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| <h3 class="section">6.55 Unnamed struct/union fields within structs/unions</h3> |
| |
| <p><a name="index-struct-3150"></a><a name="index-union-3151"></a> |
| For compatibility with other compilers, GCC allows you to define |
| a structure or union that contains, as fields, structures and unions |
| without names. For example: |
| |
| <pre class="smallexample"> struct { |
| int a; |
| union { |
| int b; |
| float c; |
| }; |
| int d; |
| } foo; |
| </pre> |
| <p>In this example, the user would be able to access members of the unnamed |
| union with code like ‘<samp><span class="samp">foo.b</span></samp>’. Note that only unnamed structs and |
| unions are allowed, you may not have, for example, an unnamed |
| <code>int</code>. |
| |
| <p>You must never create such structures that cause ambiguous field definitions. |
| For example, this structure: |
| |
| <pre class="smallexample"> struct { |
| int a; |
| struct { |
| int a; |
| }; |
| } foo; |
| </pre> |
| <p>It is ambiguous which <code>a</code> is being referred to with ‘<samp><span class="samp">foo.a</span></samp>’. |
| Such constructs are not supported and must be avoided. In the future, |
| such constructs may be detected and treated as compilation errors. |
| |
| <p><a name="index-fms_002dextensions-3152"></a>Unless <samp><span class="option">-fms-extensions</span></samp> is used, the unnamed field must be a |
| structure or union definition without a tag (for example, ‘<samp><span class="samp">struct |
| { int a; };</span></samp>’). If <samp><span class="option">-fms-extensions</span></samp> is used, the field may |
| also be a definition with a tag such as ‘<samp><span class="samp">struct foo { int a; |
| };</span></samp>’, a reference to a previously defined structure or union such as |
| ‘<samp><span class="samp">struct foo;</span></samp>’, or a reference to a <code>typedef</code> name for a |
| previously defined structure or union type. |
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