Why Are mbstowcs() and wcstombs() Not the Best Choice for Unicode String Conversions?

Converting Between Unicode String Types

The task of converting between Unicode string types can be encountered when working with various programming languages and platforms. While functions like mbstowcs() and wcstombs() may seem like viable options for conversion, their usage can be problematic.

Limitations of mbstowcs() and wcstombs()

These functions do not necessarily convert to UTF-16 or UTF-32, but rather to wchar_t, with the encoding varying based on the locale. This introduces difficulties with portability and Unicode support.

Better Methods Introduced in C 11

C 11 introduced several improved methods for converting between Unicode string types:

1. std::wstring_convert

This template class provides a convenient way to perform conversions. Once created, it can be used to easily convert between strings:

<code class="cpp">std::wstring_convert<..., char16_t> convert;
std::string utf8_string = u8"This string has UTF-8 content";
std::u16string utf16_string = convert.from_bytes(utf8_string);</code>

2. New std::codecvt Specializations

New specializations of std::codecvt are also available for specific Unicode conversions:

• std::codecvt_utf8_utf16: Converts between UTF-8 and UTF-16
• std::codecvt_utf8: Converts between UTF-8 and UTF-32

3. Definition of Subclasses

To bypass issues with protected destructors in std::codecvt specializations, custom subclasses can be defined:

<code class="cpp">template <class internT, class externT, class stateT>
struct codecvt : std::codecvt<internT, externT, stateT>
{ ~codecvt() {} };

std::wstring_convert<codecvt<char16_t, char, std::mbstate_t>, char16_t> convert16;</code>

4. std::use_facet Template Function

This function can be used to obtain existing codecvt instances, which can be helpful with Visual Studio 2010 due to specialization limitations:

<code class="cpp">std::wstring_convert<std::codecvt_utf8<char16_t>, char16_t> convert16;</code>

Note: Direct UTF-32 and UTF-16 conversion requires combining two instances of std::wstring_convert.

Criticisms of wchar_t for Unicode

While wchar_t exists for representing Unicode codepoints, its purpose and usefulness have certain limitations:

• The encoding may vary between locales, making it unsuitable for portability and direct inter-locale conversions.
• It assumes a one-to-one mapping between characters and codepoints, which is not the case with Unicode.
• This makes wchar_t unreliable for text algorithms and portable code.

For portable code, the recommended approach is to use the C 11 string conversions or appropriate encoding-specific libraries.

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