Question:
When declaring an unbounded array in my function’s parameter list, int a[], can I use the sizeof() operator to determine the array’s length at runtime?

Answer:
No, you can’t use sizeof for this purpose. The problem is that the compiler silently converts an array parameter to a pointer. Therefore, sizeof will return the size of that pointer, not the size of the array.

You can use a vector object instead of a built-in array. A vector stores the number of its elements in an internal member. You can get the vector’s size by calling the size() member function:

void func( vector  & v){ int sz = v.size();}

If you’re programming in C, you can pass the number of elements in another parameter:

void func( int arr[], int arr_size);