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Generic-Text Mappings in TCHAR.H

To simplify transporting code for international use, the Microsoft run-time library provides Microsoft-specific "generic-text" mappings for many data types, routines, and other objects. You can use these mappings, which are defined in TCHAR.H, to write generic code that can be compiled for single byte, multibyte, or Unicode, depending on a manifest constant you define using a #define statement. Generic-text mappings are Microsoft extensions that are not ANSI compatible.

Using the header file TCHAR.H, you can build single-byte, MBCS, and Unicode applications from the same sources. TCHAR.H defines macros prefixed with _tcs, which, with the correct preprocessor definitions, map to str, _mbs, or wcs functions as appropriate. To build MBCS, define the symbol _MBCS. To build Unicode, define the symbol _UNICODE. To build a single-byte application, define neither (the default). By default, _MBCS is defined for MFC applications.

The _TCHAR data type is defined conditionally in TCHAR.H. If the symbol _UNICODE is defined for your build, _TCHAR is defined as wchar_t; otherwise, for single-byte and MBCS builds, it is defined as char. (wchar_t, the basic Unicode wide character data type, is the 16-bit counterpart to an 8-bit signed char.) For international applications, use the _tcs family of functions, which operate in _TCHAR units, not bytes. For example, _tcsncpy copies n _TCHARs, not n bytes.

Because some SBCS string-handling functions take (signed) char* parameters, a type mismatch compiler warning will result when _MBCS is defined. There are three ways to avoid this warning, listed in order of efficiency:

  1. Use the "type-safe" inline function thunks in TCHAR.H. This is the default behavior.
  2. Use the "direct" macros in TCHAR.H by defining _MB_MAP_DIRECT on the command line. If you do this, you must manually match types. This is the fastest method, but is not type-safe.
  3. Use the "type-safe" statically-linked library function thunks in TCHAR.H. To do so, define the constant _NO_INLINING on the command line. This is the slowest method, but the most type-safe.

Preprocessor Directives for Generic-Text Mappings

# define Compiled Version Example
_UNICODE Unicode (wide-character) _tcsrev maps to _wcsrev
_MBCS Multibyte-character _tcsrev maps to _mbsrev
None (the default: neither _UNICODE nor _MBCS defined) SBCS (ASCII) _tcsrev maps to strrev

For example, the generic-text function _tcsrev, defined in TCHAR.H, maps to _mbsrev if you defined _MBCS in your program, or to _wcsrev if you defined _UNICODE. Otherwise _tcsrev maps to strrev. Other data type mappings are provided in TCHAR.H for programming convenience, but _TCHAR is the most useful.

Generic-Text Data Type Mappings

Data Type Name
_MBCS Not Defined
_TCHAR char char wchar_t
_TINT int int wint_t
_TSCHAR signed char signed char wchar_t
_TUCHAR unsigned char unsigned char wchar_t
_TXCHAR char unsigned char wchar_t
_T or _TEXT No effect (removed by preprocessor) No effect (removed by preprocessor) L (converts following character or string to its Unicode counterpart)

For a complete list of generic-text mappings of routines, variables, and other objects, see Generic-Text Mappings in the Run-Time Library Reference.

Note   Do not use the str family of functions with Unicode strings, which are likely to contain embedded null bytes. Similarly, do not use the wcs family of functions with MBCS (or SBCS) strings.

The following code fragments illustrate the use of _TCHAR and _tcsrev for mapping to the MBCS, Unicode, and SBCS models.

_TCHAR *RetVal, *szString;
RetVal = _tcsrev(szString);

If _MBCS has been defined, the preprocessor maps this fragment to the code:

char *RetVal, *szString;
RetVal = _mbsrev(szString);

If _UNICODE has been defined, the preprocessor maps this fragment to the code:

wchar_t *RetVal, *szString;
RetVal = _wcsrev(szString);

If neither _MBCS nor _UNICODE has been defined, the preprocessor maps the fragment to single-byte ASCII code:

char *RetVal, *szString;
RetVal = strrev(szString);

Thus you can write, maintain, and compile a single source code file to run with routines that are specific to any of the three kinds of character sets.

See Also

International Programming | Using TCHAR.H Data Types with _MBCS Code

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