Run-time error on 2D array dynamic arrays

Question

I have the following code:

int **arr = new int*[5];

for(int i = 0; i < 5; ++i)
    arr[i] = new int[];

for(int i = 0; i < 5; ++i)
    delete [] arr[i];

delete [] arr;

Now it compiles and runs successfully, however if I remove the array size '5' from the first line the code compiles but crashes with run-time error on the last line. I have the following questions that I have failed to find answers in Straustrup's C++ book, in the internet etc.

Why the code crashes in the specified case ? (My guess is that delete [] fails to find the array size to be deleted and crashes.)
If it is not allowed to allocate multidimensional arrays without indicating the size why such errors are not caught by compiler at compile time ?

This thread should be helpful. stackoverflow.com/questions/1087042/… — Mahesh
– Mahesh, Commented Sep 5, 2011 at 18:44

Seth Carnegie · Accepted Answer · 2011-09-05 18:43:02Z

3

With the [5], you're getting an array of 5 int*s.

If you remove the [5], you're saying you want a pointer to a pointer to an int. So essentially, you have new int*[1].

Then you are indexing it with numbers from 0 to 4, which is out of bounds.

answered Sep 5, 2011 at 18:43

Seth Carnegie

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1 Comment

Arik Over a year ago

Seth Do you mean that int*[] is equivalent to int*[1] ? If so then the code should crash inside the first "for" loop during second iteration, however in fact it crashes on the last line.

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