[fpc-pascal] Warning not to use the "String" type with FPC 3.x

[Graeme Geldenhuys]

On 2016-05-09 17:40, Mark Morgan Lloyd wrote:
> What, /exactly/, are you saying can be lost, and under what circumstances?

You loose “data” due to codepage based AnsiString (aka the String type)
not always supporting all code points of UTF8String or UnicodeString data.

eg:
  I write a program that assigns a UnicodeString value to an AnsiString
  variable. My program uses compiler mode OBJFPC and {$H+}. I run that
  same executable on two different Linux systems.
  NOTE: it's the same executable.

  Linux Box #1:
    The default codepage is UTF-8, thus String equals AnsiString(65001).
    No data is lost when converting from UnicodeString to String on this
    system. Essentially it’s a conversion of UTF-16 to UTF-8 - both
    support the full Unicode range.

 Linux Box #1:
   Here Linux has been setup with a default codepage of ISO-8859-1
   (Latin 1). I have a UnicodeString variable which contains BMP and
   Planes 1-12  code points. The program assigns that to my String
   type variable,  which is actually AnsiString(<latin1>). Only the
   first 255 characters of the 1.4 million Unicode code points will
   be converted. All the others will be replaced by a '?' symbol. A
   massive data loss, and that data could be critical.

What does FPC do about this? It only gives you a compiler warning when
the application was compiled, but still generates the executable as normal.

I now fully understand why Delphi 2009 and later uses UnicodeString as
the default type and their String = UnicodeString = UTF-16. It defaults
to UTF-16 on all its supported platforms (granted, Delphi support a lot
less platforms than FPC does). At least with Delphi it protects the
developers which still uses the String type everywhere (remember String
= UnicodeString there). Much safer than what FPC 3.x does now!


Now some would say, simply switch your compiler mode to DelphiUnicode.
But I don't want to do that, because I like the stricter ObjFPC mode,
and prefer ObjFPC's syntax.


Regards,
  Graeme

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