Is it acceptable to read outside the bound of an array (string)?

Question

I study a little the Delphi source code, and this look a little ugly for me (in system.pas, procedure ValLong) :

while True do
begin
  case S[I] of
    '0'..'9': Dig := Ord(S[I]) - Ord('0');
  else
    Break;
  end;
  if (Result < 0) or (Result > (High(Integer) div 10)) then
    Break;
  Result := Result*10 + Dig;
  Inc(I);
  Empty := False;
end;

As you can see the only way to get out of the loop is to read outside the boundary of S (a string). Do I miss something and this is an correct practice ?

Answer 1

Unicode strings, AnsiString and WideString types are always ending with a null character.

From Internal Data Formats-Long String Types:

The NULL character at the end of a string memory block is automatically maintained by the compiler and the built-in string handling routines. This makes it possible to typecast a string directly to a null-terminated string.

So, it is safe to rely on the fact that these types are null terminated. The code example in the question is ok.

Note: I'm assuming range checking is off, since this code is from the RTL. Otherwise, it would have caused an exception when indexing the null character.