C Language and Libraries Help (clang.hlp) (Table of Contents; Topic list)
defined
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─────C/C++ Language─────────────────────────────────────────────────────────
 
  Operator:  defined
 
  Syntax:    defined( identifier )
             !defined( identifier )
 
  Summary:   Used with the #if directive to test whether <identifier>
             is currently defined. Returns true (nonzero) if it is
             currently defined and returns false (0) if it is not. The
             logical NOT operator (!) can be used to reverse the logic
             of the defined operator. For example,
 
                  #if defined( __cplusplus )
                  #if defined( __cplusplus ) && !defined( M_I8086 )
 
  See also:  #define, #if, #ifdef, #ifndef, #undef
                                    -♦-