Boost.Locale: Text Conversions

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Text Conversions

There is a set of functions that perform basic string conversion operations: upper, lower and title case conversions, case folding and Unicode normalization. These are to_upper , to_lower, to_title, fold_case and normalize.

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

Global locale is used in all examples below.

## Case Handing

For example:

        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;

Would print:

    Upper GRÜSSEN
    Lower grüßen
    Title Grüßen
    Fold  grüssen

You may notice that there are existing functions `to_upper` and `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:

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

Would give in output:

    GRÜßEN GRÜSSEN

Where a letter "ß" was not converted correctly to double-S in first case because of a limitation of `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

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

Would modify ASCII characters only

    GRüßEN GRÜSSEN

## 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 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 this article.

## Notes

- 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.
- 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.