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/GCC 7 CPP

4.2.4 Else

The ‘#else’ directive can be added to a conditional to provide alternative text to be used if the condition fails. This is what it looks like:

#if expression
text-if-true
#else /* Not expression */
text-if-false
#endif /* Not expression */

If expression is nonzero, the text-if-true is included and the text-if-false is skipped. If expression is zero, the opposite happens.

You can use ‘#else’ with ‘#ifdef’ and ‘#ifndef’, too.

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Licensed under the GNU Free Documentation License, Version 1.3.
https://gcc.gnu.org/onlinedocs/gcc-7.1.0/cpp/Else.html