修改链表中的头指针 [英] Modifying head pointer in a linked list

问题描述

我无法理解此代码.我真正需要的是修改头指针以指向第一个元素.那么为什么 *head 不工作呢?更改 *head 的值会更改此指针指向的位置，这应该可以工作，对吗?我已经阅读了通过引用/通过值传递，但我发现它很难理解.有人可以帮助澄清这一点吗?感谢你的帮助.谢谢.

I am having trouble understanding this code. All I really need is to modify the head pointer to point to the first element. So why won't *head work ? Changing the value of *head changes where this pointer points to and that should work, right ? I have read the pass by reference/pass by value, but am finding it hard to understand. Can someone help clarify this ? Appreciate your help. Thanks.

在 C/C++ 中，指针误用更容易出错.考虑以下用于在列表前面插入元素的 C/C++ 代码:

In C/C++ it’s easier to make mistakes with pointer misuse. Consider this C/C++ code for inserting an element at the front of a list:

bool insertInFront( IntElement *head, int data ){
  IntElement *newElem = new IntElement;
  if( !newElem ) return false;

  newElem->data = data;
  head = newElem; // Incorrect!
  return true;
}

前面的代码是不正确的，因为它只更新了头指针的本地副本.正确的版本传入一个指向头指针的指针:

The preceding code is incorrect because it only updates the local copy of the head pointer. The correct version passes in a pointer to the head pointer:

bool insertInFront( IntElement **head, int data ){
  IntElement *newElem = new IntElement;
  if( !newElem ) return false;

  newElen->data = data;
  *head = newElem; // Correctly updates head
  return true;
}

推荐答案

您需要帮助来理解其中的区别，对吗?

You need help understanding the difference right?

想象第一种情况下函数的调用者:

Imagine the caller of the function in the first case:

IntElement *head;
int data;
...
insertInFront (head, data);

现在，在这种情况下，head 指向的地址被放置在堆栈中并作为参数传入 insertInFront.当 insertInFront 做 head = newElement;仅修改参数(在堆栈上).

Now, in this case, the address pointed to by head is placed on the stack and passed in as an argument to insertInFront. When insertInFront does head = newElement; only the argument (on the stack) is modified.

在第二种情况下，调用者是:

In the second case, the caller would be:

IntElement *head;
int data;
...
insertInFront (&head, data);

在这种情况下，head 的地址被放置在堆栈中并作为参数传入 insertInFront.当你做 *head = newElement 时，这个传入的地址被取消引用以获得原始列表头的地址，并被修改.

In this case, the address of head is placed on the stack and passed in as an argument to insertInFront. When you do *head = newElement, this passed in address is de-referenced to get the address of the original list head, and that is modified.

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