程序停止运行？

Question

因此，出于某种原因，我的程序在第二次排队后停止运行。它编译并没有提供任何错误信息，它只是突然退出。甚至没有中止的消息或任何东西。我有其他2人在更高的CS班比我看看，他们也看不出什么错要么...

主要如下：

using namespace std; 
#include <iostream> 
#include "Queue.h" 
#include "LinkedList.h" 

int main() 
{ 
    try 
    { 
     int type = 0; 

     cout<<"What data type do you want to work with? 1 = int, 2 = char"<<endl; 
     cin>>type; 

     if(type == 1) 
     { 
      Queue<int> q; 

      q.enqueue(1); 
      cout<<"The size is: "<< q.size() <<". And the top is: " << q.front() <<endl; 

      q.enqueue(5); 
      cout<<"The size is: "<< q.size() <<". And the top is: " << q.front() <<endl; 

      q.enqueue(3); 
      cout<<"The size is: "<< q.size() <<". And the top is: " << q.front() <<endl; 

      q.enqueue(5); 
      cout<<"The size is: "<< q.size() <<". And the top is: " << q.front() <<endl; 

      q.dequeue(); 
      cout<<"The size is: "<< q.size() <<". And the top is: " << q.front() <<endl; 

      q.dequeue(); 
      cout<<"The size is: "<< q.size() <<". And the top is: " << q.front() <<endl; 

      q.dequeue(); 
      cout<<"The size is: "<< q.size() <<". And the top is: " << q.front() <<endl; 
     } 

     if (type == 2) 
     { 
      Queue<char> q; 

      q.enqueue('a'); 
      cout<<"The size is: "<< q.size() <<". And the top is: " << q.front() <<endl; 

      q.enqueue('b'); 
      cout<<"The size is: "<< q.size() <<". And the top is: " << q.front() <<endl; 

      q.enqueue('c'); 
      cout<<"The size is: "<< q.size() <<". And the top is: " << q.front() <<endl; 

      q.dequeue(); 
      cout<<"The size is: "<< q.size() <<". And the top is: " << q.front() <<endl; 

      q.dequeue(); 
      cout<<"The size is: "<< q.size() <<". And the top is: " << q.front() <<endl; 

      q.dequeue(); 
      cout<<"The size is: "<< q.size() <<". And the top is: " << q.front() <<endl; 
     } 

     return 1; 
    } 
    catch (int e) 
    { 
     if (e == 2) 
     { 
      cout<<"Call to dequeue() generated an exception, because the queue is empty."<<endl; 
     } 
     else if (e == 3) 
     { 
      cout<<"Call to front() generated an exception, because the queue is empty."<<endl; 
     } 
    } 
} 

然后第一个模板：

using namespace std; 
#include <iostream> 
#ifndef LINKEDLIST_H 
#define LINKEDLIST_H 

template <class T> 
struct Node 
{ 
    T val; 
    Node<T> *next; 
}; 

template <class T> 
class LinkedList 
{ 
    public: 
     LinkedList(); 
     ~LinkedList(); 
     void insertAtBack(T valueToInsert); 
     bool removeFromBack(); 
     void print(); 
     bool isEmpty(); 
     int size(); 
     void clear(); 

     void insertAtFront(T valueToInsert); 
     bool removeFromFront(); 

     T& firstNum(); 

    private: 
     Node<T> *first; 
     Node<T> *last; 
}; 

/**************CPP FILE*****************/ 

template <class T> 
LinkedList<T>::LinkedList() 
{ 
    first = NULL; 
    last = NULL; 
} 

template <class T> 
LinkedList<T>::~LinkedList() 
{ 
    Node<T>* temp = first; 
    while(temp != NULL) 
    { 
     temp = temp->next; 
     delete(first); 
     first = temp; 
    } 
} 

template <class T> 
void LinkedList<T>::insertAtBack(T valueToInsert) 
{ 
    Node<T>* newNode; 
    newNode->val = valueToInsert; 
    newNode->next = NULL; 

    Node<T>* temp = first; 

    if (temp != NULL) 
    { 
     while (temp->next != NULL) 
     { 
      temp = temp->next; 
     } 

     temp->next = newNode; 
    } 
    else 
    { 
     first = newNode; 
    } 
} 

template <class T> 
bool LinkedList<T>::removeFromBack() 
{ 
    if (first == NULL && last == NULL) {return false;} 

    if (first == last) 
    { 
     cout<<"First is equal to Last."<<endl; 
     delete(first); 
     first = last = NULL; 
     return true; 
    } 

    else 
    { 
     Node<T>* temp = first; 
     int nodeCount = 0; 

     while (temp != NULL) 
     { 
      nodeCount = nodeCount + 1; 
      temp = temp->next; 
     } 

