diff options
Diffstat (limited to 'libstdc++-v3/docs/html/22_locale/codecvt.html')
| -rw-r--r-- | libstdc++-v3/docs/html/22_locale/codecvt.html | 245 |
1 files changed, 154 insertions, 91 deletions
diff --git a/libstdc++-v3/docs/html/22_locale/codecvt.html b/libstdc++-v3/docs/html/22_locale/codecvt.html index 46147d90556..64f3856d479 100644 --- a/libstdc++-v3/docs/html/22_locale/codecvt.html +++ b/libstdc++-v3/docs/html/22_locale/codecvt.html @@ -1,14 +1,22 @@ <html> <head> + <meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1" /> + <meta name="AUTHOR" content="bkoz@redhat.com (Benjamin Kosnik)" /> + <meta name="KEYWORDS" content="HOWTO, libstdc++, GCC, g++, libg++, STL" /> + <meta name="DESCRIPTION" content="Notes on the codecvt implementation." /> + <title>Notes on the codecvt implementation.</title> +<link rel="StyleSheet" href="../lib3styles.css" /> +</head> +<body> <h1> Notes on the codecvt implementation. </h1> -</head> -<I> +<p> +<em> prepared by Benjamin Kosnik (bkoz@redhat.com) on August 28, 2000 -</I> +</em> +</p> -<p> <h2> 1. Abstract </h2> @@ -28,26 +36,25 @@ wide and narrow characters and the implementation-provided extended functionality are given. </p> -<p> <h2> 2. What the standard says </h2> Around page 425 of the C++ Standard, this charming heading comes into view: -<BLOCKQUOTE> +<blockquote> 22.2.1.5 - Template class codecvt [lib.locale.codecvt] -</BLOCKQUOTE> +</blockquote> The text around the codecvt definition gives some clues: -<BLOCKQUOTE> -<I> +<blockquote> +<em> -1- The class codecvt<internT,externT,stateT> is for use when converting from one codeset to another, such as from wide characters to multibyte characters, between wide character encodings such as Unicode and EUC. -</I> -</BLOCKQUOTE> +</em> +</blockquote> <p> Hmm. So, in some unspecified way, Unicode encodings and @@ -55,18 +62,18 @@ translations between other character sets should be handled by this class. </p> -<BLOCKQUOTE> -<I> +<blockquote> +<em> -2- The stateT argument selects the pair of codesets being mapped between. -</I> -</BLOCKQUOTE> +</em> +</blockquote> <p> Ah ha! Another clue... </p> -<BLOCKQUOTE> -<I> +<blockquote> +<em> -3- The instantiations required in the Table ?? (lib.locale.category), namely codecvt<wchar_t,char,mbstate_t> and codecvt<char,char,mbstate_t>, convert the implementation-defined @@ -78,11 +85,12 @@ mbstate_t perform conversion between encodings known to the library implementor. Other encodings can be converted by specializing on a user-defined stateT type. The stateT object can contain any state that is useful to communicate to or from the specialized do_convert member. -</I> -</BLOCKQUOTE> +</em> +</blockquote> <p> At this point, a couple points become clear: +</p> <p> One: The standard clearly implies that attempts to add non-required @@ -95,7 +103,6 @@ template parameter, imply an implementation strategy that is mostly (or wholly) based on the underlying C library, and the functions mcsrtombs and wcsrtombs in particular.</p> -<p> <h2> 3. Some thoughts on what would be useful </h2> @@ -116,15 +123,19 @@ represent wide characters, and use an internal encoding of UCS4. (GNU/Linux systems using glibc, in particular.) The C programming language (and thus C++) does not specify a specific size for the type wchar_t. +</p> <p> Thus, portable C++ code cannot assume a byte size (or endianness) either. +</p> <p> Getting back to the frequently asked question: What about Unicode strings? +</p> <p> What magic spell will do this conversion? +</p> <p> A couple of comments: @@ -142,7 +153,7 @@ includes: <ul> <li> -</p> +<p> Identifiers for each of the codesets involved in the conversion. For example, using the iconv family of functions from the Single Unix Specification (what used to be called X/Open) hosted on the GNU/Linux @@ -150,10 +161,12 @@ operating system allows bi-directional mapping between far more than the following tantalizing possibilities: </p> +<p> (An edited list taken from <code>`iconv --list`</code> on a Red Hat 6.2/Intel system: +</p> -<BLOCKQUOTE> -<PRE> +<blockquote> +<pre> 8859_1, 8859_9, 10646-1:1993, 10646-1:1993/UCS4, ARABIC, ARABIC7, ASCII, EUC-CN, EUC-JP, EUC-KR, EUC-TW, GREEK-CCIcode, GREEK, GREEK7-OLD, GREEK7, GREEK8, HEBREW, ISO-8859-1, ISO-8859-2, ISO-8859-3, @@ -163,8 +176,8 @@ ISO-8859-15, ISO-10646, ISO-10646/UCS2, ISO-10646/UCS4, ISO-10646/UTF-8, ISO-10646/UTF8, SHIFT-JIS, SHIFT_JIS, UCS-2, UCS-4, UCS2, UCS4, UNICODE, UNICODEBIG, UNICODELIcodeLE, US-ASCII, US, UTF-8, UTF-16, UTF8, UTF16). -</PRE> -</BLOCKQUOTE> +</pre> +</blockquote> <p> For iconv-based implementations, string literals for each of the @@ -172,9 +185,12 @@ encodings (ie. "UCS-2" and "UTF-8") are necessary, although for other, non-iconv implementations a table of enumerated values or some other mechanism may be required. +</p> +</li> <li> Maximum length of the identifying string literal. +</li> <li> Some encodings are require explicit endian-ness. As such, some kind @@ -182,6 +198,7 @@ mechanism may be required. "Footnotes for C/C++ developers" in Haible for more information on UCS-2/Unicode endian issues. (Summary: big endian seems most likely, however implementations, most notably Microsoft, vary.) +</li> <li> Types representing the conversion state, for conversions involving @@ -189,24 +206,28 @@ mechanism may be required. conversions using iconv (such as the type iconv_t.) Note that the conversion descriptor encodes more information than a simple encoding state type. +</li> <li> Conversion descriptors for both directions of encoding. (ie, both UCS-2 to UTF-8 and UTF-8 to UCS-2.) +</li> <li> Something to indicate if the conversion requested if valid. +</li> <li> Something to represent if the conversion descriptors are valid. +</li> <li> Some way to enforce strict type checking on the internal and external types. As part of this, the size of the internal and external types will need to be known. +</li> </ul> -<p> <h2> 4. Problems with "C" code conversions : thread safety, global locales, termination. @@ -219,11 +240,13 @@ when implemented using standard "C" functions. <p> Three problems arise, one big, one of medium importance, and one small. +</p> <p> First, the small: mcsrtombs and wcsrtombs may not be multithread-safe on all systems required by the GNU tools. For GNU/Linux and glibc, this is not an issue. +</p> <p> Of medium concern, in the grand scope of things, is that the functions @@ -231,6 +254,7 @@ used to implement this specialization work on null-terminated strings. Buffers, especially file buffers, may not be null-terminated, thus giving conversions that end prematurely or are otherwise incorrect. Yikes! +</p> <p> The last, and fundamental problem, is the assumption of a global @@ -242,14 +266,15 @@ the GNU C++ library would like to offer a solution that allows multiple locales and or simultaneous usage with computationally correct results. In short, libstdc++-v3 is trying to offer, as an option, a high-quality implementation, damn the additional complexity! +</p> <p> For the required specialization codecvt<wchar_t, char, mbstate_t> , conversions are made between the internal character set (always UCS4 on GNU/Linux) and whatever the currently selected locale for the LC_CTYPE category implements. +</p> -<p> <h2> 5. Design </h2> @@ -259,25 +284,30 @@ The two required specializations are implemented as follows: <code> codecvt<char, char, mbstate_t> </code> +</p> <p> This is a degenerate (ie, does nothing) specialization. Implementing this was a piece of cake. +</p> <p> <code> codecvt<char, wchar_t, mbstate_t> </code> +</p> <p> This specialization, by specifying all the template parameters, pretty much ties the hands of implementors. As such, the implementation is straightforward, involving mcsrtombs for the conversions between char to wchar_t and wcsrtombs for conversions between wchar_t and char. +</p> <p> Neither of these two required specializations deals with Unicode characters. As such, libstdc++-v3 implements a partial specialization of the codecvt class with and iconv wrapper class, __enc_traits as the third template parameter. +</p> <p> This implementation should be standards conformant. First of all, the @@ -288,33 +318,40 @@ non-required conversions. Second of all, the standard says (in Chapter of all, the requirements for the stateT type elsewhere in the standard (see 21.1.2 traits typedefs) only indicate that this type be copy constructible. +</p> <p> As such, the type __enc_traits is defined as a