boost/libs/locale/doc/conversions.txt

//
//  Copyright (c) 2009-2011 Artyom Beilis (Tonkikh)
//
//  Distributed under the Boost Software License, Version 1.0. (See
//  accompanying file LICENSE_1_0.txt or copy at
//  http://www.boost.org/LICENSE_1_0.txt)
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// vim: tabstop=4 expandtab shiftwidth=4 softtabstop=4 filetype=cpp.doxygen
/*!
\page conversions Text Conversions

There is a set of functions that perform basic string conversion operations: 
upper, lower and \ref term_title_case "title case" conversions, \ref term_case_folding "case folding"
and Unicode \ref term_normalization "normalization". These are \ref boost::locale::to_upper "to_upper" , \ref boost::locale::to_lower "to_lower", \ref boost::locale::to_title "to_title", \ref boost::locale::fold_case "fold_case" and \ref boost::locale::normalize "normalize".

All these functions receive an \c std::locale object as parameter or use a global locale by default.

Global locale is used in all examples below.

\section conversions_case Case Handing

For example:
\code
    std::string grussen = "grüßEN";
    std::cout   <<"Upper "<< boost::locale::to_upper(grussen) << std::endl
                <<"Lower "<< boost::locale::to_lower(grussen) << std::endl
                <<"Title "<< boost::locale::to_title(grussen) << std::endl
                <<"Fold  "<< boost::locale::fold_case(grussen) << std::endl;
\endcode

Would print:

\verbatim
Upper GRÜSSEN
Lower grüßen
Title Grüßen
Fold  grüssen
\endverbatim

You may notice that there are existing functions \c to_upper and \c to_lower in the Boost.StringAlgo library.
The difference is that these function operate over an entire string instead of performing incorrect character-by-character conversions.

For example:

\code
    std::wstring grussen = L"grüßen";
    std::wcout << boost::algorithm::to_upper_copy(grussen) << " " << boost::locale::to_upper(grussen) << std::endl;
\endcode

Would give in output:

\verbatim
GRÜßEN GRÜSSEN
\endverbatim

Where a letter "ß" was not converted correctly to double-S in first case because of a limitation of \c std::ctype facet.

This is even more problematic in case of UTF-8 encodings where non US-ASCII are not converted at all. 
For example, this code 

\code
    std::string grussen = "grüßen";
    std::cout << boost::algorithm::to_upper_copy(grussen) << " " << boost::locale::to_upper(grussen) << std::endl;
\endcode

Would modify ASCII characters only

\verbatim
GRüßEN GRÜSSEN
\endverbatim

\section conversions_normalization Unicode Normalization

Unicode normalization is the process of converting strings to a standard form, suitable for text processing and
comparison. For example, character "ü" can be represented by a single code point or a combination of the character "u" and the
diaeresis "¨". Normalization is an important part of Unicode text processing.

Unicode defines four normalization forms. Each specific form is selected by a flag passed
to \ref boost::locale::normalize() "normalize" function:

- NFD - Canonical decomposition - boost::locale::norm_nfd
- NFC - Canonical decomposition followed by canonical composition - boost::locale::norm_nfc or boost::locale::norm_default
- NFKD - Compatibility decomposition - boost::locale::norm_nfkd
- NFKC - Compatibility decomposition followed by canonical composition - boost::locale::norm_nfkc

For more details on normalization forms, read <a href="http://unicode.org/reports/tr15/#Norm_Forms">this article</a>.

\section conversions_notes Notes

-   \ref boost::locale::normalize() "normalize" operates only on Unicode-encoded strings, i.e.: UTF-8, UTF-16 and UTF-32 depending on the
    character width. So be careful when using non-UTF encodings as they may be treated incorrectly.
-   \ref boost::locale::fold_case() "fold_case" is generally a locale-independent operation, but it receives a locale as a parameter to
    determine the 8-bit encoding.
-   All of these functions can work with an STL string, a NUL terminated string, or a range defined by two pointers. They always
    return a newly created STL string.
-   The length of the string may change, see the above example.
*/