     Node<T>* temp2 = first; 

     for(int i = 1; i < (nodeCount - 1); i++) 
     { 
      temp2 = temp2->next; 
     } 

     cout << temp2->val<<endl; 
     delete(temp2->next); 

     last = temp2; 
     last->next = NULL; 

     return true; 
    } 
} 

template <class T> 
void LinkedList<T>::print() 
{ 
    Node<T>* temp = first; 

    if (temp == NULL) 
    { 
     cout<<""; 
    } 

    if (temp->next == NULL) 
    { 
     cout<<temp->val; 
    } 
    else 
    { 
     while (temp != NULL) 
     { 
      cout<< temp->val; 
      temp = temp->next; 
      cout<< ","; 
     } 
    } 
} 

template <class T> 
bool LinkedList<T>::isEmpty() 
{ 
    if (first == NULL && last == NULL) {return true;} 
    else {return false;} 
} 

template <class T> 
int LinkedList<T>::size() 
{ 
    if (first == NULL && last == NULL) {return 0;} 

    Node<T>* temp = first; 
    int nodeCounter = 0; 

    while (temp != NULL) 
    { 
     nodeCounter = nodeCounter + 1; 
     temp = temp->next; 
    } 
    return nodeCounter; 
} 

template <class T> 
void LinkedList<T>::clear() 
{ 
    Node<T>* temp = first; 
    while(temp != NULL) 
    { 
     temp = temp->next; 
     first = temp; 
     delete(temp); 
    } 
} 

template <class T> 
void LinkedList<T>::insertAtFront(T valueToInsert) 
{ 
    Node<T>* newNode; 

    newNode->val = valueToInsert; 

    if(first == NULL) 
    { 
     first = newNode; 
    } 
    else 
    { 
     newNode->next = first; 
     first = newNode; 
    } 
} 

template <class T> 
bool LinkedList<T>::removeFromFront() 
{ 
    if (first == NULL && last == NULL) {return false;} 

    else 
    { 
     Node<T>* temp; 

     temp = first; 
     first = first->next; 

     delete(temp); 

     return true; 
    } 
} 

template <class T> 
T& LinkedList<T>::firstNum() 
{ 
    return first->val; 
} 

#endif 

而第二个模板：

#include <iostream> 
#include "LinkedList.h" 
#ifndef QUEUE_H 
#define QUEUE_H 

template <class T> 
class Queue: public LinkedList<T> 
{ 
    public: 
     Queue(); 
     ~Queue(); 

     void enqueue(T value); 
     T dequeue(); 
     T& front(); 
}; 

/**************CPP FILE*****************/ 

template <class T> 
Queue<T>::Queue(){} 

template <class T>  
Queue<T>::~Queue(){} 

template <class T>  
void Queue<T>::enqueue(T value) 
{ 
    LinkedList<T>::insertAtBack(value); 
} 

template <class T> 
T Queue<T>::dequeue() 
{ 
    if(LinkedList<T>::isEmpty()) 
    { 
     throw 2; 
    } 
    else 
    { 
     T firstElmnt = LinkedList<T>::firstNum(); 
     LinkedList<T>::removeFromFront(); 

     return firstElmnt; 
    } 
} 

template <class T> 
T& Queue<T>::front() 
{ 
    if(LinkedList<T>::isEmpty()) 
    { 
     throw 3; 
    } 
    else 
    { 
     return LinkedList<T>::firstNum(); 
    } 
} 

#endif 

Answer 1

让我们来看看在发生了什么并添加代码一些意见：

template <class T> 
void LinkedList<T>::insertAtBack(T valueToInsert) 
{ 
    // Let's declare a pointer that is uninitialized and points to who 
    // knows where. 
    Node<T>* newNode; 

    // Now let's dereference this pointer. Remember, it points somewhere 
    // random. Maybe this will crash, maybe this won't. It's undefined 
    // behavior. 
    newNode->val = valueToInsert; 

    // We are well inside undefined behavior land now. I hope you brought 
    // your passport and a fresh pair of pants. 
    newNode->next = NULL; 

    Node<T>* temp = first; 

    if (temp != NULL) 
    { 
     while (temp->next != NULL) 
     { 
      temp = temp->next; 
     } 

     temp->next = newNode; 
    } 
    else 
    { 
     // If we get here, first will be changed and end up pointing to 
     // some random location in memory. And that's bad, mmkay? 
     first = newNode; 
    } 
} 

道德这个故事是解引用未初始化的指针是坏。