#if defined and # if!DEFINED (macro definition for C language)

We're going to check if a is defined.

#if defined A
#undef A
#define a 200
#endif

The above statement verifies whether a is defined and, if defined, is defined with the #undef statement and redefined a as 200

#ifndef a //If A is not defined
#define a 100
#endif

#undef为解除定义

#ifndef是if the abbreviation for not defined, that is, if there is no definition

Role

1 conditional compilation

#if!defined xxx_xxx
#define Xxx_xxx
#endif

2 Error message

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

#if defined and # if!DEFINED (macro definition for C language)