How do I delete dynamic non-rectangular 2-dimensional Arrays in CPP

Question

This is some code I wrote just to understand working with pointers. Here I want to allocate and display a 2-dimensional Pointerarray

int main()
{
    constexpr int size = 10;
    int** _2darray = new int* [size];

    for ( int i = 0; i < size; i++ )
    {
        *(_2darray + i) = new int[i];
        for ( int j = 0; j <= i; j++ )
        {
            *(*(_2darray + i) + j) = j;
        }
    }

    for ( int i = 0; i < size; i++ )
    {
        for ( int j = 0; j <= i; j++ )
        {
            std::cout << _2darray[i][j] << " ";
        }
        std::cout << std::endl;
    }
}

This will result in printing this:

0
0 1
0 1 2
0 1 2 3
0 1 2 3 4
0 1 2 3 4 5
0 1 2 3 4 5 6
0 1 2 3 4 5 6 7
0 1 2 3 4 5 6 7 8
0 1 2 3 4 5 6 7 8 9

But if I want to prevent a memory leak now, I would want to do this:

for ( int i = 0; i < size; i++ )
  {
       delete[] _2darray[i];
  }
  delete[] _2darray;

Unfortunately this will give me an exception that the heap was corrupted

I guess that it crashes because it doesen't know the exact size of the subarrays, but I also heard that delete always keeps track of how much bytes are being allocated for a array.

Thank you for any short answer.

The delete logic is correct. Unfortunately sometime after creating the arrays and before deleting them the program marches out of bounds and corrupts some important book-keeping information stored near the array. Find where that happens and you'll solve the problem. — user4581301, Mar 06 '20 at 22:49
I always worry when I see a loop with a `<=` in it. Check the range of `j` that results from `for ( int j = 0; j <= i; j++ )` to make certain it's what you expect. — user4581301, Mar 06 '20 at 22:51
the problem is `*(_2darray + i) = new int[i];` given that the for loop is initialized with i=0. You are allocating an array with zero items in it and then writing to it — jwezorek, Mar 06 '20 at 23:00
`int** _2darray = new int* [size];` allocates for `size` **pointers**. `*(_2darray + i) = new int[i];` allocates for `i int` and assigns the starting address for that block to your pointers in turn. When you want to free the memory, you must first loop over your **pointers** freeing the allocated blocks of `int` before freeing the block of pointers. `for (int i = 0; i < size; i++) delete[] _2darray[i];` then free your block of pointers, `delete[] _2darray;`. (note: there are no "arrays" involved, you have allocated blocks of memory) Also note: `_2darray[i]` and `_2darray[i][j]` are readable. — David C. Rankin, Mar 06 '20 at 23:00
Turns out the j <= i was really the problem, now it is running smoothly, thank you! — sigmund, Mar 06 '20 at 23:03
EDIT: Also @jwezorek comment is true, i corrected it with: *(_2darray + i) = new int[i + 1]; — sigmund, Mar 06 '20 at 23:24

jwezorek · Accepted Answer · 2020-03-10T16:58:10.197

Your delete logic is correct.

The error message means that your program is writing to memory that is out of bounds. The comparison j <= i is not what is incorrect here however -- I mean assuming the program is currently generating the output you want.

Just note that you want your ith array to contain i+1 items. Specifically notice the zeroth array must contain one item (namely zero) so *(_2darray + i) = new int[i] should be *(_2darray + i) = new int[i+1].

With that change in place the for loops with the j <= i comparisons are correct. Index j is going to iterate over each item in the ith array which contains i+1 items. The comparison j < i will yield only i iterations when starting from zero so you do need j <= i in this case even though such a comparison is non-standard looking. Consider j < i+1, which is equivalent but might be clearer.

score 0 · Answer 2 · answered Mar 06 '20 at 23:00

for ( int i = 0; i < size; i++ )
{
    *(_2darray + i) = new int[i];
     //...
}

For first iteration, when i=0 you do:

*(_2darray + i) = new int[0];

This is not a good way to allocate memory. see this C++ new int[0] — will it allocate memory?
And after you do:

for ( int j = 0; j <= i; j++ )

it should be:

for ( int j = 0; j < i; j++ )

How do I delete dynamic non-rectangular 2-dimensional Arrays in CPP

2 Answers2