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114 lines
4.5 KiB
HTML
114 lines
4.5 KiB
HTML
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any later version published by the Free Software Foundation. A copy of
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(a) The FSF's Front-Cover Text is:
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<head>
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<title>Directives Within Macro Arguments (The C Preprocessor)</title>
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<link href="Index-of-Directives.html#Index-of-Directives" rel="index" title="Index of Directives">
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<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
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<link href="Macros.html#Macros" rel="up" title="Macros">
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<link href="Macro-Pitfalls.html#Macro-Pitfalls" rel="next" title="Macro Pitfalls">
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<a name="Directives-Within-Macro-Arguments"></a>
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<p>
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Next: <a href="Macro-Pitfalls.html#Macro-Pitfalls" accesskey="n" rel="next">Macro Pitfalls</a>, Previous: <a href="Undefining-and-Redefining-Macros.html#Undefining-and-Redefining-Macros" accesskey="p" rel="prev">Undefining and Redefining Macros</a>, Up: <a href="Macros.html#Macros" accesskey="u" rel="up">Macros</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Index-of-Directives.html#Index-of-Directives" title="Index" rel="index">Index</a>]</p>
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</div>
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<hr>
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<a name="Directives-Within-Macro-Arguments-1"></a>
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<h3 class="section">3.9 Directives Within Macro Arguments</h3>
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<a name="index-macro-arguments-and-directives"></a>
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<p>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. GNU CPP
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processes 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>
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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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</p>
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<div class="smallexample">
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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></div>
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<p>which expands to
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</p>
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<div class="smallexample">
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<pre class="smallexample">1 2 1 2
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</pre></div>
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<p>with the semantics described above.
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</p>
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