C++ Pointers and Dynamic Arrays and Delete Operator

Question

Ok so I am going to lay out two programs. Both are dynamic arrays using pointers and the new operator. But one doesn't seem to like the delete operator.

#include <iostream>
int main()
{
  int *p;
  p = new int[5];

  for (int i = 0; i < 5; i++)
  {
    p[i] = 25 + (i * 10);
    std::cout << p[i] << " ";
  }
  std::cout << std::endl;

  delete [] p;
  p = NULL;

  return 0;
}

That's the first program. It likes the delete operator just fine. Now the program that dislikes the delete operator:

#include <iostream>
int main()
{
  int x;
  int *p;
  p = new int[5];

  *p = 4;

  for (int i = 0; i < 5; i++)
  {
    std::cout << *p << " ";
    x = *p;
    p++;
    *p = x + 1;
  }
  std::cout << std::endl;

  delete [] p;
  p = NULL;

  return 0;
}

This program compiles just fine. But during execution, it throws an error - free(): invalid pointer: 0xfdb038 .. or whatever the memory address is for that particular execution. So, the question is: Why can't the delete operator be used in the second case? I don't want to have memory leak; I don't want the pointer to be dangling. If I just say p = NULL;, then p = 0, but I believe the pointer is still dangling?, but I'm not sure. Thanks in advance.

Because in the second case you change the p pointer and it doesn't point to the array beginning before the deleting. — Edgar Rokjān
– Edgar Rokjān, Commented Jun 18, 2016 at 21:00

templatetypedef · Accepted Answer · 2016-06-18 21:01:48Z

Look at this loop in the second piece of code:

for (int i = 0; i < 5; i++)
{
  std::cout << *p << " ";
  x = *p;
  p++;
  *p = x + 1;   // <--- Here
}

Notice that in this line, you write to the memory address currently pointed at by p. Since you always increment p and then write to it, you end up writing off past the end of the region that you allocated for p. (If we imagine pOrig as a pointer to where p initially points, then this writes to pOrig[1], pOrig[2], pOrig[3], pOrig[4], and pOrig[5], and that last write is past the end of the region). This results in undefined behavior, meaning that literally anything can happen. This is Bad News.

Additionally, delete[] assumes that you are passing in a pointer to the very first element of the array that you allocated. Since you've incremented p so many times in that loop, you're trying to delete[] a pointer that wasn't at the base of the allocated array, hence the issue.

To fix this, don't write to p after incrementing it, and store a pointer to the original array allocated with new[] so that you can free that rather than the modified pointer p.

nwp · Accepted Answer · 2016-06-18 21:02:35Z

0

You have to delete the pointer that you got from new. However, in your second code you did p++ which changed the pointer. Therefore you tried to delete a pointer you didn't get from new and delete crashes.

To fix this type of error never use new. Instead use std::vector<int> p;. Since you never need new you cannot forget a delete.

answered Jun 18, 2016 at 21:02

nwp

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6 Comments

nwp Over a year ago

@EdgarRokyan How is that strange? You will notice that you will not find manual memory management in modern C++. There is no need to subject yourself to this error-prone, outdated and unnecessary practice.

templatetypedef Over a year ago

@EdgarRokyan There actually is a trend in modern C++ to avoid explicit new and delete operators. Look at, for example, make_shared and make_unique in conjunction with smart pointer types.

Edgar Rokjān Over a year ago

@templatetypedef I mean that you can't avoid using explicitly new/delete because of many C libraries, for example, which deal with arrays and raw pointers. So it's much more better to learn how to use new/delete appropriately instead of ignoring them. In such context the phrase "never use new" looks strange for me. Besides the aforementioned advice doesn't answer the question directly.

nwp Over a year ago

@EdgarRokyan C doesn't use new or delete, therefore you never need new or delete to work with C libraries. You just use a std::vector and pass pointers to the data to the C library. The first paragraph answers the question directly, the second in a general manner, I don't see how it doesn't.

Edgar Rokjān Over a year ago

@templatetypedef What if C library needs a preallocated buffer to fill in? Do you claim that it's better to create a vector and use pointers to it's buffer instead of simply using new and delete?

|

P. Dmitry · Accepted Answer · 2016-06-18 21:06:11Z

0

Problem is changing p in p++. You should always store (to delete) original pointer. Like this:

#include <iostream>
int main()
{
  int *original = new int[5];
  int *p = original;

  for (int i = 0; i < 5; i++)
  {
    std::cout << *p << " ";
    int x = *p;
    p++;
    *p = x + 1;
  }
  std::cout << std::endl;

  delete [] original;    
  return 0;
}

answered Jun 18, 2016 at 21:06

P. Dmitry

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