#if
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Used when evaluating an expression.
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#ifdef
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Used to check if an identifier is defined.
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#ifndef
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Used to check if an identifier is not defined.
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#elif
#elseif
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Used when evaluating an expression. Only evaluated when the preceding
if, elif or elseif directives evaluate to false.
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#else
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Used only used when the preceding #if, #ifdef, #ifndef,
#elif or elseif directives evaluate to false.
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#endif
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Used to end #if, #ifdef and #ifndef directives.
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expression
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the expression to evaluate.
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identifier
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the identifier to check for existence.
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statement
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the statement(s) to compile when the preceding #if, #ifdef,
#ifndef, #elif or elseif evaluates to true.
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