Wide characters support in D

[Ruslan Nikolaev]

> 
> Generally Linux systems use UTF-8 so I guess the "system
> encoding" there will be UTF-8. But then if you start to use
> QT you have to use UTF-16, but you might have to intermix
> UTF-8 to work with other libraries in the backend (libraries
> which are not necessarily D libraries, nor system
> libraries). So you may have a UTF-8 backend (such as the
> MySQL library), UTF-8 "system encoding" glue code, and
> UTF-16 GUI code (QT). That might be a good or a bad choice,
> depending on various factors, such as whether the glue code
> send more strings to the backend or the GUI.
> 
> Now try to port the thing to Windows where you define the
> "system encoding" as UTF-16. Now you still have the same
> UTF-8 backend, and the same UTF-16 GUI code, but for some
> reason you're changing the glue code in the middle to
> UTF-16? Sure, it can be made to work, but all the string
> conversions will start to happen elsewhere, which may change
> the performance characteristics and add some potential for
> bugs, and all this for no real reason.
> 
> The problem is that what you call "system encoding" is only
> the encoding used by the system frameworks. It is relevant
> when working with the system frameworks, but when you're
> working with any other API, you'll probably want to use the
> same character type as that API does, not necessarily the
> "system encoding". Not all programs are based on extensive
> use of the system frameworks. In some situations you'll want
> to use UTF-16 on Linux, or UTF-8 on Windows, because you're
> dealing with libraries that expect that (QT, MySQL).
> 

Agreed. True, system encoding is not always that clear. Yet, usually UTF-8 is common for Linux (consider also Gtk, wxWidgets, system calls, etc.) At the same time, UTF-16 is more common for Windows (consider win32api, DFL, system calls, etc.). Some programs written in C even tend to have their own 'tchar' so that they can be compiled differently depending on platform.

> A compiler switch is a poor choice there, because you can't
> mix libraries compiled with a different compiler switches
> when that switch changes the default character type.

Compiler switch is only necessary for system programmer. For instance, gcc also has 'fshort-wchar' that changes width of wchar_t to 16 bit. It also DOES break the code casue libraries normally compiled for wchar_t to 32 bit. Again, it's generally not for application programmer.

> 
> In most cases, it's much better in my opinion if the
> programmer just uses the same character type as one of the
> libraries it uses, stick to that, and is aware of what he's
> doing. If someone really want to deal with the complexity of

Programmer should not know generally what encoding he works with. For both UTF-8 and UTF-16, it's easy to determine number of bytes (words) in multibyte (word) sequence by just looking at first code point. This can also be builtin function (e.g. numberOfChars(tchar firstChar)). Size of each element can easily be determined by sizeof. Conversion to UTF-32 and back can be done very transparently.

The only problem it might cause - bindings with other libraries (but in this case you can just use fromUTFxx and toUTFxx; you do this conversion anyway). Also, transferring data over the network - again you can just stick to a particular encoding (for network and files, UTF-8 is better since it's byte order free).

> supporting both character types depending on the environment
> it runs on, it's easy to create a "tchar" and "tstring"
> alias that depends on whether it's Windows or Linux, or on a
> custom version flag from a compiler switch, but that'll be
> his choice and his responsibility to make everything work.

If it's a choice of programmer, then almost all advantages of tchar are lost. It's like garbage collector - if used by everybody, you can expect advantages of using it. However, if it's optional - everybody will write libraries assuming no GC is available, thus - almost all performance advantages are lost.

And after all, one of the goals of D (if I am not wrong) to be flexible, so that performance gains will be available for particular configurations if they can be achieved (it's fully compiled language). It does not stick to something particular and say 'you must use UTF-8' or 'you must use UTF-16'.

> michel.fortin at michelf.com
> http://michelf.com/
> 
>