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comp.lang.c++

std::wstringbuf and imbue to convert from utf-8 to wchar_t?

Boris Dušek

11/2/2008 7:27:00 PM

Hi,

I have an API that returns UTF-8 encoded strings. I have a utf8 codevt
facet available to do the conversion from UTF-8 to wchar_t encoding
defined by the platform. I have no trouble converting when a UTF-8
encoded string comes from file - I just create a std::wifstream and
imbue it with a locale that uses the utf-8 facet for
std::locale::ctype. Then I just use operator>> to get wstring properly
decoded from UTF-8. I thought I could create something similar for
std::wstringstream or std::wstringbuf, but I have a hard time with it.

I imagine the situation that if a std::wstringstream is imbued with
UTF-8, then it stored an array of char (not wchar_t) which is encoded
with UTF-8. I can push to it or get from it wide string like I like,
and the result is encoded in UTF-8 in some internal buffer.

What I now need is to be able to supply my UTF-8 buffer prefilled with
the values I need in UTF-8 to act as the internal UTF-8 encoded buffer
for the std::wstingbuf, and then call operator>>(..., std::wstring &),
to get the wide-string representation converted from the UTF-8 to the
proper wide encoding. Also while I am at it, I would like to know the
reverse - how to get this internal UTF-8 encoded buffer (so I can push
wstrings into it as I like and get a "char *" encoded in UTF-8).

Sample code (how I would imagine it):
char name[] = "Boris Du" "\xc5\xa1" "ek"; // my name - Boris Dušek
std::wstringstream conv;
conv.rdbuf()->pubsetcharbuf(name, 11); // pubsetbuf only accepts
"wchar_t *", not "char *"
std::wstring wname;
conv >> wname; // now my name should be properly decoded from UTF-8

Thanks for any suggestions,
Boris
4 Answers

Boris Dušek

11/2/2008 7:30:00 PM

0

> Sample code (how I would imagine it):
> char name[] = "Boris Du" "\xc5\xa1" "ek"; // my name - Boris Dušek
> std::wstringstream conv;
> conv.rdbuf()->pubsetcharbuf(name, 11); // pubsetbuf only accepts
> "wchar_t *", not "char *"
> std::wstring wname;
> conv >> wname; // now my name should be properly decoded from UTF-8

Please imagine that I imbued conv with a UTF-8 locale; I forgot to put
that into the above code.

Sam

11/2/2008 11:53:00 PM

0

Boris Dušek writes:

>> Sample code (how I would imagine it):
>> char name[] = "Boris Du" "\xc5\xa1" "ek"; // my name - Boris Dušek
>> std::wstringstream conv;
>> conv.rdbuf()->pubsetcharbuf(name, 11); // pubsetbuf only accepts
>> "wchar_t *", not "char *"
>> std::wstring wname;
>> conv >> wname; // now my name should be properly decoded from UTF-8
>
> Please imagine that I imbued conv with a UTF-8 locale; I forgot to put
> that into the above code.

You need to instantiate a std::locale("en_US.utf-8"). Then, invoke
std::use_facet< std::codecvt<wchar_t, char> >(locale) to obtain a reference
to a std::codecvt<wchar_t, char> object. Then, use the object's in() and
out() methods to convert between utf-8 encoded chars, and wchar_t.

Yes, this is a rather convoluted. I don't understand why doing these kinds
of things have to be so complicated, but, that's how it is.

James Kanze

11/3/2008 11:57:00 AM

0

On Nov 2, 8:27 pm, Boris Du?ek <boris.du...@gmail.com> wrote:

> I have an API that returns UTF-8 encoded strings. I have a
> utf8 codevt facet available to do the conversion from UTF-8 to
> wchar_t encoding defined by the platform. I have no trouble
> converting when a UTF-8 encoded string comes from file - I
> just create a std::wifstream and imbue it with a locale that
> uses the utf-8 facet for std::locale::ctype. Then I just use
> operator>> to get wstring properly decoded from UTF-8. I
> thought I could create something similar for
> std::wstringstream or std::wstringbuf, but I have a hard time
> with it.

It won't work, because wstringbuf doesn't take input or generate
output in the form of char's. wstringbuf uses a wstring. The
code translation in wfilebuf takes place in the wfilebuf, not in
any of the base classes, and it takes place because all file IO
in C++ involves char's; it's there to allow you to transfer
char's to and from the disk, while only seeing wchar_t at the
interface with the class.

> I imagine the situation that if a std::wstringstream is imbued
> with UTF-8, then it stored an array of char (not wchar_t)
> which is encoded with UTF-8. I can push to it or get from it
> wide string like I like, and the result is encoded in UTF-8 in
> some internal buffer.

> What I now need is to be able to supply my UTF-8 buffer
> prefilled with the values I need in UTF-8 to act as the
> internal UTF-8 encoded buffer for the std::wstingbuf, and then
> call operator>>(..., std::wstring &), to get the wide-string
> representation converted from the UTF-8 to the proper wide
> encoding. Also while I am at it, I would like to know the
> reverse - how to get this internal UTF-8 encoded buffer (so I
> can push wstrings into it as I like and get a "char *" encoded
> in UTF-8).

> Sample code (how I would imagine it):
> char name[] = "Boris Du" "\xc5\xa1" "ek"; // my name - Boris Du?ek
> std::wstringstream conv;
> conv.rdbuf()->pubsetcharbuf(name, 11); // pubsetbuf only accepts
> "wchar_t *", not "char *"

There is no pubsetcharbuf function. It's the str() function
you'd be interested in. But in all cases; the character type of
a wstringbuf is always wchar_t; the class does not support
conversion to any other basic type. (That is, in a way, the
price we pay for it being a template.)

--
James Kanze (GABI Software) email:james.kanze@gmail.com
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Boris Dušek

11/3/2008 9:27:00 PM

0

Thanks to both of you. I now see while filebuf is special. I found a
wbuffer_convert template at Dinkumware's site, and also found that it
will be in C++0x. I also discovered Boost.Iostreams' code_converter (I
am already using Boost.Iostreams in my project, so using it is easy).