By: Max_Power 8/20/2004

This tutorial gives very brief information about the C++ Preprocessor, it is meant to be used as a reference. It also lists and gives a brief description of pragma directives that are specific to popular compilers.

Before your C++ code is compiled it gets run through the C++ preprocessor, which is used to perform processing functions to prepare the code for compiling. The preprocessor's job can be broken down into three steps:

1. Removing the comments.
2. Including any files.
3. Replacing names with their macros introduced using #define.

The preprocessor does the majority of its job by following the explicit instructions given to it, also called directives. A preprocessor directive should always be prepended with a #. As dictated by the American National Standards Institute (ANSI) standard, comments are the only text that should follow a preprocessor directive. Here are examples of valid preprocess directives:

#    define speed 1
    #define speed 1
#define speed 1 //The speed of a turtal

The following table details the 12 standard ANSI compliant preprocessor directives. Examples are only given for the directives the author feels are not self explanatory.

#define	Used to define a macro.
#elif	Combination of the #else and #if directives.
#else	Used to define an else condition for a conditional preprocessor block.
#endif	Used to end a conditional preprocessor block.
#error	Used to report errors, usually used in accordance with conditional directives.
#if	Makes it so you can have certain pieces of code only be compiled under a certain condition. Ex. #if sizeof(int) = 4       typedef int Integer #endif
#ifdef	Makes is so you can have a certian piece of code only be compiled if a macro with the given name is defined. Ex. #define debug 1 #ifdef debug       ... #endif
#ifndef	Makes is so you can have a certian piece of code only be compiled if a macro with the given name is not defined.
#include	Used to include the contents of another file. The contents of the included file are copied to the location were the include directive appears.
#line	Allows you to make the compiler believe the current line is a specified line in a specified file. This directive is normally used to make syntax error messages provided by the compiler more meaningful. Ex. #line 90 "blah.h"
#pragma	Used by compilers to implement custom directives.
#undef	Used to undefine a previously defined macro. Useful if you need to redefine a macro.

The following table briefly details Visual C++ specific preprocessor directives, which are used in accordance with the #pragma directive. Consult your Visual C++ documentation for more details. Some descriptions are word for word what they are in the Microsoft documentation. You will need to visit the documentation for help on actually using these.

alloc_text	Used to name the code section a the given function is to reside in.
auto_inline	Used to turn on or off consideration for inline functions. If you use this to turn of inline candidate checking explicitly defined inline functions will still be made inline.
bss_seg	Used to specify the section for uninitialized data.
check_stack	Used to turn on or off probes used to check the stack.
code_seg	Used to specify the section for your programs code.
const_seg	Used to specify the section for your programs constant data.
comment	Places a comment record in an object file or executable file.
component	Turns on or off the collecting of dependency or browser information from within your source files.
data_seg	Used to specify the section for your data.
function	Specifies that calls to functions specified in the pragma’s argument list be generated.
hdrstop	Used to stop header precompilation for a specific header.
include_alias	See documentation.
init_seg	Used to affect the way startup code is executed.
inline_depth	Used to limit inline expansion.
inline_recursion	Used to control the inline expansion of recursive function calls.
intrinsic	See documentation.
message	Used to display messages in the output window during a compile.
once	Used to specify that a file should only be included once by the compiler.
optimize	Used to specify a specific type of optimization for a specific function, see documentation.
pack	See documentation.
pointers_to_members	See documentation.
setlocale	Used to define the country and language used during translation of literal strings and wide-character constants.
vtordisp	See documentation.
warning	Used to modify compiler warning messages.

The following table briefly details Borland C++ specific preprocessor directives, which are used in accordance with the #pragma directive. Consult your Borland C++ documentation for more details. Some descriptions are word for word what they are in the Borland documentation. You will need to visit the documentation for help on actually using these.

alignment	Used to display a message in the status window giving the current alignment and enum size.
anon_struct	Allows you to compile anonymous structures embedded in classes.
argsused	Allows you to disable the compiler warning message stating "Parameter name is never used in function func-name" for a specific function.
checkoption	See documentation.
codeseg	Allows you to name the the segment, class, or group where functions are allocated.
comment	Allows you to write a comment record into an output file.
defineonoption	Allows you to alias a command line option to a name.
exit	Allows you to specify a function that should be called before the program exits.
hdrfile	Lets you sets the name of the file in which to store precompiled headers.
hdrstop	Lets you terminate the list of header files eligible for precompilation.
inline	See documentation.
intrinsic	Used to override command-line switches or IDE options to control the inlining of functions.
link	Instructs the linker to link the file into an executable file.
message	Used to specify messages to display during compilation.
nopushoptwarn	Used to to include command-line options within program code.
obsolete	See documentation.
option	See nopushoptwarn.
pack	See documentation.
package	Used to assure that packaged units are initialized in the order determined by their dependencies.
resource	Causes the file to be marked as a form unit and requires matching .dfm and header files.
startup	Allows you to specify functions to be called before the main function.
undefineonoption	Used to undefine aliases defined using defineonoption.
warn	Used to override compiler warnings.

The following table details predefined constants used by the preprocessor.

__DATE__	Gives the date the source file was compiled.
__FILE__	Gives the name of the source file.
__LINE__	Gives the line number of the current source file.
__TIME__	Gives the time the source file was compiled.
__STDDC__	Used to indication the implementation is ANSI C compliant .