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| 21 | <meta http-equiv="Content-Type" content="text/html; charset=utf-8">
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| 22 | <title>Auto Type (GNU C Language Manual)</title>
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| 23 |
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| 24 | <meta name="description" content="Auto Type (GNU C Language Manual)">
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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="Variables.html" rel="up" title="Variables">
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| 33 | <link href="Local-Variables.html" rel="next" title="Local Variables">
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| 34 | <link href="Designated-Inits.html" rel="prev" title="Designated Inits">
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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="Auto-Type"></span><div class="header">
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| 59 | <p>
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| 60 | Next: <a href="Local-Variables.html" accesskey="n" rel="next">Local Variables</a>, Previous: <a href="Designated-Inits.html" accesskey="p" rel="prev">Designated Inits</a>, Up: <a href="Variables.html" accesskey="u" rel="up">Variables</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="Referring-to-a-Type-with-_005f_005fauto_005ftype"></span><h3 class="section">20.4 Referring to a Type with <code>__auto_type</code></h3>
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| 64 | <span id="index-_005f_005fauto_005ftype"></span>
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| 65 | <span id="index-typeof"></span>
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| 66 | <span id="index-macros_002c-types-of-arguments"></span>
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| 67 |
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| 68 | <p>You can declare a variable copying the type from
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| 69 | the initializer by using <code>__auto_type</code> instead of a particular type.
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| 70 | Here’s an example:
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| 71 | </p>
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| 72 | <div class="example">
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| 73 | <pre class="example">#define max(a,b) \
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| 74 | ({ __auto_type _a = (a); \
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| 75 | __auto_type _b = (b); \
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| 76 | _a > _b ? _a : _b })
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| 77 | </pre></div>
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| 78 |
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| 79 | <p>This defines <code>_a</code> to be of the same type as <code>a</code>, and
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| 80 | <code>_b</code> to be of the same type as <code>b</code>. This is a useful thing
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| 81 | to do in a macro that ought to be able to handle any type of data
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| 82 | (see <a href="Macros-and-Auto-Type.html">Macros and Auto Type</a>).
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| 83 | </p>
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| 84 | <p>The original GNU C method for obtaining the type of a value is to use
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| 85 | <code>typeof</code>, which takes as an argument either a value or the name of
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| 86 | a type. The previous example could also be written as:
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| 87 | </p>
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| 88 | <div class="example">
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| 89 | <pre class="example">#define max(a,b) \
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| 90 | ({ typeof(a) _a = (a); \
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| 91 | typeof(b) _b = (b); \
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| 92 | _a > _b ? _a : _b })
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| 93 | </pre></div>
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| 94 |
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| 95 | <p><code>typeof</code> is more flexible than <code>__auto_type</code>; however, the
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| 96 | principal use case for <code>typeof</code> is in variable declarations with
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| 97 | initialization, which is exactly what <code>__auto_type</code> handles.
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| 98 | </p>
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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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