NewStr/AssignStr dont conform to Delphi standard

[dje]

Delphi's NewStr returns a valid pointer for empty strings. FreePascal returns nil.

Code: Pascal  [Select][+][-]

1. S := NewStr('')

Delphi's AssignStr accepts nil and allocates new memory. FreePascal throws an "Access violation" on nil.

Code: Pascal  [Select][+][-]

1. AssignStr(S, 'New String');

Effectively it means NewStr('')'s must be replaced with at lease a space, or the following code.

Code: Pascal  [Select][+][-]

1.     P := NewStr(' ');
2.     AssignStr(P, '');  

Since empty strings return a nil, all code which de-references empty strings must check for nil before doing so.

Tested against Delphi 11 and FPC 3.2.2

[dje]

Code: Pascal  [Select][+][-]

1. {$mode delphi}
2.  
3. program StrTest;
4.  
5. uses
6.   SysUtils;
7.  
8. var
9.   P: PString;
10.  
11. begin
12.   try
13.     P := NewStr(''); // Replace with space to make it work
14.     AssignStr(P, 'This will crash');
15.     Writeln('Before: P = "', P^, '"');
16.     AssignStr(P, 'A Second ansistring');
17.     Writeln('After : P = "', P^, '"');
18.     DisposeStr(P);
19.   except
20.     on E: Exception do WriteLn(E.Message);
21.   end;
22.   ReadLn;
23. end.

[dje]

Just an update: It would appear GetStr/DisposeStr originate from TP7 via the "Objects" unit.
TP7 also returns nil for GetStr('') but doesn't contain a AssignStr function.

I checked the FreePascals Object unit, and the documentation is clear:

If the string is empty then Nil is returned.

https://www.freepascal.org/docs-html/rtl/objects/newstr.html

But, it then contains the procedure SetStr which does accept a nil, so this code doesn't crash:

Code: Pascal  [Select][+][-]

1. {$mode delphi}
2. program StrTest;
3. uses SysUtils, Objects;
4. var P: PString;
5. begin
6.   try
7.     P := NewStr('');
8.     SetStr(P, 'This now wont crash');
9.     Writeln('Before: P = "', P^, '"');
10.     SetStr(P, 'A Second ansistring');
11.     Writeln('After : P = "', P^, '"');
12.     DisposeStr(P);
13.   except
14.     on E: Exception do WriteLn(E.Message);
15.   end;
16.   ReadLn;
17. end.  

I guess the 3 GetStr/AssignStr/DisposeStr functions in SysUtils could be removed if they arn't going to be updated.
The Objects unit is enough for legacy code.
 

[nanobit]

If functions need more nil checks, you could report them in the bug tracker.

NewStr('') returning nil (but in Delphi: global NullStr) could be seen as preferable:
Spreading the NullStr (PString) via pointer variables is risky because they should be used only for read access, whereas write access (p^:= s;) would change the global NullStr.

[marcov]

The sysutils ones are probably for backwards compatibility with 16-bit Delphi 1, and should be treated separate from unit objects.

Please file a bug with a reproduction case.