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[英]C++ node assign error: Thread 1: EXC_BAD_ACCESS (code=1, address=0x0)
[英]C++: Attempting to add new node to linked list yields “Thread 1: EXC_BAD_ACCESS (code=1, address=0x0)” error
我正在编写一个用于作业的程序,该程序可以创建和操作链表。 我在Node.cpp的Node :: SetData函数以及List.cpp的List :: Add_End行(特别是“ current-> SetData(data);”)中遇到一行“ EXC_BAD_ACCESS”错误,并且由于某种原因,main.cpp中的行(特别是“ // Add_End节点到列表”)。 我假设一旦修复了Node :: SetData错误,这些其他错误将自行解决。
在搜索Stack Overflow和Google之后,我无法确定为什么会发生此错误。 我认为这个问题( C ++的新功能,我不理解的“ EXC_BAD_ACCESS”错误 )会有所帮助,但是我仍然遇到问题。
我犯了什么编码错误?
main.cpp
#include <iostream>
#include <cstddef>
using namespace std;
#include "List.h"
int main()
{
// New list
List list;
Node *answer;
// Add_End nodes to the list
list.Add_End(111);
list.Print();
list.Add_End(222);
list.Print();
list.Add_End(333);
list.Print();
list.Add_End(444);
list.Print();
list.Add_End(555);
list.Print();
// Delete nodes from the list
list.Delete(444);
list.Print();
list.Delete(333);
list.Print();
list.Delete(222);
list.Print();
list.Delete(555);
list.Print();
list.Delete(111);
list.Print();
cout << "Testing Add_Front: and others" << endl;
list.Add_Front(888);
list.Print();
list.Add_Front(999);
list.Print();
list.Add_Front(49);
list.Print();
cout << "Checking find function" << endl;
answer = list.Find(888);
cout << "Value for node returned by find function call with 888 is " << answer->Data() << "." << endl;
cout << "Checking find function" << endl;
answer = list.Find(999);
cout << "Value for node returned by find function call with 888 is " << answer->Data() << "." << endl;
cout << "Checking find function" << endl;
answer = list.Find(49);
cout << "Value for node returned by find function call with 888 is " << answer->Data() << "." << endl;
cout << "Call find function with value not in list." << endl;
answer = list.Find(7);
if (answer == NULL)
{
cout << "returned null pointer since 7 not found" << endl;
}
else
{
cout << "in else of answer == NULL where Value for node returned by find function call with 7 is " << answer->Data() << "." << endl;
}
cout << "testing delete_front: " << endl;
list.Delete_Front();
list.Print();
cout << "testing delete_end: " << endl;
list.Delete_End();
list.Print();
return 0;
}
清单.h
#ifndef LIST_H
#define LIST_H
#include <cstddef>
#include "Node.h"
class List
{
private:
Node* head;
public:
List();
void Add_End(int data);
void Delete(int data);
void Delete_Front();
void Add_Front(int data);
void Delete_End();
Node* Find(int data);
void Print();
};
#endif
List.cpp
#include <iostream>
#include <cstddef>
using namespace std;
#include "List.h"
List::List()
{
head = NULL;
return;
}
void List::Add_End(int data)
{
Node* current;
Node* newEnd = new Node();
for (current = head; current != NULL; current = current->Next())
{}
current->SetData(data);
current->SetNext(newEnd);
newEnd->SetData(NULL);
newEnd->SetNext(NULL);
return;
}
void List::Delete(int data) {
/*
FILL IN CODE (will do later)
*/
return;
}
void List::Delete_Front()
{
/*
FILL IN CODE (will do later)
*/
return;
}
void List::Add_Front(int data)
{
Node* newNode = new Node();
newNode->SetData(data);
newNode->SetNext(head);
head = newNode;
return;
}
void List::Delete_End()
{
if (head == NULL)
{
cout << "List has no member so cannot delete end" << endl;
return;
}
// check if one in length
if (head->Next() == NULL)
{
head = NULL;
return;
}
// 2 or greater in length
Node* current;
Node* prev;
prev = head;
for (current = head->Next(); current->Next() != NULL; current = current->Next())
{
prev = current;
}
prev->SetNext(NULL);
return;
}
Node* List::Find(int data)
{
Node* current;
for (current = head; current != NULL && current->Data() != data; current = current->Next())
{}
if(current == NULL)
{
cout << "Did not find " << data << "." << endl;
return NULL;
}
else // found
{
cout << "Found " << data << "." << endl;
return current;
}
}
void List::Print()
{
Node* current;
for (current = head; current != NULL; current = current->Next())
{
cout << current->Data() << " ";
}
cout << endl;
return;
}
节点
#ifndef NODE_H
#define NODE_H
class Node
{
private:
int data;
Node* next;
public:
Node();
void SetData(int aData);
void SetNext(Node* aNext);
int Data();
Node* Next();
};
#endif
Node.cpp
#include <cstddef>
#include "Node.h"
Node::Node()
{
this->SetData(NULL);
this->SetNext(NULL);
return;
}
void Node::SetData(int aData)
{
this->data = aData;
return;
}
void Node::SetNext(Node* aNext)
{
this->next = aNext;
return;
}
int Node::Data()
{
return data;
}
Node* Node::Next()
{
return next;
}
第一次调用current-> SetData时(请参见下文),current为NULL,因此在访问它时会出现页面错误(页面错误是现代操作系统在尝试访问未分配的内存时给您的错误。访问冲突。)
void List::Add_End(int data)
{
Node* current;
