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

[Bart]

> 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.

Thanks for explaining.

>
> You can report a bug with the following sample to hopefully improve this:
>
Reported as issue #40106

On Thu, Jan 12, 2023 at 9:39 PM Sven Barth <pascaldragon at googlemail.com> wrote:
>
> 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



-- 
Bart