Generic-Text Mappings in Tchar.h
To simplify the transporting of 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 character sets, depending on a manifest constant that you define by using a #define statement. Generic-text mappings are Microsoft extensions that are not ANSI compatible.
By using the Tchar.h, you can build single-byte, Multibyte Character Set (MBCS), and Unicode applications from the same sources. Tchar.h defines macros (which have the prefix _tcs) that, 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 Single Byte Character Set (SBCS) string-handling functions take (signed) char* parameters, a type mismatch compiler warning results when _MBCS is defined. There are three ways to avoid this warning:
Use the type-safe inline function thunks in Tchar.h. This is the default behavior.
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.
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
_tcsrev maps to _wcsrev
_tcsrev maps to _mbsrev
None (the default has neither _UNICODE nor _MBCS defined)
_tcsrev maps to strrev
For example, the generic-text function _tcsrev, which is 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
_T or _TEXT
No effect (removed by preprocessor)
No effect (removed by preprocessor)
L (converts the following character or string to its Unicode counterpart)
For a list of generic-text mappings of routines, variables, and other objects, see Generic-Text Mappings in the Run-Time Library Reference.
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 this code:
char *RetVal, *szString; RetVal = _mbsrev(szString);
If _UNICODE has been defined, the preprocessor maps this fragment to this code:
wchar_t *RetVal, *szString; RetVal = _wcsrev(szString);
If neither _MBCS nor _UNICODE have been defined, the preprocessor maps the fragment to single-byte ASCII code, as follows:
char *RetVal, *szString; RetVal = strrev(szString);
Therefore, 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.