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Unicode and Multibyte Character Set (MBCS) Support

  • Artikkeli

Some languages, for example, Japanese and Chinese, have large character sets. To support programming for these markets, the Microsoft Foundation Class Library (MFC) enables two different approaches to handling large character sets:

  • Unicode, wchar_t based wide-characters, and strings encoded as UTF-16.

  • Multibyte Character Sets (MBCS), char based single or double-byte characters and strings encoded in a locale-specific character set.

Note

Microsoft recommends the MFC Unicode libraries for all new development.
The MBCS libraries were deprecated in Visual Studio 2013 and Visual Studio 2015. This is no longer the case.
Starting with Visual Studio 2017, the MBCS libraries are no longer deprecated and don't generate deprecation warnings.

MFC Support for Unicode Strings

The entire MFC class library is conditionally enabled for Unicode characters and strings stored in wide characters as UTF-16. In particular, class CString is Unicode-enabled.

These library, debugger, and DLL files are used to support Unicode in MFC:

MFCversionU.LIB
MFCversionUD.LIB
MFCMversionU.LIB
MFCMversionUD.LIB
MFCSversionU.LIB
MFCSversionUD.LIB
UAFXCW.LIB
UAFXCWD.LIB

MFCversionU.PDB
MFCversionUD.PDB
MFCMversionU.PDB
MFCMversionUD.PDB
MFCSversionU.PDB
MFCSversionUD.PDB
UAFXCW.PDB
UAFXCWD.PDB

MFCversionU.DLL
MFCversionUD.DLL
MFCMversionU.DLL
MFCMversionUD.DLL

(version represents the version number of the file; for example, '140' means version 14.0.)

CString is based on the TCHAR data type. If the symbol _UNICODE is defined for a build of your program, TCHAR is defined as type wchar_t, a 16-bit character encoding type. Otherwise, TCHAR is defined as char, the normal 8-bit character encoding. Therefore, under Unicode, a CString is composed of 16-bit characters. Without Unicode, it's composed of characters of type char.

To complete Unicode programming of your application, you must also:

  • Use the _T macro to conditionally code literal strings to be portable to Unicode.

  • When you pass strings, pay attention to whether function arguments require a length in characters or a length in bytes. The difference is important if you're using Unicode strings.

  • Use portable versions of the C run-time string-handling functions.

  • Use the following data types for characters and character pointers:

    • Use TCHAR where you would use char.

    • Use LPTSTR where you would use char*.

    • Use LPCTSTR where you would use const char*. CString provides the operator LPCTSTR to convert between CString and LPCTSTR.

CString also supplies Unicode-aware constructors, assignment operators, and comparison operators.

The Run-Time Library Reference defines portable versions of all its string-handling functions. For more information, see the category Internationalization.

MFC Support for MBCS Strings

The class library is also enabled for multibyte character sets, but only for double-byte character sets (DBCS).

In a multibyte character set, a character can be one or 2 bytes wide. If it's 2 bytes wide, its first byte is a special "lead byte" that is chosen from a particular range, depending on which code page is in use. Taken together, the lead and "trail bytes" specify a unique character encoding.

If the symbol _MBCS is defined for a build of your program, type TCHAR, on which CString is based, maps to char. It's up to you to determine which bytes in a CString are lead bytes and which are trail bytes. The C run-time library supplies functions to help you determine this.

Under DBCS, a given string can contain all single-byte ANSI characters, all double-byte characters, or a combination of the two. These possibilities require special care in parsing strings. This includes CString objects.

Note

Unicode string serialization in MFC can read both Unicode and MBCS strings regardless of which version of the application that you are running. Your data files are portable between Unicode and MBCS versions of your program.

CString member functions use special "generic text" versions of the C run-time functions they call, or they use Unicode-aware functions. Therefore, for example, if a CString function would typically call strcmp, it calls the corresponding generic-text function _tcscmp instead. Depending on how the symbols _MBCS and _UNICODE are defined, _tcscmp maps as follows:

Symbols Function
_MBCS defined _mbscmp
_UNICODE defined wcscmp
Neither symbol defined strcmp

Note

The symbols _MBCS and _UNICODE are mutually exclusive.

Generic-text function mappings for all of the run-time string-handling routines are discussed in C Run-Time Library Reference. For a list, see Internationalization.

Similarly, CString methods are implemented by using generic data type mappings. To enable both MBCS and Unicode, MFC uses TCHAR for char or wchar_t, LPTSTR for char* or wchar_t*, and LPCTSTR for const char* or const wchar_t*. These ensure the correct mappings for either MBCS or Unicode.

See also

Strings (ATL/MFC)
String Manipulation