如何为对象超载cout？

Question

I have a this struct:

struct DigitNode{
  char digit;
  DigitNode *prev;
  DigitNode *next; 
};

My BigInt class has private member variables: Bigint *head and Bigint *tail.

What I am trying to do is implement a BigInt datatype. My struct is a doubly linked list with each node containing a digit of a number. The number it should represent is the number you would get from the linked list if you read each character left to right.

This is my constructor:

BigInt::BigInt(const string &numIn) throw(BigException)
{  

  DigitNode *ptr = new DigitNode;
  DigitNode *temp;
  ptr->prev = NULL;
  if (numIn[0] == '-' || numIn[0] == '+') ptr->digit = numIn[0];
  else ptr->digit = numIn[0] - '0';
  this->head = ptr;
  for (int i = 1; numIn[i] != '\0'; i++)
    {
      ptr->next = new DigitNode;
      temp = ptr;
      ptr = ptr->next;
      ptr->digit = numIn[i] - '0';
      ptr->prev = temp;
    }
  ptr->next = NULL;
  this->tail = ptr;

}

This is my attempt at an operator<< overloader:

ostream&  operator<<(ostream & stream, const BigInt & bigint)
{ 

  DigitNode *ptr = bigint.head;
  string num = "";
  while (ptr != NULL)
    {
      num += ptr->digit;
    }

  return  stream << num; 
}

This is my error:

terminate called after throwing an instance of 'std::bad_alloc' what(): std::bad_alloc Aborted (core dumped)

Answer 1

The problem is that in your while loop the ptr variable never contains NULL . This causes an endless loop that exhausts all of your memory while it builds the string. To fix this you need to advance ptr to the next linked element. The second problem is that you are storing values 0 through 9 in digit instead of actual their respective characters representations. You need to adjust the value when you append it to the string, like so.

while (ptr != NULL)
{
    num += ptr->digit + '0';
    ptr = ptr->next;
}