#elif

Starts an optional, alternative conditional-assembly block, adding another boolean-expression condition.

  #elif bool-expr statement-group
  
  

bool-expr

Any boolean expression.

statement-group

Any valid assembly statements.

This directive must be part of an #if ... #elif ... [#else] ... #endif conditional structure (with each of these directives starting a new line). The preprocessor implements the assembly statements that #elif introduces only if (1) the bool-expr condition of the #if directive is false, and (2) the bool-expr condition of the #elif directive is true.

For a logical structure of multiple levels, you can use the #elif directive several times, as in this pattern:

  #if bool-expr-1
     statement-group-1
   #elif bool-expr-2
     statement-group-2
   #elif bool-expr-3
     statement-group-3
   #elif bool-expr-4
     statement-group-4
   #else
     statement-group-5
   #endif

• If this structure's bool-expr-1 is true, the preprocessor executes the statement-group-1 statements, then goes to the #endif directive.
• If bool-expr-1 is false, the preprocessor skips statement-group-1, executing the first #elif directive. If bool-expr-2 is true, the preprocessor executes statement-group-2, then goes to the #endif directive.
• If bool-expr-2 also is false, the preprocessor skips statement-group-2, executing the second #elif directive.
• The preprocessor continues evaluating the boolean expressions of succeeding #elif directives until it comes to a boolean expression that is true.
• If none of the boolean expressions are true, the preprocessor processes statement-group-5, because this structure includes an #else directive.
• If none of the boolean values were true and there were no #else directive, the preprocessor would not process any of the statement groups.)