What is the difference between allocating array like int a[10][10] and allocating array using new keyword in c++?

Question

I am presently using Windows 10 with gcc version 6.3.0 (MinGW.org GCC-6.3.0-1).
Code 1

#include <bits/stdc++.h>
using namespace std;

int main(){
    int** a = new int*[100000];
    for(int i = 0; i < 100000; ++i)
        a[i] = new int[1000];
    cout << "Array allocation Ok!!!\n";
    return 0;
}

//Output
Array allocation Ok!!!

Code 2

#include <bits/stdc++.h>
using namespace std;

int main(){
    int arr[100000][1000];
    cout << "Array allocation Ok!!!\n";
    return 0;
}

//I got no Output, it just return the console back to me

Someone suggested me the difference could be that in Code 2 row i and i+1 are contiguous (row major representation) i.e. int arr[1000][100] is same as int arr[100000] in terms of memory. But in Code 1 the column entries are contiguous in nature but not the rows. But if that could be the case then what about pointer arithmetics?

The second program crashes due to stack overflow. The other differences are not relevant to your problem. You will see the same output if you use a smaller array, say `int arr[10][10];`. — R Sahu, Sep 18 '18 at 03:23
Doesn't appear to be a duplicate with regard to the link. The OP was asking about memory alignment of heap allocations versus stack allocations, not stack overflow — Matthew Fisher, Sep 18 '18 at 03:41

score 0 · Accepted Answer · answered Sep 18 '18 at 03:19

0

The short answer is that a[10][10] is allocated on the stack and the array being allocated with new is allocated on the heap.

answered Sep 18 '18 at 03:19

James Folk

46
3

What is the difference between allocating array like int a[10][10] and allocating array using new keyword in c++?

1 Answers1