non-templatized, POD type to be used as the third type of a codecvt instantiation. This type is just a wrapper class for iconv, and provides an easy interface to iconv functionality. +</p> <p> There are two constructors for __enc_traits: +</p> <p> <code> __enc_traits() : __in_desc(0), __out_desc(0) </code> +</p> <p> This default constructor sets the internal encoding to some default (currently UCS4) and the external encoding to whatever is returned by nl_langinfo(CODESET). +</p> <p> <code> __enc_traits(const char* __int, const char* __ext) </code> +</p> <p> This constructor takes as parameters string literals that indicate the desired internal and external encoding. There are no defaults for either argument. +</p> <p> One of the issues with iconv is that the string literals identifying @@ -325,24 +362,28 @@ inducing) strategy was implemented: end-users can specify any string (subject to a pre-determined length qualifier, currently 32 bytes) for encodings. It is up to the user to make sure that these strings are valid on the target system. +</p> <p> <code> void _M_init() </code> +</p> <p> Strangely enough, this member function attempts to open conversion descriptors for a given __enc_traits object. If the conversion descriptors are not valid, the conversion descriptors returned will not be valid and the resulting calls to the codecvt conversion functions will return error. +</p> <p> <code> bool _M_good() </code> +</p> <p> Provides a way to see if the given __enc_traits object has been properly initialized. If the string literals describing the desired @@ -351,57 +392,60 @@ fail, and this will return false. If the internal and external encodings are valid, but iconv_open could not allocate conversion descriptors, this will also return false. Otherwise, the object is ready to convert and will return true. +</p> <p> <code> __enc_traits(const __enc_traits&) </code> +</p> <p> As iconv allocates memory and sets up conversion descriptors, the copy constructor can only copy the member data pertaining to the internal and external code conversions, and not the conversion descriptors themselves. +</p> <p> Definitions for all the required codecvt member functions are provided for this specialization, and usage of codecvt<internal character type, external character type, __enc_traits> is consistent with other codecvt usage. +</p> -<p> <h2> 6. Examples </h2> <ul> - <li> - a. conversions involving string literals + <li> + a. conversions involving string literals <pre> - typedef codecvt_base::result result; - typedef unsigned short unicode_t; - typedef unicode_t int_type; - typedef char ext_type; - typedef __enc_traits enc_type; - typedef codecvt<int_type, ext_type, enc_type> unicode_codecvt; - - const ext_type* e_lit = "black pearl jasmine tea"; - int size = strlen(e_lit); - int_type i_lit_base[24] = + typedef codecvt_base::result result; + typedef unsigned short unicode_t; + typedef unicode_t int_type; + typedef char ext_type; + typedef __enc_traits enc_type; + typedef codecvt<int_type, ext_type, enc_type> unicode_codecvt; + + const ext_type* e_lit = "black pearl jasmine tea"; + int size = strlen(e_lit); + int_type i_lit_base[24] = { 25088, 27648, 24832, 25344, 27392, 8192, 28672, 25856, 24832, 29184, 27648, 8192, 27136, 24832, 29440, 27904, 26880, 28160, 25856, 8192, 29696, 25856, 24832, 2560 }; - const int_type* i_lit = i_lit_base; + const int_type* i_lit = i_lit_base; const ext_type* efrom_next; const int_type* ifrom_next; - ext_type* e_arr = new ext_type[size + 1]; - ext_type* eto_next; - int_type* i_arr = new int_type[size + 1]; - int_type* ito_next; + ext_type* e_arr = new ext_type[size + 1]; + ext_type* eto_next; + int_type* i_arr = new int_type[size + 1]; + int_type* ito_next; // construct a locale object with the specialized facet. - locale loc(locale::classic(), new unicode_codecvt); + locale loc(locale::classic(), new unicode_codecvt); // sanity check the constructed locale has the specialized facet. VERIFY( has_facet<unicode_codecvt>(loc) ); const unicode_codecvt& cvt = use_facet<unicode_codecvt>(loc); @@ -409,70 +453,80 @@ codecvt usage. unicode_codecvt::state_type state01("UNICODE", "ISO_8859-1"); initialize_state(state01); result r1 = cvt.in(state01, e_lit, e_lit + size, efrom_next, - i_arr, i_arr + size, ito_next); + i_arr, i_arr + size, ito_next); VERIFY( r1 == codecvt_base::ok ); VERIFY( !int_traits::compare(i_arr, i_lit, size) ); VERIFY( efrom_next == e_lit + size ); VERIFY( ito_next == i_arr + size ); </pre> - <li> - b. conversions involving std::string - <li> - c. conversions involving std::filebuf and std::ostream + </li> + <li> + b. conversions involving std::string + </li> + <li> + c. conversions involving std::filebuf and std::ostream + </li> </ul> More information can be found in the following testcases: <ul> -<li> testsuite/22_locale/codecvt_char_char.cc -<li> testsuite/22_locale/codecvt_unicode_wchar_t.cc -<li> testsuite/22_locale/codecvt_unicode_char.cc -<li> testsuite/22_locale/codecvt_wchar_t_char.cc +<li> testsuite/22_locale/codecvt_char_char.cc </li> +<li> testsuite/22_locale/codecvt_unicode_wchar_t.cc </li> +<li> testsuite/22_locale/codecvt_unicode_char.cc </li> +<li> testsuite/22_locale/codecvt_wchar_t_char.cc </li> </ul> -<p> <h2> 7. Unresolved Issues </h2> <ul> <li> - a. things that are sketchy, or remain unimplemented: - do_encoding, max_length and length member functions - are only weakly implemented. I have no idea how to do - this correctly, and in a generic manner. Nathan? + a. things that are sketchy, or remain unimplemented: + do_encoding, max_length and length member functions + are only weakly implemented. I have no idea how to do + this correctly, and in a generic manner. Nathan? +</li> <li> - b. conversions involving std::string - - <ul> - <li> - how should operators != and == work for string of - different/same encoding? - - <li> - what is equal? A byte by byte comparison or an - encoding then byte comparison? - - <li> - conversions between narrow, wide, and unicode strings - </ul> + b. conversions involving std::string + + <ul> + <li> + how should operators != and == work for string of + different/same encoding? + </li> + + <li> + what is equal? A byte by byte comparison or an + encoding then byte comparison? + </li> + + <li> + conversions between narrow, wide, and unicode strings + </li> + </ul> +</li> <li> - c. conversions involving std::filebuf and std::ostream - <ul> - <li> - how to initialize the state object in a - standards-conformant manner? - - <li> - how to synchronize the "C" and "C++" - conversion information? - - <li> - wchar_t/char internal buffers and conversions between - internal/external buffers? - </ul> + c. conversions involving std::filebuf and std::ostream + <ul> + <li> + how to initialize the state object in a + standards-conformant manner? + </li> + + <li> + how to synchronize the "C" and "C++" + conversion information? + </li> + + <li> + wchar_t/char internal buffers and conversions between + internal/external buffers? + </li> + </ul> +</li> </ul> -<p> <h2> 8. Acknowledgments </h2> @@ -480,7 +534,6 @@ Ulrich Drepper for the iconv suggestions and patient answering of late-night questions, Jason Merrill for the template partial specialization hints, language clarification, and wchar_t fixes. -<p> <h2> 9. Bibliography / Referenced Documents </h2> @@ -489,34 +542,44 @@ Drepper, Ulrich, GNU libc (glibc) 2.2 manual. In particular, Chapters "6. C <p> Drepper, Ulrich, Numerous, late-night email correspondence +</p> <p> Feather, Clive, "A brief description of Normative Addendum 1," in particular the parts on Extended Character Sets http://www.lysator.liu.se/c/na1.html +</p> <p> Haible, Bruno, "The Unicode HOWTO" v0.18, 4 August 2000 ftp://ftp.ilog.fr/pub/Users/haible/utf8/Unicode-HOWTO.html +</p> <p> ISO/IEC 14882:1998 Programming languages - C++ +</p> <p> ISO/IEC 9899:1999 Programming languages - C +</p> <p> Khun, Markus, "UTF-8 and Unicode FAQ for Unix/Linux" http://www.cl.cam.ac.uk/~mgk25/unicode.html +</p> <p> Langer, Angelika and Klaus Kreft, Standard C++ IOStreams and Locales, Advanced Programmer's Guide and Reference, Addison Wesley Longman, Inc. 2000 +</p> <p> Stroustrup, Bjarne, Appendix D, The C++ Programming Language, Special Edition, Addison Wesley, Inc. 2000 +</p> <p> System Interface Definitions, Issue 6 (IEEE Std. 1003.1-200x) The Open Group/The Institute of Electrical and Electronics Engineers, Inc. http://www.opennc.org/austin/docreg.html +</p> - +</body> +</html> |