Re: [fpc-devel] Why: "Can't take the address of constant expressions" here?

[Sven Barth via fpc-devel]

Am 11.01.2023 um 23:58 schrieb Bart via fpc-devel:
Given the following program (an excerpt form a test program for a
bugreport about the fpwidestring unit):
===
program test;
{$codepage utf8}
{$mode objfpc}
{$h+}

uses
   FpWideString;

var
   WSource: WideString = 'source';
   USource: UnicodeString = 'source';
   WDest: WideString = '' ;
   UDest: UnicodeString = '';
   ASource: AnsiString = 'source';
   ADest: AnsiString = '';
   P: array[0..99] of AnsiChar;

begin
   with WideStringManager do
   begin
     writeln(1);
     Wide2AnsiMoveProc(pwidechar(WSource),RawByteString(ADest),
CP_UTF8, Length(WSource));
     writeln(2);
     Ansi2WideMoveProc(PChar(ASource), CP_UTF8, UDest,
Length(ASource));   //<< test.lpr(24,53) Error: Can't take the address
of constant expressions (caret behind UDest)
end.
====
C:\Users\Bart\LazarusProjecten\bugs\Console\fpwidestring>fpc test.lpr
Free Pascal Compiler version 3.3.1 [2022/10/11] for i386
Copyright (c) 1993-2022 by Florian Klaempfl and others
Target OS: Win32 for i386
Compiling test.lpr
test.lpr(24,53) Error: Can't take the address of constant expressions
...
UDest is of the wrong type here, it compiles fine with WDest (WideString).
I just don't understand the "Can't take the address of constant expressions"
I would have expected something like "Call by var for arg no. 3 has to
match exactly: Got "UnicodeString" expected "WideString""

The issue is indeed the conversion from UnicodeString to WideString 
which is not allowed for a var/out parameter. That the compiler doesn't 
use the error “Call by var for arg no. 3 has to match exactly: Got 
"UnicodeString" expected "WideString"” is due to Ansi2WideMoveProc() 
being a function pointer. The code that determines that error is not 
called in that case, instead the other is caused essentially as a last 
resort.

You can report a bug with the following sample to hopefully improve this:

=== code begin ===

program tunicode;

var
  Str: UnicodeString = 'Foobar';

procedure Test(var aArg: WideString);
begin

end;

var
  Func: procedure(var aArg: WideString) = Nil;
begin
  Test(Str);
  Func(Str);
end.

=== code end ===

Regards,
Sven
_______________________________________________
fpc-devel maillist  -  fpc-devel@lists.freepascal.org
https://lists.freepascal.org/cgi-bin/mailman/listinfo/fpc-devel