Question

i have this problem, look this code of example:

src := '';
for iindex := 0 to 255 do
  begin
    src := src + char(byte(iindex));
    writeln (iindex:5, byte(src[iindex]):5);
  end;

where src is a string type and iindex is a integer type and as output i should to have something as:

iIndex   byte(src[iIndex])
 0         0
 1         1
 2         2
 3         3
 ..................
 254       254
 255       255   

and so was too in all lasts version of delphi until to delphi xe; now with delphi xe2, i have this output:

iIndex   byte(src[iIndex])
 0         0
 1         0
 2         1
 3         2
 ..................
 254       253
 255       254   

Now i ask, need use different type of string? I have found a workaround for solve it of course, updating byte(src[iindex]) to byte(src[succ(iindex)]) but so when iIndex = 255, i have byte(src[256]) that raise a exception and so need change byte in word.
Of course, i don’t think that it is the best solution, so i ask you if there is a better solution to it.
Thanks very much.

Answer 1

You are accessing the string out-of-bounds. String access is 1-based in Delphi. You should be getting a range check error. Also, I think you should prefer using chr() and ord() instead of the casts.

src := '';
for iindex := 0 to 255 do
begin
  src := src + chr(iindex);
  writeln (iindex:5, ord(src[iindex+1]):5);
end;

I don’t see that the code could work in earlier versions of Delphi. Indeed Delphi 2010 behaves exactly as you describe XE2 behaving once range checking is disabled. This is yet another example of why range checking is so valuable.