The C programming language-2nd-- notes -- 4.11.3

4.11.3 Conditions include

         Conditional statements can be used to control the preprocessing itself , The value of this conditional statement is calculated during the execution of preprocessing . This method provides a means for selectively including different codes according to the calculated condition values in the compilation process .

        #if Statement on the constant integer expression in it （ It cannot contain sizeof、 Type converter or enum Constant ） evaluation , If the value of the expression is not equal to 0, Then include the following lines , Until I met #endif、#elif or #else The statement so far （ Preprocessing statement #elif class else if）. stay #if Expressions available in statements defined( name ), The value of this expression follows ： When the name is defined , Its value is 1; otherwise , Its value is 0.

        example , To guarantee hdr.h The content of is only included once , You can include the contents of this file in the following conditional statements ：

#if !defined(HDR)
#define HDR
/*hdr.h Put the contents of here */
#endif

First inclusion hdr.h When , Will define the name HDR; Then include this again .h when , The name is already defined , Jump directly to #endif It's about . A similar method can also be used to avoid repeatedly including the same file . If multiple header files can use this method consistently , Then each header file can include any header file it depends on , Users do not have to consider the various dependencies between and processing header files .

        The following preprocessing code tests the system variables first SYSTEM, Then determine which version of header file to include according to the value of this variable ;

#if SYSTEM == SYSV
    #define HDR "sysv.h"
#elif SYSTEM == BSD
    #define HDR "bsd.h"
#elif SYSTEM == MSDOS
    #define HDR "msdos.h"
#else
    #define HDR "default.h"
#endif
#include HDR

        C Language specifically defines two preprocessing statements #ifdef And #ifndef, Used to test whether a name has been defined . It's about #if The first example of can be changed to ：

#ifndef HDR
#define HDR

/*hdr.h File contents here */

#endif