Text Conversions

Boost.Locale provides several functions for basic string manipulation:

• to_upper: convert a string to upper case
• to_lower: convert a string to lower case
• to_title: convert a string to title case
• fold_case: makes a string case-agnostic (see case folding)
• normalize: convert equivalent code points to a consistent binary form (normalization)

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

There are existing functions to_upper and to_lower in the Boost.StringAlgo library, however the Boost.Locale functions operate on 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 report on unicode.org.

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 NULL 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.