[fpc-devel] Trying to understand the wiki-Page "FPC Unicode support"

[Hans-Peter Diettrich]

Michael Schnell schrieb:
> On 11/26/2014 12:09 PM, Sven Barth wrote:
>>  In Delphi (and FPC) CP_ACP corresponds by default with the current 
>> system codepage (e.g. CP1252 on a German Windows). 
> 
> OK. So in Delphi XE (in Germany) String(CP_ACP) is the same as 
> String(CP1252) but different from String without brackets which in turn 
> is the same as String(CP_UTF16) ? Correct ?

CP_ACP (and CP_NONE) describes a *static* encoding, and has an fixed 
value (CP_ACP=0, CP_NONE=$FFFF). The dynamic encoding of strings, kept 
in AnsiString(0) or RawByteString variables, must be obtained from the 
string itself. When the string is empty, StringCodepage returns 
DefaultSystemCodePage (for CP_ACP).


>> CP_UTF16 is not supported, because AnsiString only supports 1-Byte 
>> character strings (and UTF-8 as the odd one) and not 2-Byte character 
>> strings.
> 
> I still don't understand. The wiki article seems to suggest that it is 
> about a type called "ANSIString" that features a dynamically settable 
> "code page information". From discussions about Delphi and FPC, I only 
> know a String type with a dynamically settable "code page information" 
> that also features a dynamically settable "Bytes per Character 
> information" and hence does support 1, 2 and 4 "Bytes per Character". 
> (e.g. UTF-8, UTF-16, and UTF-32).

You should have noticed that there exists no String or Char type, that 
would allow for arbitrary bytes/char counts (see my other answer for 
details).


>> The difference to Delphi currently is that for FPC 
>> String=AnsiString(CP_ACP) and for Delphi String=UnicodeString (aka 
>> 2-Byte string).
>>
> 
> I understand that you mean (e.g.) Delphi XE. But what version of FPC is 
> "currently". Am I wrong assuming that in the svn we do have the 
> "NewStrings" library that supports dynamical code-page *and* 
> byte-per-character settings and hence supports e.g. CP1251, UTF-8, 
> UTF-16, and UTF-32 ?

The byte-per-character field is read-only, just like for any dynamic array.

> So I seem to understand the meaning of 
> String(CP1252), String(CP_UTF8), and String(CP_UTF16) (which seems do be 
> the Delphi notation), but I seemingly don't get the exact meaning of 
> "AnsiString(CP_ACP)" or "AnsiString(CP1251)"

The Delphi notation is the same, e.g. AnsiString(CP_ACP).

> In the end, what the definition of "String" without brackets is, might 
> be due to a settable compiler option and/or the OS the compiler is set 
> to create code for.

Right, the *generic* String type can be mapped to either ShortString, 
AnsiString(0) or UnicodeString, depending on compiler versions and 
switches. A raw guess can be derived from sizeof(Char).

DoDi