一个例子

Linus大神举了一个指针的例子，解释了什么才是core low-level coding，这个例子让我警醒——什么才是修炼真正的内功。

At the opposite end of the spectrum, I actually wish more people understood the really core low-level kind of coding. Not big, complex stuff like the lockless name lookup, but simply good use of pointers-to-pointers etc. For example, I’ve seen too many people who delete a singly-linked list entry by keeping track of the “prev” entry, and then to delete the entry, doing something like:

if(prev)
  prev->next = entry->next;
else
  list_head = entry->next;

and whenever I see code like that, I just go “This person doesn’t understand pointers”. And it’s sadly quite common.

People who understand pointers just use a “pointer to the entry pointer”, and initialize that with the address of the list_head. And then as they traverse the list, they can remove the entry without using any conditionals, by just doing a “*pp = entry->next”.

So there’s lots of pride in doing the small details right. It may not be big and important code, but I do like seeing code where people really thought about the details, and clearly also were thinking about the compiler being able to generate efficient code (rather than hoping that the compiler is so smart that it can make efficient code despite the state of the original source code).

具体来说，大神给了这样的代码：

如果我们需要写一个remove_if(link, rm_cond_func)的函数，也就是传入一个单向链表，和一个自定义的是否删除的函数，然后返回处理后的链接。

这个代码不难，基本上所有的教科书都会提供下面的代码示例，而这种写法也是大公司的面试题标准模板：

typedef struct node
{
    struct node * next;
    ....
} node;
 
typedef bool (* remove_fn)(node const * v);
 
// Remove all nodes from the supplied list for which the
// supplied remove function returns true.
// Returns the new head of the list.
node * remove_if(node * head, remove_fn rm)
{
    for (node * prev = NULL, * curr = head; curr != NULL; )
    {
        node * const next = curr->next;
        if (rm(curr))
        {
            if (prev)
                prev->next = next;
            else
                head = next;
            free(curr);
        }
        else
            prev = curr;
        curr = next;
    }
    return head;
}

这里remove_fn由调用查提供的一个是否删除当前实体结点的函数指针，其会判断删除条件是否成立。这段代码维护了两个节点指针prev和curr，标准的教科书写法——删除当前结点时，需要一个previous的指针，并且还要这里还需要做一个边界条件的判断——curr是否为链表头。于是，要删除一个节点（不是表头），只要将前一个节点的next指向当前节点的next指向的对象，即下一个节点（即：prev->next = curr->next），然后释放当前节点。

但在Linus看来，这是不懂指针的人的做法。那么，什么是core low-level coding呢？那就是有效地利用二级指针，将其作为管理和操作链表的首要选项。代码如下：

void remove_if(node ** head, remove_fn rm)
{
    for (node** curr = head; *curr; )
    {
        node * entry = *curr;
        if (rm(entry))
        {
            *curr = entry->next;
            free(entry);
        }
        else
            curr = &entry->next;
    }
}

不需要prev指针了，也不需要再去判断是否为链表头了，但是，curr变成了一个指向指针的指针。这正是这段程序的精妙之处。

可以只用一个二级指针来操作链表，对所有节点都一样。

这个代码使用递归和指针引用写出来，思想是一样的，便于理解对比一下:

void delete(list_node*& L,ElemType x)
{
  list_node*p;
  if(!L)
    return;
  if(L->data == x)
  {
    p = L;
    L = L->next;
    free(p);
    delete(L,x);
  }
  else
    delete(L->next,x);
}

异曲同工，可对比思考一下。

这个例子中，二级指针的使用就是内功的体现，也是科班计算机工程师的核心战斗力。

通过本文希望惊醒自己，停止无脑地写垃圾代码，三脚猫的皮毛功夫在几年后将会由机器接管，我将会失业。