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(cpp-295.info)Conditional Syntax

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Syntax of Conditionals
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   A conditional in the C preprocessor begins with a "conditional
directive": `#if', `#ifdef' or `#ifndef'.  Note: Conditionals-Macros,
for information on `#ifdef' and `#ifndef'; only `#if' is explained here.

If

#if Directive. Basic conditionals using `#if' and `#endif'.

Else

#else Directive. Including some text if the condition fails.

Elif

#elif Directive. Testing several alternative possibilities.