c++队列实现无法按预期工作

Question

今天，我希望使用指针在c ++中创建自己的去队列。在这个程序中，我想要创建两个排程，在其中添加不同的元素。不幸的是，事实并非如此。屏幕上只有6 5，只有6。为什么?我不知道怎么排成两队？我想我的问题之一是*L = NULL, * R = NULL，但我不确定？所以到目前为止我的密码-

#include <iostream>

struct elem
{
    int key;
    elem* next;
}*L = NULL, * R = NULL;

void push_l(int n, elem*& p)
{
    p = L;
    L = new elem;
    L->key = n;
    L->next = p;

    if (R == NULL)
        R = L;
}

int pop_l(int& n, elem*& p)
{
    if (L)
    {
        p = L;
        n = L->key;
        L = L->next;

        if (L == NULL)
            R = NULL;
        delete p;

        return 1;
    }
    else
    {
        return 0;
    }
}



int main()
{
    int k;
    elem* p = new elem;
    elem* q = new elem;
    push_l(5, p);
    push_l(6, q);

    while (pop_l(k, q))
    {
        std::cout << k<< " ";
    }
}

我不确定如何在屏幕上只打印6，因为这个元素只被推送到排队列中？此外，这些只是脱队列的一些特性，因为我想从一开始就了解它！

Answer 1

这样的类可能如下所示：

class /* or struct, if you really want to... */ Dequeue 
{
public:
   void push(int value);
   bool pop(int& value); // if you return 1 for success and 0 for error
                         // bool is the more appropriate type...

private:
   struct elem // it's an implementation detail, so recommendable
               // to have it as a nested class...
   {
        // just as you had before...

        // you might additionally provide a constructor:
        // this makes creating new instances easier/simpler
        elem(int k, elem* n) : key(k), next(n) { }
   };

   elem* m_head = nullptr;
   elem* m_tail = nullptr; 
}

void Dequeue::push(int value)
{
//   auto tmp = new elem;
//   tmp->key = value;
//   tmp->next = m_head;
//   m_head = tmp;

// with constructor:
   m_head = new elem(value, m_head);

   if(!m_tail)
       m_tail = m_head;
}

bool Dequeue::pop(int& value)
{
   // transform analogously using m_head and m_tail instead of L and R
   // return true and false instead of 1 and 0 
}

您会注意到，该函数看起来和以前一样-不过，与现在的成员函数一样，它访问调用该函数的实例的类成员：

Dequeue d1; 
Dequeue d2;
// each of these have their own, separate m_head and m_tail pointers!

d1.push(12);
d2.push(10);
// now pushed to separate instances 

附带注意:未经测试的代码，如果你发现了一个错误，请自己修复.