arrays declaration and addressing

I have a few straightforward questions:-

1. Is the following correct according to a normal c++ compiler?

   int arr[3][4];
   void func(int *a, int m, int n)
   {
    int i,j;
    cin>>i>>j; 
    cout<< a[i*n + j]; //is this way of addressing correct provided 0<=i<m and 0<=j<n
   } 
   
   int main()
   {
    func((int*)arr, 3,4);
   }
   

2. If the bounds of an array strictly has to be a constant expression, why doesn't the following generate compiler errors?

   int func(int m, int n)
   {
    int arr[m][n]; //m and n are not known until run time
   }
   

Is the following correct according to a normal c++ compiler?

Yes, because the Standard specifies that even arrays with more than 1 dimensions are contigous in memory. BUT! You're misunderstanding array indexing and length a bit - i and j have to be strictly less than 3 and 4, respectively. The 3 and 4 are the sizes of the array in its two dimensions, and not the maximal possible subscripts.

If the bounds of an array strictly has to be a constant expression, why doesn't the following generate compiler errors?

Maybe you're using a compiler that supports this as an extension (think of GCC, Clang, etc.)