Predefined Macros

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The compiler recognizes seven predefined ANSI C macros (see Table 1.1), and the Microsoft C++ implementation provides several more (see Table 1.2). These macros take no arguments and cannot be redefined. Their value (except for __LINE__ and __FILE__) must be constant throughout compilation. Some of the predefined macros listed below are defined with multiple values. Their values can be set by selecting the corresponding menu option in the Visual C++ development environment, or by using a command-line switch. See the tables below for more information.

Table 1.1   ANSI-Compliant Predefined Macros

Macro Description
__DATE__ The compilation date of the current source file. The date is a string literal of the form Mmm dd yyyy. The month name Mmm is the same as for dates generated by the library function asctime declared in TIME.H.
__FILE__ The name of the current source file. __FILE__ expands to a string surrounded by double quotation marks.
__FUNCTION__ Valid only within a function and returns the undecorated name of the enclosing function (as a string). __FUNCTION__ is not expanded if you use /EP or /P compiler options.
__LINE__ The line number in the current source file. The line number is a decimal integer constant. It can be altered with a #line directive.
__STDC__ Indicates full conformance with the ANSI C standard. Defined as the integer constant 1 only if the /Za compiler option is given and you are not compiling C++ code; otherwise is undefined.
__TIME__ The most recent compilation time of the current source file. The time is a string literal of the form hh:mm:ss.
__TIMESTAMP__ The date and time of the last modification of the current source file, expressed as a string literal in the form Ddd Mmm Date hh:mm:ss yyyy, where Ddd is the abbreviated day of the week and Date is an integer from 1 to 31.

Table 1.2   Microsoft-Specific Predefined Macros

Macro Description
_CHAR_UNSIGNED Default char type is unsigned. Defined when /J is specified.
__cplusplus Defined for C++ programs only.
_CPPRTTI Defined for code compiled with /GR (Enable Run-Time Type Information).
_CPPUNWIND Defined for code compiled with /GX (Enable Exception Handling).
_DLL Defined when /MD or /MDd (Multithread DLL) is specified.
_M_ALPHA Defined for DEC ALPHA platforms. It is defined as 1 by the ALPHA compiler, and it is not defined if another compiler is used.
_M_IX86 Defined for x86 processors. See Table 1.3 for more details.
_M_MPPC Defined for Power Macintosh platforms (no longer supported).
_M_MRX000 Defined for MIPS platforms (no longer supported).
_M_PPC Defined for PowerPC platforms (no longer supported).
_MFC_VER Defines the MFC version. Defined as 0x0600 for Microsoft Foundation Class Library 6.0 or later. Always defined.
_MSC_EXTENSIONS This macro is defined when compiling with the /Ze compiler option (the default). Its value, when defined, is 1.
_MSC_VER Defines the compiler version. Defined as 1200 for Microsoft Visual C++ 6.0 or later. Always defined.
_MT Defined when /MD or /MDd (Multithreaded DLL) or /MT or /MTd (Multithreaded) is specified.
_WIN32 Defined for applications for Win32®. Always defined.

As shown in following tables, the compiler generates a value for the preprocessor identifiers that reflect the processor option specified.

Table 1.3   Values for _M_IX86

Option in Development Environment Command-Line Option Resulting Value
Blend /GB _M_IX86 = 500 (Default. Future compilers will emit a different value to reflect the dominant processor.)
Pentium /G5 _M_IX86 = 500
Pentium Pro /G6 _M_IX86 = 600
80386 /G3 _M_IX86 = 300
80486 /G4 _M_IX86 = 400