An implementation must allow a pointer to one past the end to
exist. How it does this is its business. On many machines, you
can safely put any value into a pointer, without risk (unless
you dereference it); on such systems, the one past the end
pointer may point to unmapped memory—I’ve
actually encountered a case under Windows where it did.
On other machines, just loading a pointer to unmapped memory
into a register will trap, causing the program to crash. On
such machines, the implementation must ensure that this doesn’t
happen, either by refusing to use the last byte or word of
allocated memory, or by ensuring that all use of the pointer
other than dereferencing it avoids any instructions which might
cause the hardware to treat it as an invalid pointer. (Most such
systems have separate address and data registers, and will only
trap if the pointer is loaded into an address register. If the
data registers are large enough, the compiler can safely load
the pointer into a data register for e.g. comparison. This is
often necessary anyway, as the address registers don’t always
support comparison.)
Re your last question: C and C++ are exactly identical in this respect; C++ simply took over the rules from C.