How do you convert between different types of Unicode strings in C 11?

Unicode String Conversion Methods

Converting between different types of Unicode strings can be necessary in various programming scenarios. However, the existing method of using mbstowcs() and wcstombs() has its limitations. While these methods perform conversions between multibyte character sets and wide character strings, they do not necessarily work with UTF-16 or UTF-32, and they depend on the locale's wchar_t encoding.

Better Approaches in C 11

C 11 introduced several new options for Unicode string conversions, including:

1. std::wstring_convert

This template class provides a convenient interface for converting between strings. It can be used with different codecvt facets to handle various conversions, such as UTF-8 to UTF-16 or UTF-8 to UTF-32:

<code class="cpp">std::wstring_convert<std::codecvt_utf8_utf16<char16_t>, 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 Codecvt Specializations

C 11 also introduced new codecvt specializations that are easier to use:

<code class="cpp">std::codecvt_utf8_utf16<char16_t> // converts between UTF-8 and UTF-16
std::codecvt_utf8<char32_t> // converts between UTF-8 and UTF-32
std::codecvt_utf8<char16_t> // converts between UTF-8 and UCS-2</code>

These specializations can be used with std::wstring_convert to facilitate conversions:

<code class="cpp">std::wstring_convert<std::codecvt_utf8_utf16<char16_t>, char16_t> convert16;
std::string a = convert16.to_bytes(u"This string has UTF-16 content");</code>

Note: Visual Studio 2010 may have issues using these specializations due to template specialization limitations with typedef'd types. In such cases, it's recommended to define a subclass of codecvt with a destructor or use the std::use_facet template function.

3. Converting Between UTF-32 and UTF-16

Since C 11 does not provide a direct conversion between UTF-32 and UTF-16, you can combine two instances of std::wstring_convert:

<code class="cpp">std::wstring_convert<std::codecvt_utf8_utf32<char32_t>, char32_t> convert32;
std::wstring_convert<std::codecvt_utf8_utf16<char16_t>, char16_t> convert16;
std::u32string utf32_string = u"This string has UTF-32 content";
std::string utf8_string = convert32.to_bytes(utf32_string);
std::u16string utf16_string = convert16.from_bytes(utf8_string);</code>

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