[fpc-devel] Strange Problem!

Martin lazarus at mfriebe.de
Wed Apr 25 22:48:59 CEST 2012

On 25/04/2012 21:11, Amir wrote:
> I am not sure what you mean be "your program is not valid". I am 
> initializing Result. Modify its content and ...
> I already observed that if I use either shortstring or use indices to 
> modify Result, this problem does not occur. But still I can't see what 
> causes this problem. And I believe it is a bug, isn't it?

Try the program at the end.
it will print "x" !!!

Strings a pointers. And they do "copy on write"

    s2 := s1;
means that s1 and s2 point to the same memory (not only the same 
content, but the same location in mem)

Internally the strig knows, that the memory is used by 2 strings
   s2[1] := 'a';
will trigger "copy on write". So s2 gets copied first then modified

using pchar does not trigger such copies.

so the cod modifies the constant you use for initialization.


program Project1;
function Foo: AnsiString;
   Result:= '                ';

   s: String;
   p: PChar;
   s := Foo;
   p := @s[1];
   p^ := 'x';

More information about the fpc-devel mailing list