队列的指针问题

Question

我正在做家庭作业，一切都快完成了。我的代码中的某个部分遇到了问题。我尝试了一些事情，但似乎无法弄清楚我做错了什么或我需要做什么才能让它发挥作用。

我正在尝试实现一个接受名为 QueueData 的特定对象的队列。当我执行该程序时，它将一直运行到 P2Queue.cpp 文件中的第 44 行。代码此时的目的是将新的 QueueData 对象添加到队列的尾部，然后将 temp 的指针设置为空，表示队列的末尾。程序在文件 P2Queue.cpp 的第 44 行处停止运行。我做错了什么，我需要改变什么，这简直要了我的命。

提前致谢！如果您需要进一步说明，请告诉我。

---

P2Queue.cpp
#include"P2Queue.h"
#include<iostream>
#include<string.h>

using namespace std;

P2Queue::P2Queue() //: head(NULL), tail(NULL), count(0)
{
    head=NULL;
    tail=NULL;
    count=0;
}

P2Queue::~P2Queue()
{
    QueueData* temp;
    while(head!=NULL)
    {
        temp=head;
        head=temp->next;
        delete temp;
    }
    head=tail=NULL;
}

void P2Queue::Enqueue(int x, char *y)
{
    cout<<"I'm HERE in the function call"<<endl;
    QueueData* temp;
    temp=new QueueData();
    temp->num=x;
    cout<<temp->num<<endl;
    strcpy(temp->data, y);
    cout<<temp->data<<endl;
    temp->next=NULL;
    cout<<"LALALA"<<endl;
    tail->next=temp;
    cout<<"I'm here11!!"<<endl;
    tail=temp;
    cout<<"I'm here!!"<<endl;
}

void P2Queue::Enqueue(QueueData* ptr)
{
    tail->next=ptr;
    tail=ptr;
    tail->next=NULL;
}

QueueData* P2Queue::Dequeue()
{
    QueueData* temp;
    temp=new QueueData();
    temp=head;
    head=temp->next;
    return temp;
}

int P2Queue::QueueSize()
{
    QueueData* temp;
    temp=head;
    while(temp!=NULL)
    {
        temp=temp->next;
        count++;
    }
    delete temp;
    return count;
}

QueueData* P2Queue::getHead()
{
    return head;
}

QueueData* P2Queue::getTail()
{
    return tail;
}

---

P2Queue.h
#ifndef P2QUEUE_H
#define P2QUEUE_H

struct QueueData
{
    int num;
    char data[128];
    QueueData* next;
};

class P2Queue
{
    private:
        int count;
        QueueData* head;
        QueueData* tail;
    public:
        QueueData* getHead();
        QueueData* getTail();
        P2Queue();
        ~P2Queue();
        void Enqueue(int x, char* y);
        void Enqueue(QueueData* ptr);
        QueueData* Dequeue();
        int QueueSize();
};

#endif 

---

BSSim.cpp
#include"BSSim.h"
#include<stdio.h>
#include<string.h>
#include <sys/types.h>     
#include <sys/timeb.h>     
#include <time.h> 

using namespace std;

BSSim::BSSim()
{
    A = new P2Queue();
    B = new P2Queue();
    C = new P2Queue();
}

BSSim::~BSSim()
{
    delete A;
    delete B;
    delete C;
}

bool BSSim::getNextLine(char *line, int lineLen)
{
     bool    done = false;

    while(!done)
    {
        inFile.getline(line, lineLen);  // Read a line from the file 
        // Note: inFile is a private class variable

        if(inFile.good())    // If a line was successfully read
        {
           if(strlen(line) == 0)  // Skip any blank lines
                continue;
            else if(line[0] == '#')  // Skip any comment lines
                continue;
            else done = true;    // Got a valid data line so return with this line
        }
        else
        {
            strcpy(line, "");  // Clear the buffer array
            return false;      // Flag end of file
        }
    } // end while
    return true; // Flag a successful read
}

bool BSSim::runSimulation(char *cmdFile)
{
    inFile.open(cmdFile, ifstream::in);
    int    x;
    char   ch;
    char   ch2;
    char   cArray[32];
    char   str[128];
    struct _timeb   tStruct;
    double currenttime;
    double nexttime=0;
    bool done = false;

