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| 3 | <!-- Copyright (C) 2022 Richard Stallman and Free Software Foundation, Inc.
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| 5 | (The work of Trevis Rothwell and Nelson Beebe has been assigned or
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| 6 | licensed to the FSF.)
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| 8 | Permission is granted to copy, distribute and/or modify this document
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| 20 | <head>
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| 21 | <meta http-equiv="Content-Type" content="text/html; charset=utf-8">
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| 22 | <title>Macros and Auto Type (GNU C Language Manual)</title>
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| 23 |
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| 24 | <meta name="description" content="Macros and Auto Type (GNU C Language Manual)">
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| 29 | <link href="index.html" rel="start" title="Top">
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| 30 | <link href="Symbol-Index.html" rel="index" title="Symbol Index">
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| 31 | <link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
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| 32 | <link href="Macro-Pitfalls.html" rel="up" title="Macro Pitfalls">
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| 33 | <link href="Self_002dReferential-Macros.html" rel="next" title="Self-Referential Macros">
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| 34 | <link href="Duplication-of-Side-Effects.html" rel="prev" title="Duplication of Side Effects">
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| 55 | </head>
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| 56 |
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| 57 | <body lang="en">
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| 58 | <span id="Macros-and-Auto-Type"></span><div class="header">
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| 59 | <p>
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| 60 | Next: <a href="Self_002dReferential-Macros.html" accesskey="n" rel="next">Self-Referential Macros</a>, Previous: <a href="Duplication-of-Side-Effects.html" accesskey="p" rel="prev">Duplication of Side Effects</a>, Up: <a href="Macro-Pitfalls.html" accesskey="u" rel="up">Macro Pitfalls</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Symbol-Index.html" title="Index" rel="index">Index</a>]</p>
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| 61 | </div>
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| 62 | <hr>
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| 63 | <span id="Using-_005f_005fauto_005ftype-for-Local-Variables"></span><h4 class="subsubsection">26.5.10.5 Using <code>__auto_type</code> for Local Variables</h4>
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| 64 | <span id="index-local-variables-in-macros"></span>
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| 65 | <span id="index-variables_002c-local_002c-in-macros"></span>
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| 66 | <span id="index-macros_002c-local-variables-in"></span>
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| 67 |
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| 68 | <p>The operator <code>__auto_type</code> makes it possible to
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| 69 | define macros that can work on any data type even though they need to
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| 70 | generate local variable declarations. See <a href="Auto-Type.html">Auto Type</a>.
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| 71 | </p>
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| 72 | <p>For instance, here’s how to define a safe “maximum” macro that
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| 73 | operates on any arithmetic type and computes each of its arguments
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| 74 | exactly once:
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| 75 | </p>
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| 76 | <div class="example">
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| 77 | <pre class="example">#define max(a,b) \
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| 78 | ({ __auto_type _a = (a); \
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| 79 | __auto_type _b = (b); \
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| 80 | _a > _b ? _a : _b; })
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| 81 | </pre></div>
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| 82 |
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| 83 | <p>The ‘<samp>({ … })</samp>’ notation produces <em>statement
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| 84 | expression</em>—a statement that can be used as an expression
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| 85 | (see <a href="Statement-Exprs.html">Statement Exprs</a>). Its value is the value of its last
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| 86 | statement. This permits us to define local variables and store each
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| 87 | argument value into one.
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| 88 | </p>
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| 89 | <span id="index-underscores-in-variables-in-macros"></span>
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| 90 | <span id="index-_005f-in-variables-in-macros"></span>
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| 91 |
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| 92 | <p>The reason for using names that start with underscores for the local
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| 93 | variables is to avoid conflicts with variable names that occur within
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| 94 | the expressions that are substituted for <code>a</code> and <code>b</code>.
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| 95 | Underscore followed by a lower case letter won’t be predefined by the
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| 96 | system in any way.
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| 97 | </p>
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| 98 |
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| 99 |
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| 100 |
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| 101 |
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| 102 | </body>
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| 103 | </html>
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