如何删除 CPP 中的动态非矩形二维数组

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.

Answer 1

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 i th 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 i th 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.

Answer 2

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++ )