Node* newEnd = new Node();
for (current = head; current != NULL; current = current->Next())
{}
current->SetData(data);
current->SetNext(newEnd);
newEnd->SetData(NULL);
newEnd->SetNext(NULL);
return;
}
我设法更正了代码,因此在其他人遇到相同问题的情况下,我将做些解释。
变更:在我解释此修复程序之前,让我解释一下我所做的更改。 链表的最后一个节点本身可以保存数据值,而不仅仅是NULL
(即,最后一个节点的data
不必为NULL
,而其next
应该为NULL
),所以我认为这样会更好。 该代码在涉及的每个位置都反映了这一点,例如List::Add_End(int data)
函数。
解决方法:我修改了List
构造函数以为列表创建头节点。 因此,即使列表为空,链接列表也将始终至少具有一个节点。 稍后,我将说明程序如何识别空列表和非空列表。
这是原始的构造函数:
List::List()
{
head = NULL;
return;
}
这是新的构造函数:
List::List()
{
Node* headNode = new Node();
head = headNode;
return;
}
为什么要进行此修改? 据我所知,我遇到了EXC_BAD_ACCESS错误,因为List::Add_End(int data)
函数试图操纵链接列表的head
,就好像它是节点对象一样,而实际上并非如此。 (我相信这是他回答这个问题的意思。)这就是为什么我更改了编码,使得即使列表为空,列表也始终包含头节点。
区分空列表和非空列表。 我更改了Node
构造函数以将data
设置为整数-1122334455
,而不是像我最初那样将其设置为NULL
。 因此,如果列表为空,则head->Data()
(即头节点的data
)为-112233455
而head->Next()
(即头节点的next
)为NULL
。 这种方法的缺点是不可能有一个包含整数 -1122334455
的单项列表 ,但是我认为这个数字很可能是不必要的。 只要列表中至少包含两项, head->Data()
可以为-1122334455
。
新代码:其余代码反映了这些修改。 由于我仅对List.cpp和Node.cpp进行了重大更改,因此下面仅复制了它们。 其他三个程序文件基本上未更改。 仅供参考,有很多多余的return
, this
是我不想删除的。
List.cpp
#include <iostream>
#include <cstddef>
using namespace std;
#include "List.h"
// -1122334455 is an arbitrary integer that is likely to never be needed by the user
List::List()
{
Node* headNode = new Node();
head = headNode;
return;
}
Node* List::Add_End(int data)
{
// if list is empty (i.e., list has only head node with data == -1122334455 & next == NULL)
if (head->Data() == -1122334455 && head->Next() == NULL)
{
head->SetData(data);
return head;
}
// if list is nonempty
else
{
Node* current;
Node* newEnd = new Node();
for (current = head; current->Next() != NULL; current = current->Next())
{}
current->SetNext(newEnd);
newEnd->SetData(data);
newEnd->SetNext(NULL);
return newEnd;
}
}
void List::Delete(int data)
{
Node* prev;
Node* current;
// if list is empty
if (head->Data() == -1122334455 && head->Next() == NULL)
{
cout << "Cannot delete this datum because list is empty." << endl;
return;
}
// if list contains 1 element
if (head->Data() == data && head->Next() == NULL)
{
head->SetData(-1122334455);
return;
}
else if (head->Data() != data && head->Next() == NULL)
{
cout << "Datum not found in list." << endl;
return;
}
// if list contains 2 or more elements
prev = head;
for (current = head->Next(); current->Data() != data && current->Next() != NULL; current = current->Next())
{
prev = prev->Next();
}
if (current->Data() == data && current->Next() != NULL)
{
prev->SetNext(current->Next());
delete current;
return;
}
else if (current->Data() == data && current->Next() == NULL)
{
prev->SetNext(NULL);
delete current;
return;
}
else
{
cout << "Datum not found in list." << endl;
return;
}
}
void List::Delete_Front()
{
Node* origHead = head;
Node* newHead = head->Next();
head = newHead;
delete origHead;
return;
}
void List::Add_Front(int data)
{
// if list is empty
if (head->Data() == -1122334455 && head->Next() == NULL)
{
head->SetData(data);
return;
}
// if list is nonempty
Node* newNode = new Node();
newNode->SetData(data);
newNode->SetNext(head);
head = newNode;
return;
}
void List::Delete_End()
{
if (head->Data() == -1122334455 && head->Next() == NULL)
{
cout << "List has no member so cannot delete end" << endl;
return;
}
// check if one in length
else if (head->Data() != -1122334455 && head->Next() == NULL)
{
head->SetData(-1122334455);
return;
}
// 2 or greater in length
else
{
Node* current;
Node* prev;
prev = head;
for (current = head->Next(); current->Next() != NULL; current = current->Next())
{
prev = current;
}
prev->SetNext(NULL);
return;
}
}
Node* List::Find(int data)
{
Node* current;
for (current = head; current != NULL && current->Data() != data; current = current->Next())
{}
if (current == NULL)
{
cout << "Did not find " << data << "." << endl;
return NULL;
}
else // found
{
cout << "Found " << data << "." << endl;
return current;
}
}
void List::Print()
{
if (head->Data() == -1122334455 && head->Next() == NULL)
{
cout << "List is empty." << endl;
return;
}
Node* current;
for (current = head; current != NULL; current = current->Next())
{
cout << current->Data() << " ";
}
cout << endl;
return;
}
Node.cpp
#include <cstddef>
#include "Node.h"
Node::Node()
{
// -1122334455 is an arbitrary integer that is likely to never be needed by the user
this->SetData(-1122334455);
this->SetNext(NULL);
return;
}
void Node::SetData(int aData)
{
this->data = aData;
return;
}
void Node::SetNext(Node* aNext)
{
this->next = aNext;
return;
}
int Node::Data()
{
return this->data;
}
Node* Node::Next()
{
return this->next;
}
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