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<title>Directives Within Macro Arguments - The C Preprocessor</title>
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<a name="Directives-Within-Macro-Arguments"></a>
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Next: <a rel="next" accesskey="n" href="Macro-Pitfalls.html#Macro-Pitfalls">Macro Pitfalls</a>,
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Previous: <a rel="previous" accesskey="p" href="Undefining-and-Redefining-Macros.html#Undefining-and-Redefining-Macros">Undefining and Redefining Macros</a>,
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Up: <a rel="up" accesskey="u" href="Macros.html#Macros">Macros</a>
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<h3 class="section">3.9 Directives Within Macro Arguments</h3>
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<p><a name="index-macro-arguments-and-directives-71"></a>
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Occasionally it is convenient to use preprocessor directives within
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the arguments of a macro. The C and C++ standards declare that
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behavior in these cases is undefined.
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<p>Versions of CPP prior to 3.2 would reject such constructs with an
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error message. This was the only syntactic difference between normal
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functions and function-like macros, so it seemed attractive to remove
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this limitation, and people would often be surprised that they could
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not use macros in this way. Moreover, sometimes people would use
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conditional compilation in the argument list to a normal library
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function like ‘<samp><span class="samp">printf</span></samp>’, only to find that after a library upgrade
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‘<samp><span class="samp">printf</span></samp>’ had changed to be a function-like macro, and their code
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would no longer compile. So from version 3.2 we changed CPP to
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successfully process arbitrary directives within macro arguments in
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exactly the same way as it would have processed the directive were the
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function-like macro invocation not present.
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<p>If, within a macro invocation, that macro is redefined, then the new
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definition takes effect in time for argument pre-expansion, but the
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original definition is still used for argument replacement. Here is a
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pathological example:
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<pre class="smallexample"> #define f(x) x x
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f (1
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#undef f
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#define f 2
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f)
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</pre>
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<p class="noindent">which expands to
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<pre class="smallexample"> 1 2 1 2
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</pre>
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<p class="noindent">with the semantics described above.
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