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(cpp-295.info)#elif Directive

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The `#elif' Directive
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   One common case of nested conditionals is used to check for more
than two possible alternatives.  For example, you might have

     #if X == 1
     ...
     #else /* X != 1 */
     #if X == 2
     ...
     #else /* X != 2 */
     ...
     #endif /* X != 2 */
     #endif /* X != 1 */

   Another conditional directive, `#elif', allows this to be abbreviated
as follows:

     #if X == 1
     ...
     #elif X == 2
     ...
     #else /* X != 2 and X != 1*/
     ...
     #endif /* X != 2 and X != 1*/

   `#elif' stands for "else if".  Like `#else', it goes in the middle
of a `#if'-`#endif' pair and subdivides it; it does not require a
matching `#endif' of its own.  Like `#if', the `#elif' directive
includes an expression to be tested.

   The text following the `#elif' is processed only if the original
`#if'-condition failed and the `#elif' condition succeeds.  More than
one `#elif' can go in the same `#if'-`#endif' group.  Then the text
after each `#elif' is processed only if the `#elif' condition succeeds
after the original `#if' and any previous `#elif' directives within it
have failed.  `#else' is equivalent to `#elif 1', and `#else' is
allowed after any number of `#elif' directives, but `#elif' may not
follow `#else'.