Question

This is a simplified code to explain the problem:

int *nums[10];

*nums[0] = 5;

cout << *nums[0] << endl;

The code compiled, but it fails when it executes. So I tried this:

int *nums[10];

*nums[1] = 5;

cout << *nums[1] << endl;

and it prints out fine. I figured out that array was starting from *nums[1] to *nums[10]
instead of the usual *nums[0] to *nums[10]. I’ve checked with others who use the Netbeans C/C++ compiler and theirs work as it should. I assume that it is some preference changed within the specific compiler. How do I change it so it works the way it should?

Answer 1

Arrays are 0-based. You’re running into undefined behavior.

int *nums[10];

creates an array of 10 uninitialized pointers to int.

*nums[0] = 5;

dereferences an uninitialized pointer. Anything can happen. For it to behave as expected, allocate memory before accessing the pointers:

for ( int i = 0 ; i < 10 ; i++ )
   nums[i] = new int;

and delete it at the end:

for ( int i = 0 ; i < 10 ; i++ )
   delete nums[i];

For example, I get a warning in MSVS:

warning C4700: uninitialized local variable ‘nums’ used

and also a crash :).