    if(!inFile.is_open())   // If the file was not opened successfully, bail out.
    {
        // inFile.is_open() returns false if the file could not be found or
        //    if for some other reason the open failed.
        cout << "Unable to open command file.\nProgram terminating.\n";
        return 0;
    }
    else
    {
        while(!done)
        {
            _ftime(&tStruct);
            currenttime = (tStruct.time*1000) + tStruct.millitm;
                if(currenttime>=nexttime)
                {
                    getNextLine(line, 128);
                    sscanf(line, "%s", cArray);
                    cout<<cArray<<endl;
                    cout<<"I'm here!"<<endl;
                    if(strcmp(cArray, "ENQUEUE")==0)
                        {
                            cout<<"I'm in the Enqueue if"<<endl;
                            sscanf(line, "%s %d %s", cArray, &x, str);
                            cout<<"HERE!"<<endl;
                            cout<<x<<" "<<str<<endl;
                            A->Enqueue(x, str);
                            cout<<"Enqueue A - ID="<<x<<", Data="<<str<<", A="<<A->QueueSize()<<", B="<<B->QueueSize()<<", C="<<C->QueueSize()<<endl;
                            nexttime=currenttime;
                            nexttime+=0.5;
                        }
                    else if(strcmp("DEQUEUE", cArray)==0)
                        {
                            sscanf(line, "%s %c", cArray, &ch);
                            if(ch=='A')
                            {
                                if(A->QueueSize()==0)
                                {
                                    cout<<"Queue A is empty"<<endl;
                                }
                                else
                                {
                                    sscanf(line, "%s %c %s %c", cArray, &ch, str, &ch2);
                                    if(ch2=='B')
                                    {
                                    B->Enqueue(A->Dequeue());
                                    cout<<"Dequeue from A into B - ID="<<B->getTail()->num<<", Data="<<B->getTail()->data<<", A="<<A->QueueSize()<<", B="<<B->QueueSize()<<", C="<<C->QueueSize()<<endl;
                                    nexttime=currenttime;
                                    nexttime+=0.5;
                                    }
                                    else
                                    {
                                    C->Enqueue(A->Dequeue());
                                    cout<<"Dequeue from A into C - ID="<<C->getTail()->num<<", Data="<<C->getTail()->data<<", A="<<A->QueueSize()<<", B="<<B->QueueSize()<<", C="<<C->QueueSize()<<endl;
                                    nexttime=currenttime;
                                    nexttime+=0.5;
                                    }
                                }
                            if(ch=='B')
                            {
                                if(B->QueueSize()==0)
                                {
                                    cout<<"Queue B is empty"<<endl;
                                }
                                else
                                {
                                B->Dequeue();
                                cout<<"Dequeue from B - ID="<<B->getTail()->num<<", Data="<<B->getTail()->data<<", A="<<A->QueueSize()<<", B="<<B->QueueSize()<<", C="<<C->QueueSize()<<endl;
                                nexttime=currenttime;
                                nexttime+=0.5;
                                }
                            }
                            else
                            {
                                if(C->QueueSize()==0)
                                {
                                    cout<<"Queue C is empty"<<endl;
                                }
                                else
                                {
                                    C->Dequeue();
                                    cout<<"Dequeue from C - ID="<<C->getTail()->num<<", Data="<<C->getTail()->data<<", A="<<A->QueueSize()<<", B="<<B->QueueSize()<<", C="<<C->QueueSize()<<endl;
                                    nexttime=currenttime;
                                    nexttime+=0.5;
                                }
                            }
                        }
                    else if(strcmp("NOACTION", cArray)==0)
                        {
                            cout<<"NO ACTION"<<endl;
                            nexttime=currenttime;
                            nexttime+=0.5;
                        }
                }
                else
                {
                    inFile.close();
                    cout<<"The simulation has terminated normally."<<endl;
                    return done;
                }

            }
        }
    }
}

---

BSSim.h
#ifndef BSSIM_H
#define BSSIM_H

#include"P2Queue.h"
#include<iostream>
#include<fstream>

using namespace std;

class BSSim
{
    private:
        P2Queue* A;
        P2Queue* B;
        P2Queue* C;
        ifstream inFile;
        char line[128];
    public:
        BSSim();
        ~BSSim();
        bool runSimulation(char* cmdFile);
        bool getNextLine(char* line, int lineLen);
};

#endif 

---

#include"BSSim.h"
#include"P2Queue.h"

using namespace std;
void main()
{
    BSSim x;
    x.runSimulation("SimData.txt");
}

---

Example data file:
ENQUEUE 1234 QueueTest_01
ENQUEUE 2345 QueueTest_02
ENQUEUE 3456 QueueTest_03
ENQUEUE 4567 QueueTest_04
ENQUEUE 5678 QueueTest_05
ENQUEUE 6789 QueueTest_06
NOACTION
DEQUEUE A ENQUEUE B
DEQUEUE A ENQUEUE C
DEQUEUE A ENQUEUE B
DEQUEUE A ENQUEUE C
DEQUEUE B
DEQUEUE C
DEQUEUE B
DEQUEUE C
DEQUEUE A ENQUEUE B
DEQUEUE A ENQUEUE C
DEQUEUE A ENQUEUE B
DEQUEUE A ENQUEUE C
DEQUEUE B
DEQUEUE C
DEQUEUE B
DEQUEUE C
NOACTION

Answer 1

最初队列是空的并且 head == tail == NULL。

将第一个项目放入队列会发生什么？

Answer 2

据我所知，您对队列的实现是完全错误的。您可以在此处找到有关如何实现它的信息：http://en.wikipedia.org/ wiki/Queue_（数据结构）。

例如：这没有任何意义

void P2Queue::Enqueue(QueueData* ptr)
{
    tail->next=ptr;
    tail=ptr;
    tail->next=NULL;
}

首先，当列表为空时，它会压碎或做一些奇怪的事情。其次，它仅添加一个元素并从队列中删除所有现有元素。

它至少应该是这样的：

void P2Queue::Enqueue(QueueData* ptr)
{
    ptr->next = tail;
    tail = ptr;
}

Answer 3

我相信您的问题，就像其他人所说的那样，是您的程序在队列为空时不进行处理。您需要做的是在函数中添加检查队列是否为空。如果队列为空，则需要以稍微不同的方式处理入队。我相信这会解决您的问题。下面是一些可能对您有帮助的伪代码。让我知道这是否有帮助！ ＜3

void P2Queue::Enqueue(int x, char *y)
{
    QueueData* temp;
    temp=new QueueData();
    temp->num=x;
    strcpy(temp->data, y);
    temp->next=NULL;

    if(queue is empty) {
        head = temp;
        tail = temp;
    }
    else {
        tail->next = temp;
        tail = temp;
    }
}

void P2Queue::Enqueue(QueueData* ptr)
{
    ptr->next = NULL
    if(queue is empty) {
        head = ptr;
        tail = ptr;
    }
    else {
        tail->next = ptr;
        tail = ptr;
    }
}