1. This site uses cookies. By continuing to use this site, you are agreeing to our use of cookies. Learn More.

Commonly used C Preprocessor Directives

Discussion in 'C' started by shabbir, Sep 22, 2006.

  1. shabbir

    shabbir Administrator Staff Member

    Joined:
    Jul 12, 2004
    Messages:
    15,293
    Likes Received:
    365
    Trophy Points:
    83
    Commonly used C Preprocessor Directives

    #define


    You can use the #define directive to give a meaningful name to a constant in your program. The two forms of the syntax are:

    Syntax :
    #define identifier token-string

    #define identifier[( identifier, ... , identifier )] token-string

    The #define directive substitutes token-string for all subsequent occurrences of an identifier in the source file.

    #error



    Syntax :
    #error token-string

    When #error directives are encountered, compilation terminates. These directives are most useful for detecting programmer inconsistencies and violation of constraints during preprocessing.

    Example :
    Code:
    #if !defined(__cplusplus)
    #error C++ compiler required.
    #endif
    

    #undef


    As its name implies, the #undef directive removes (undefines) a name previously created with #define.

    Syntax

    #undef identifier

    #if, #elif, #else, #endif


    The #if directive, with the #elif, #else, and #endif directives, controls compilation of portions of a source file. If the expression you write (after the #if) has a nonzero value, the line group immediately following the #if directive is retained in the translation unit.

    Syntax
    Code:
    #if
        line text
    #elif
        line text
    #else
        line text
    #endif
    

    #include


    The #include directive tells the preprocessor to treat the contents of a specified file as if those contents had appeared in the source program at the point where the directive appears.

    Syntax

    #include "path-spec"

    This form instructs the preprocessor to look for include files in the same directory of the file that contains the #include statement

    #include <path-spec>

    This form instructs the preprocessor to search for include files first along the path specified by the /I compiler option, then along the path specified by the INCLUDE environment variable.

    #ifdef, #ifndef


    The #ifdef and #ifndef directives perform the same task as the #if directive when it is used with defined( identifier ).

    Syntax

    #ifdef identifier

    #ifndef identifier


    is equivalent to

    #if defined identifier

    #if !defined identifier


    #pragma



    Each implementation of C and C++ supports some features unique to its host machine or operating system. Some programs, for instance, need to exercise precise control over the memory areas where data is placed or to control the way certain functions receive parameters. The #pragma directives offer a way for each compiler to offer machine- and operating-system-specific features while retaining overall compatibility with the C and C++ languages. Pragmas are machine- or operating-system-specific by definition, and are usually different for every compiler.

    Syntax

    #pragma token-string

    token-string can be any one of the following.

    alloc_text
    comment
    init_seg1
    optimize
    auto_inline
    component
    inline_depth
    pack
    bss_seg
    data_seg
    inline_recursion
    pointers_to_members1
    check_stack
    function
    intrinsic
    setlocale
    code_seg
    hdrstop
    message
    vtordisp1
    const_seg
    include_alias
    once
    warning

    Instead of going into the details of what each one of them is I would discuss the most commonly used one here as thats the title of the article and that is once.

    It specifies that the file, in which the pragma resides, will be included (opened) only once by the compiler in a building of a particular file.

    #pragma once

    There is one big confusion among the developers that header file is included once if it contains #pragma once only for the entire build but actually thats not the case. When you have two files code1.cpp and code2.cpp and one header file header.h. Put #pragma once and then a variable declaration in the header file as int iTest = 10; and try including the header file from both the cpp files and you will see an error like iTest already defined in xxxx.obj.

    This is not because #pragma is not working but actually the concept is for the compilation of the .cpp file a header file can be included multiple times from different locations and that is prevented using the #pragma once
     
  2. rahul.mca2001

    rahul.mca2001 New Member

    Joined:
    Feb 13, 2008
    Messages:
    103
    Likes Received:
    0
    Trophy Points:
    0
    can we #define something to be used latter for setting values
     

Share This Page