Pointer types, such as opaque, interface, class, and array, translate into 64-bit integers to enable FORTRAN 77 code portability between systems with 32-bit and 64-bit address spaces. On a 32-bit system, the upper 32 bits of these quantities are ignored. Systems with more than 64-bit pointers aren't currently supported.
Generally, clients should treat opaque, interface, class, and array values as black boxes. However, there is one value that is special. A value of zero for any of these quantities indicates the pointer does not refer to an object, thus making zero the FORTRAN 77 equivalent of NULL. Any nonzero value is or should be a valid object reference. Developers should initialize values to be passed as in or inout parameters to zero or a valid object reference.