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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="Intertwined-Incomplete-Types.html" rel="next" title="Intertwined Incomplete Types">
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| 58 | <span id="Incomplete-Types"></span><div class="header">
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| 59 | <p>
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| 60 | Next: <a href="Intertwined-Incomplete-Types.html" accesskey="n" rel="next">Intertwined Incomplete Types</a>, Previous: <a href="Unnamed-Types-as-Fields.html" accesskey="p" rel="prev">Unnamed Types as Fields</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="Incomplete-Types-1"></span><h3 class="section">15.18 Incomplete Types</h3>
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| 64 | <span id="index-incomplete-types"></span>
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| 65 | <span id="index-types_002c-incomplete"></span>
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| 66 |
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| 67 | <p>A type that has not been fully defined is called an <em>incomplete
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| 68 | type</em>. Structure and union types are incomplete when the code makes a
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| 69 | forward reference, such as <code>struct foo</code>, before defining the
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| 70 | type. An array type is incomplete when its length is unspecified.
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| 71 | </p>
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| 72 | <p>You can’t use an incomplete type to declare a variable or field, or
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| 73 | use it for a function parameter or return type. The operators
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| 74 | <code>sizeof</code> and <code>_Alignof</code> give errors when used on an
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| 75 | incomplete type.
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| 76 | </p>
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| 77 | <p>However, you can define a pointer to an incomplete type, and declare a
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| 78 | variable or field with such a pointer type. In general, you can do
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| 79 | everything with such pointers except dereference them. For example:
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| 80 | </p>
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| 81 | <div class="example">
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| 82 | <pre class="example">extern void bar (struct mysterious_value *);
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| 83 |
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| 84 | void
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| 85 | foo (struct mysterious_value *arg)
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| 86 | {
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| 87 | bar (arg);
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| 88 | }
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| 89 |
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| 90 | <span class="roman">…</span>
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| 91 |
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| 92 | {
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| 93 | struct mysterious_value *p, **q;
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| 94 |
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| 95 | p = *q;
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| 96 | foo (p);
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| 97 | }
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| 98 | </pre></div>
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| 99 |
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| 100 | <p>These examples are valid because the code doesn’t try to understand
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| 101 | what <code>p</code> points to; it just passes the pointer around.
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| 102 | (Presumably <code>bar</code> is defined in some other file that really does
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| 103 | have a definition for <code>struct mysterious_value</code>.) However,
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| 104 | dereferencing the pointer would get an error; that requires a
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| 105 | definition for the structure type.
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| 106 | </p>
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| 107 |
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| 108 |
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| 109 |
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| 110 | </body>
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