it were a
//dependent call. We do not recommend using -fpermissive to work around invalid code, and it will also
//only catch cases where functions in base classes are called, not where variables in base classes are
//used (as in the example above).
//
//Note that some compilers (including G++ versions prior to 3.4) get these examples wrong and accept above
//code without an error. Those compilers do not implement two-stage name lookup correctly.
//
接下来是我在stack overflow 网站上得到的解答：
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 << current->info << " ";
current = current->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;
}
#endifunorderedLinkedList.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 << "Cannot delete from an empty list."
<< endl;
else
{
if (first->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 << "The item to be deleted is not in "
<< "the list." << endl;
}//end else
}//end else
}//end deleteNode
#endif主程序如下main.cpp：
//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 << "Line 3: Enter integers ending "
<< "with -999" << endl; //Line 3
cin >> num; //Line 4
while (num != -999) //Line 5
{
list1.insertLast(num); //Line 6
cin >> num; //Line 7
}
cout << endl; //Line 8
cout << "Line 9: list1: "; //Line 9
list1.print(); //Line 10
cout << endl; //Line 11
cout << "Line 12: Length of list1: "
<< list1.length() << endl; //Line 12
list2 = list1; //test the assignment operator Line 13
cout << "Line 16: list2: "; //Line 14
list2.print(); //Line 15
cout << endl; //Line 16
cout << "Line 17: Length of list2: "
<< list2.length() << endl; //Line 17
cout << "Line 18: Enter the number to be "
<< "deleted: "; //Line 18
cin >> num; //Line 19
cout << endl; //Line 20
list2.deleteNode(num); //Line 21
cout << "Line 22: After deleting " << num
<< " list2: " << endl; //Line 22
list2.print(); //Line 23
cout << endl; //Line 24
cout << "Line 25: Length of list2: "
<< list2.length() << endl; //Line 25
cout << endl << "Line 26: Output list1 "
<< "using an iterator" << endl; //LIne 26
linkedListIterator<int> it; //Line 27
for (it = list1.begin(); it != list1.end();
++it) //Line 28
cout << *it << " "; //Line 29
cout << endl; //Line 30
return 0;
}编译结果error message 如下：
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？
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。
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：
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。
#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 << "Cannot delete from an empty list."
<< endl;
else
{
if (this -> 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 << "The item to be deleted is not in "
<< "the list." << endl;
}//end else
}//end else
}//end deleteNode
#endif编译通过， 运行结果如下：
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：
//To make the code valid either use this->f(), or Base::f(). Using the -fpermissive flag will also
//let the compiler accept the code, by marking all function calls for which no declaration is visible
//at the time of definition of the template for later lookup at instantiation time, as if it were a
//dependent call. We do not recommend using -fpermissive to work around invalid code, and it will also
//only catch cases where functions in base classes are called, not where variables in base classes are
//used (as in the example above).
//
//Note that some compilers (including G++ versions prior to 3.4) get these examples wrong and accept above
//code without an error. Those compilers do not implement two-stage name lookup correctly.
//
接下来是我在stack overflow 网站上得到的解答：
内容来自用户分享和网络整理，不保证内容的准确性，如有侵权内容，可联系管理员处理 
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