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| 22 | <title>Structure Assignment (GNU C Language Manual)</title>
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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="Structures.html" rel="up" title="Structures">
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| 33 | <link href="Unions.html" rel="next" title="Unions">
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| 56 |
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| 57 | <body lang="en">
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| 58 | <span id="Structure-Assignment"></span><div class="header">
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| 59 | <p>
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| 60 | Next: <a href="Unions.html" accesskey="n" rel="next">Unions</a>, Previous: <a href="Overlaying-Structures.html" accesskey="p" rel="prev">Overlaying Structures</a>, Up: <a href="Structures.html" accesskey="u" rel="up">Structures</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="Structure-Assignment-1"></span><h3 class="section">15.12 Structure Assignment</h3>
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| 64 | <span id="index-structure-assignment"></span>
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| 65 | <span id="index-assigning-structures"></span>
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| 66 |
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| 67 | <p>Assignment operating on a structure type copies the structure. The
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| 68 | left and right operands must have the same type. Here is an example:
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| 69 | </p>
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| 70 | <div class="example">
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| 71 | <pre class="example">#include <stddef.h> /* <span class="roman">Defines <code>NULL</code>.</span> */
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| 72 | #include <stdlib.h> /* <span class="roman">Declares <code>malloc</code>.</span> */
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| 73 | <span class="roman">…</span>
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| 74 |
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| 75 | struct point { double x, y; };
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| 76 |
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| 77 | struct point *
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| 78 | copy_point (struct point point)
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| 79 | {
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| 80 | struct point *p
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| 81 | = (struct point *) malloc (sizeof (struct point));
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| 82 | if (p == NULL)
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| 83 | fatal ("Out of memory");
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| 84 | *p = point;
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| 85 | return p;
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| 86 | }
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| 87 | </pre></div>
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| 88 |
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| 89 | <p>Notionally, assignment on a structure type works by copying each of
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| 90 | the fields. Thus, if any of the fields has the <code>const</code>
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| 91 | qualifier, that structure type does not allow assignment:
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| 92 | </p>
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| 93 | <div class="example">
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| 94 | <pre class="example">struct point { const double x, y; };
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| 95 |
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| 96 | struct point a, b;
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| 97 |
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| 98 | a = b; /* <span class="roman">Error!</span> */
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| 99 | </pre></div>
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| 100 |
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| 101 | <p>See <a href="Assignment-Expressions.html">Assignment Expressions</a>.
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| 102 | </p>
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| 103 |
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| 104 |
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| 105 |
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