C5_#undef

#undef
 Remove a #define macro definition
------------------------------------------------------------------------------
 Syntax

     #undef <identifier>

 Arguments

     <identifier> is the name of the manifest constant or pseudofunction
     to remove.

 Description

     #undef removes an identifier defined with the #define directive. After
     an #undef, the specified identifier becomes undefined.  Use #undef to
     remove an identifier before you redefine it with #define, preventing the
     compiler warning that occurs when an existing identifier is redefined.
     Also, use #undef to make conditional compilation specific to certain
     sections of a program.

 Examples

     .  To define and then undefine a manifest constant and a
        pseudofunction:

        #define K_ESC  27
        #define MAX(x, y)   IF(x > y, x, y)
        .
        . <statements>
        .
        #undef K_ESC
        #undef MAX

     .  To use #undef to undefine an identifier before redefining it:

        #define DEBUG
        .
        . <statements>
        .
        #undef DEBUG
        #define DEBUG   .T.

     .  To undefine an identifier if it exists, and otherwise define
        it for later portions of the program file:

        #ifdef TEST
           #undef TEST
        #else
           #define TEST
        #endif

---

See Also: #define #ifdef #ifndef

---

C5 Directives

#command        Specify a user-defined command or translation directive
#define         Define a manifest constant or pseudofunction
#error          Generate a compiler error and display a message
#ifdef          Compile a section of code if an identifier is defined
#ifndef         Compile a section of code if an identifier is undefined
#include        Include a file into the current source file
#stdout         Send literal text to the standard output device
#translate      Specify a user-defined command or translation directive
#undef          Remove a #define macro definition 
#xcommand       Specify a user-defined command or translation directive
#xtranslate     Specify a user-defined command or translation directive

What is Preprocessor ?

What is Preprocessor and How it Works ?

Preprocessor  Primer ( .pdf )

Preprocessor Primer

What is Preprocessor and How it Works ?

Preprocessor Primer ( .pdf )

C5 Pre-processor Directives

#command | #translate :

Specify a user-defined command or translation directive

#command <matchPattern> => <resultPattern>
#translate <matchPattern> => <resultPattern>

#define :

Define a manifest constant or pseudofunction

#define <idConstant> [<resultText>]
#define <idFunction>([<arg list>]) [<exp>]

#error :

Generate a compiler error and display a message

#error [<messageText>]

#ifdef :

Compile a section of code if an identifier is defined

#ifdef <identifier>
    <statements>...
[#else]
    <statements>...
#endif

#ifndef :

Compile a section of code if an identifier is undefined

#ifndef <identifier>
    <statements>...
[#else]
    <statements>...
#endif

#include :

Include a file into the current source file

#include "<otherSourceFileSpec>"

#stdout :

Send literal text to the standard output device

#stdout [<messageText>]

#undef :

Remove a #define macro definition

#undef <identifier>

#xcommand | #xtranslate :

Specify a user-defined command or translation directive

#xcommand <matchPattern> => <resultPattern>
#xtranslate <matchPattern> => <resultPattern>

Pre-processor

What is Pre-processor and how it works ?

The Pre-processor is a translation program that prepares source code for compilation by  applying pre-defined text replacements. The replacements to be made are specified by  directives in the source file. The preprocessor operates transparently as a part of the  compilation process.

First phase of of compilation process is pre-processing. The portion of compile engine  that perform pre-processing phase is called pre-processor.

Before the later compilation  phases take place, pre-processor scan the source file from top to bottom for certain  directives and translate them into regular source code that can be compiled. The output of  the pre-processor is then used as input to the next step of compilation. The /P compiler  option can be used to write this pre-processor output (.ppo) file to disk so that you can  see the source code that was used as input to the next compilation phase. This option  is especially useful if you have used the #command and #translate directives to build  user-defined commands. You can look at the (.ppo) file to see whether or not commands  translated as you expected.

The following table summarize the pre-processor directives.

Directive              Meaning
--------------------   --------------------------------------------------------------
#define                Define a manifest constant or pseudo-function
#undef                 Remove a #define definition

#include               Include a file into the current source file

#command, #xcommand,   Specify a user defined command or translation 
#translate,            directive (This four directives are same as each
#xtranslate            others with some minor differences) . 

#ifdef                 Compile a section of code if an identifier is defined
#ifndef                Compile a section of code if an identifier is undefined

#error                 Generate a compiler error and display a message
#stdout                Send literal text to the standard output device

Here you can find pre-processor terms.