Why std::sort giving errors in a function?

I am using std::sort in main function, but when use in a function, it is giving errors.

int main()
{
   int arr[5] = {1,4,2,3,5};
   std::sort(std::begin(arr), std::end(arr));
   ...
}

above code works fine. but when I do

void my_sort(int a[], int length)
{
   std::sort(std::begin(a), std::end(a)); 
}
int main()
{
    int arr[5] = {1,4,2,3,5};
    my_sort(arr, 5);
}

It gives error that

error: no matching function for call to ‘begin(int*&)’
     std::sort(std::begin(a), std::end(a));
                           ^

Why is it so ? I am using g++