How to delete completely 2D pointer array

Question

I have a 2D pointer matrix in C++ such as,

typedef unsigned char  U8;
typedef unsigned int   U32;
int M=10;
int m_L=8;
U8** A = new U8*[M];
for (U32 i = 0; i < M; ++i)
{
    A[i] = new U8[m_L];
}

After setting value in A0, I will write a function which decide delete or not delete M-2 rows in A, depends on the random number is 0 or 1

void delete_or_not(U8** A,int M) 
{
  if (rand_num==1){
     for (U32 index = M-2; index < M; ++index){
        delete[] A[index];
     }
  }
}

Now, in main function (which contains A memory allocation), I want to free/delete the memory which allocated for A. I can use the code

//free A matrix
for (U32 i = 0; i < M; ++i)
{
    if (i < m_L)
    {
      delete[] A[i];
      A[i] = NULL;
    }
}
delete[] A;
A = NULL;

My problem is that, I don't know that A is delete (M-2) rows or not. Hence, above code does clearly delete all memory, if my random number is 0. That means, above code only delete correct memory if M-2 rows is deleted in the delete_or_not function. How can delete the A matrix perfectly. Thanks

Finaly, my full code is

typedef unsigned char  U8;
typedef unsigned int   U32;
int M=10;
int m_L=8;
U8** A = new U8*[M];
for (U32 i = 0; i < M; ++i)
{
    A[i] = new U8[m_L];
}
delete_or_not(A,M);
//free A matrix 
//Way 1: will miss M-2 row if delete_or_not function did not delete 2 rows.
// It only correct if rand_num=1
for (U32 i = 0; i < M; ++i)
{
    if (i < m_L)
    {
      delete[] A[i];
      A[i] = NULL;
    }
}
delete[] A;
A = NULL;

//Way 2- It will correct if the size of A is M by M

for (U32 i = 0; i < M; ++i)
{
      delete[] A[i];
      A[i] = NULL;
}
delete[] A;
A = NULL;

There are more readable names to chose from than U32, U8, M, m_L etc — Paul Evans
– Paul Evans, Commented Dec 31, 2015 at 7:05
Don't use raw arrays, pointers, new or new[], and you won't have to worry about them. — n. m. could be an AI
– n. m. could be an AI, Commented Dec 31, 2015 at 7:08
Also notice that your description and your code is not the same. Your description states: delete or not delete M-2 rows but your code actually deletes 2 elements because you have U32 index = M-2 as initialization of index in the for-loop. — 4386427
– 4386427, Commented Dec 31, 2015 at 7:43

Amit · Accepted Answer · 2015-12-31 07:21:43Z

4

Just set deleted elements to NULL and everything will work fine:

void delete_or_not(U8** A,int M) 
{
  if (rand_num==1){
     for (U32 index = M-2; index < M; ++index){
        delete[] A[index];
        A[index] = NULL;
     }
  }
}

Also, this is not very useful:

for (U32 i = 0; i < M; ++i)
{
    if (i < m_L)
    {
      delete[] A[i];
      A[i] = NULL;
    }
}

There's no point advancing i from 0 to M if you then only "do work" when i is smaller then m_L.

But really, in C++ you should probably use std::vector<std::vector<U8>> instead and simply erase or pop_back to get rid of "rows".

edited Dec 31, 2015 at 7:21

answered Dec 31, 2015 at 7:07

Amit

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2 Comments

Jame Over a year ago

Sorry, maybe I was wrong explaination. My expected is how can I delete A matrix after call the delete_or_not function. I will update

Amit Over a year ago

@user8430 - did you try my suggested solution?

4386427 · Accepted Answer · 2015-12-31 07:32:18Z

1

In void delete_or_notyou shall set deleted elements to NULL as already proposed by another answer by Amit.

Then Way 2 of your posted code is correct in both cases.

Calling delete on a NULL is perfectly legal and doesn't hurt at all.

Way 1 is not working and should be removed.

In summary:

void delete_or_not(U8** A,int M) 
{
  if (rand_num==1){
     for (U32 index = M-2; index < M; ++index){
        delete[] A[index];
        A[index] = NULL;    // Add this to your code
     }
  }
} 


// In main somewhere...

// free A
for (U32 i = 0; i < M; ++i)
{
      delete[] A[i];   // Not a problem if the element has already been deleted
                       // because the pointer will be NULL and calling
                       // delete with NULL is OK (and changes nothing)
      A[i] = NULL;
}
delete[] A;
A = NULL;

edited Dec 31, 2015 at 7:32

answered Dec 31, 2015 at 7:24

4386427

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3 Comments

Jame Over a year ago

Thanks. But way 2 will crashed if random_num=1. Thus, 2 rows already deleted in the delete_or_not function. Hence, for =1 to M is not correct

4386427 Over a year ago

No, it will not crash if you make the changed proposed by Amit.

Jame Over a year ago

I added it again. Thanks. It worked now. Thanks for your explaination

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