使用链接列表创建FIFO队列，不使用全局变量

Question

我只是编程的入门者，我要做一些功课。 基本上，它应该是链接列表队列，而不使用全局变量。

有时我的程序崩溃，有时不会崩溃。

我不知道我在做什么错。

非常感谢您的帮助。

我认为问题可能出在del和print函数中。

#include <iostream>
using namespace std;

void menu()
{
    cout << "Choose one of options" << endl;
    cout << "1. New queue" << endl;
    cout << "2. Attach" << endl;
    cout << "3. Delete" << endl;
    cout << "4. Print" << endl;
    cout << "5. End" << endl;
}

struct el_FIFO
{
    int x;
    struct el_FIFO *nast;
};

struct FIFO
{
    struct el_FIFO *front;
    struct el_FIFO *rear;
};

void attach(struct FIFO *k, int el)
{
    struct el_FIFO *nowy = new el_FIFO;
    nowy->x = el;
    nowy->nast = NULL;
    if (k->front == NULL)
    {
        k->front = nowy;
        k->rear = nowy;
    }
    else
    {
        k->rear->nast = nowy;
        k->rear = nowy;
    }
}

void del(struct FIFO *k)
{
    struct el_FIFO *temp;
    if (k->front != NULL)
    {
        temp = k->front->nast;
        delete k->front;
        k->front = temp;
    }
    else
    {
        cout << "There is no element to delete";
    }
}

void new_queue(struct FIFO *k)
{
    if (k->front == NULL)
    {
        cout << "queue is already empty";
    }
    else
    {
        while (k->front != NULL)
        {
            del(k);
        }
        cout << "queue is empty";

    }
}

void print(struct FIFO *k)
{
    struct FIFO *temp_FIFO;
    if (k->front)
    {
        cout << "queue: " << endl;
        while (k->front)
        {
            if (temp_FIFO->front == NULL)
            {
                temp_FIFO->front = k->front;
                temp_FIFO->rear = k->front;
            }
            else
            {
                temp_FIFO->rear->nast = k->front;
                temp_FIFO->rear = k->front;
            }
            cout << k->front->x << endl;
            del(k);
        }
        while (temp_FIFO->front)
        {
            if (k->front == NULL)
            {
                k->front = temp_FIFO->front;
                k->rear = temp_FIFO->front;
            }
            else
            {
                k->rear->nast = temp_FIFO->front;
                k->rear = temp_FIFO->front;
            }
            del(temp_FIFO);
        }
    }
    else
    {
        cout << "queue is empty";
    }
}

int rear()
{
    cout << "The end";
    return 0;
}

int main()
{
    struct FIFO *queue = new FIFO;
    queue->front = NULL;
    queue->rear = NULL;
    int el = 0;
    int c = 0;
    do
    {
        menu();
        cin >> c;
        cout << endl;
        switch (c)
        {
            case 1:
                new_queue(queue);
                cout << endl << endl;
                break;
            case 2:
                cout << "Add element" << endl;
                cin >> el;
                attach(queue, el);
                cout << endl;
                break;
            case 3:
                cout << "Element will be deleted" << endl;
                del(queue);
                cout << endl << endl;
                break;
            case 4:
                print(queue);
                cout << endl;
                break;
            case 5:
                rear();
                break;
            default:
                cout << "You chose wrong" << endl << endl;
                break;
        }
    } while (c != 5);
    return 0;
}