Delphi Copy-on-Write for String

Question

I have a code like this:

function Test: string;
var
   r, s: string;
begin
   r := 'Hello';
   Writeln(NativeInt(PChar(@r[1])));
   s := r;
   Writeln(NativeInt(PChar(@s[1])));
   Result := r;
   Writeln(NativeInt(PChar(@Result[1])));
end;

People say that delphi uses copy-on-write for strings. but the above function prints 3 different Addresses for variable, r, s, and Result. So this is confusing.. Is there only a copy of 'Hello' string in memory?

Answer 1

Whenever you take the address of an element of a string, that counts as a write in the eyes of the compiler. As far as it is concerned, you now have a raw pointer to the internals of the string, and who knows what you plan to do with it. So, from its perspective, it plays safe. It decides to make a unique copy of the string, so that you are free to do whatever dastardly deed you plan to do with your raw pointer.

The code you have compiles to this:

Project2.dpr.13: r := 'Hello';
00419EF8 8D45FC           lea eax,[ebp-$04]
00419EFB BAA89F4100       mov edx,$00419fa8
00419F00 E827D2FEFF       call @UStrLAsg
Project2.dpr.14: Writeln(NativeInt(PChar(@r[1])));
00419F05 8D45FC           lea eax,[ebp-$04]
00419F08 E883D3FEFF       call @UniqueStringU
00419F0D 8BD0             mov edx,eax
00419F0F A18CE64100       mov eax,[$0041e68c]
00419F14 E853B2FEFF       call @Write0Long
00419F19 E82EB5FEFF       call @WriteLn
00419F1E E845A1FEFF       call @_IOTest
Project2.dpr.15: s := r;
00419F23 8D45F8           lea eax,[ebp-$08]
00419F26 8B55FC           mov edx,[ebp-$04]
00419F29 E8FED1FEFF       call @UStrLAsg
Project2.dpr.16: Writeln(NativeInt(PChar(@s[1])));
00419F2E 8D45F8           lea eax,[ebp-$08]
00419F31 E85AD3FEFF       call @UniqueStringU
00419F36 8BD0             mov edx,eax
00419F38 A18CE64100       mov eax,[$0041e68c]
00419F3D E82AB2FEFF       call @Write0Long
00419F42 E805B5FEFF       call @WriteLn
00419F47 E81CA1FEFF       call @_IOTest
Project2.dpr.17: Result := r;
00419F4C 8BC3             mov eax,ebx
00419F4E 8B55FC           mov edx,[ebp-$04]
00419F51 E88ED1FEFF       call @UStrAsg
Project2.dpr.18: Writeln(NativeInt(PChar(@Result[1])));
00419F56 8BC3             mov eax,ebx
00419F58 E833D3FEFF       call @UniqueStringU
00419F5D 8BD0             mov edx,eax
00419F5F A18CE64100       mov eax,[$0041e68c]
00419F64 E803B2FEFF       call @Write0Long
00419F69 E8DEB4FEFF       call @WriteLn
00419F6E E8F5A0FEFF       call @_IOTest

The calls to UniqueStringU are performing the copy in copy-on-write.