| <!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"> |
| <html> |
| <!-- Copyright (C) 1987-2015 Free Software Foundation, Inc. |
| |
| Permission is granted to copy, distribute and/or modify this document |
| under the terms of the GNU Free Documentation License, Version 1.3 or |
| any later version published by the Free Software Foundation. A copy of |
| the license is included in the |
| section entitled "GNU Free Documentation License". |
| |
| This manual contains no Invariant Sections. The Front-Cover Texts are |
| (a) (see below), and the Back-Cover Texts are (b) (see below). |
| |
| (a) The FSF's Front-Cover Text is: |
| |
| A GNU Manual |
| |
| (b) The FSF's Back-Cover Text is: |
| |
| You have freedom to copy and modify this GNU Manual, like GNU |
| software. Copies published by the Free Software Foundation raise |
| funds for GNU development. --> |
| <!-- Created by GNU Texinfo 5.2, http://www.gnu.org/software/texinfo/ --> |
| <head> |
| <title>The C Preprocessor: Directives Within Macro Arguments</title> |
| |
| <meta name="description" content="The C Preprocessor: Directives Within Macro Arguments"> |
| <meta name="keywords" content="The C Preprocessor: Directives Within Macro Arguments"> |
| <meta name="resource-type" content="document"> |
| <meta name="distribution" content="global"> |
| <meta name="Generator" content="makeinfo"> |
| <meta http-equiv="Content-Type" content="text/html; charset=utf-8"> |
| <link href="index.html#Top" rel="start" title="Top"> |
| <link href="Index-of-Directives.html#Index-of-Directives" rel="index" title="Index of Directives"> |
| <link href="index.html#SEC_Contents" rel="contents" title="Table of Contents"> |
| <link href="Macros.html#Macros" rel="up" title="Macros"> |
| <link href="Macro-Pitfalls.html#Macro-Pitfalls" rel="next" title="Macro Pitfalls"> |
| <link href="Undefining-and-Redefining-Macros.html#Undefining-and-Redefining-Macros" rel="prev" title="Undefining and Redefining Macros"> |
| <style type="text/css"> |
| <!-- |
| a.summary-letter {text-decoration: none} |
| blockquote.smallquotation {font-size: smaller} |
| div.display {margin-left: 3.2em} |
| div.example {margin-left: 3.2em} |
| div.indentedblock {margin-left: 3.2em} |
| div.lisp {margin-left: 3.2em} |
| div.smalldisplay {margin-left: 3.2em} |
| div.smallexample {margin-left: 3.2em} |
| div.smallindentedblock {margin-left: 3.2em; font-size: smaller} |
| div.smalllisp {margin-left: 3.2em} |
| kbd {font-style:oblique} |
| pre.display {font-family: inherit} |
| pre.format {font-family: inherit} |
| pre.menu-comment {font-family: serif} |
| pre.menu-preformatted {font-family: serif} |
| pre.smalldisplay {font-family: inherit; font-size: smaller} |
| pre.smallexample {font-size: smaller} |
| pre.smallformat {font-family: inherit; font-size: smaller} |
| pre.smalllisp {font-size: smaller} |
| span.nocodebreak {white-space:nowrap} |
| span.nolinebreak {white-space:nowrap} |
| span.roman {font-family:serif; font-weight:normal} |
| span.sansserif {font-family:sans-serif; font-weight:normal} |
| ul.no-bullet {list-style: none} |
| --> |
| </style> |
| |
| |
| </head> |
| |
| <body lang="en" bgcolor="#FFFFFF" text="#000000" link="#0000FF" vlink="#800080" alink="#FF0000"> |
| <a name="Directives-Within-Macro-Arguments"></a> |
| <div class="header"> |
| <p> |
| 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> |
| </div> |
| <hr> |
| <a name="Directives-Within-Macro-Arguments-1"></a> |
| <h3 class="section">3.9 Directives Within Macro Arguments</h3> |
| <a name="index-macro-arguments-and-directives"></a> |
| |
| <p>Occasionally it is convenient to use preprocessor directives within |
| the arguments of a macro. The C and C++ standards declare that |
| behavior in these cases is undefined. |
| </p> |
| <p>Versions of CPP prior to 3.2 would reject such constructs with an |
| error message. This was the only syntactic difference between normal |
| functions and function-like macros, so it seemed attractive to remove |
| this limitation, and people would often be surprised that they could |
| not use macros in this way. Moreover, sometimes people would use |
| conditional compilation in the argument list to a normal library |
| function like ‘<samp>printf</samp>’, only to find that after a library upgrade |
| ‘<samp>printf</samp>’ had changed to be a function-like macro, and their code |
| would no longer compile. So from version 3.2 we changed CPP to |
| successfully process arbitrary directives within macro arguments in |
| exactly the same way as it would have processed the directive were the |
| function-like macro invocation not present. |
| </p> |
| <p>If, within a macro invocation, that macro is redefined, then the new |
| definition takes effect in time for argument pre-expansion, but the |
| original definition is still used for argument replacement. Here is a |
| pathological example: |
| </p> |
| <div class="smallexample"> |
| <pre class="smallexample">#define f(x) x x |
| f (1 |
| #undef f |
| #define f 2 |
| f) |
| </pre></div> |
| |
| <p>which expands to |
| </p> |
| <div class="smallexample"> |
| <pre class="smallexample">1 2 1 2 |
| </pre></div> |
| |
| <p>with the semantics described above. |
| </p> |
| |
| |
| |
| </body> |
| </html> |
