Accessing Array Out of Bounds in C/C++

In a language such as Java, an exception such as java.lang.ArrayIndexOutOfBoundsException may occur if an array is accessed out of bounds. But there is no such functionality in C or C++, and undefined behavior may occur if an array is accessed out of bounds.

In this example, we have an array of size n. We will try to access the element and assign memory out of bounds and see the behavior in the output. Our task is to access the array element out of bounds in C/C++.

Accessing Out of Bound Memory

Accessing out of bounds means trying to access the array index that is greater than the array size. It returns any garbage value while we try to access an array element greater than the array size or out of bounds in C/C++. Here is an example:

Example

In this example, we have 5 elements in the array. We are printing the array elements up to 7 elements, but we have only 5 elements. In this case, it will print the first 5 array elements, then it will print the garbage value. Here is the C and C++ code example:

#include<stdio.h>
int main()
{
    int arr[] = {1, 2, 3, 4, 5};
    printf("The elements of array : ");
    for (int i = 0; i < 7; i++)
        printf(" %d", arr[i]);
    return 0;
}

The elements of array :  1 2 3 4 5 32765

#include <iostream>
using namespace std;

int main()
{
   int arr[] = {1, 2, 3, 4, 5};
   cout << "The elements of array : ";
   for (int i = 0; i < 7; i++)
      cout << arr[i] << " ";
   return 0;
}

The elements of array : 1 2 3 4 5 0 -724676608

Out of Bound Memory Allocation

In this section, we are trying to assign and access an array element at an index whose index is greater than the size of the element. It returns a segmentation fault. The segmentation fault means that we are trying to access the memory that is not allocated to the array.

Example

Here is an example of allocating out-of-bounds memory to an array in C and C++ that returns a segmentation fault in both cases:

#include<stdio.h>
int main()
{
    int arr[] = {1, 2, 3, 4, 5};
    arr[10] = 57;
    printf(" %d", arr[10]);
    return 0;
}

Segmentation fault (core dumped)

#include <iostream>
using namespace std;

int main()
{
   int arr[] = {1, 2, 3, 4, 5};
   arr[10] = 57;
   cout << arr[10];
   return 0;
}

Segmentation fault (core dumped)

Accessing Out of Bound Memory Using vector

When we access an array element greater than the array size, it returns a garbage value, as we have seen in the previous section. But the vector returns an error 'out_of_range' instead of giving any garbage value.

Example

In this example, we have a vector having five elements, and we are trying to print seven elements. This will throw an error as we are trying to access out-of-bounds elements.

#include <vector>
#include <iostream>
using namespace std;

int main()
{
   vector<int> v = {1, 2, 3, 4, 5};
   for (size_t i = 0; i < 7; ++i)
   {
      cout << v.at(i) << " ";
   }
   cout << endl;
   return 0;
}

The output of the above code will be an error that is displayed in the snippet below: