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| <h3 class="section">6.59 Unnamed struct/union fields within structs/unions</h3> |
| |
| <p><a name="index-g_t_0040code_007bstruct_007d-3484"></a><a name="index-g_t_0040code_007bunion_007d-3485"></a> |
| As permitted by ISO C11 and 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 class="noindent">In this example, you are 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, in this structure: |
| |
| <pre class="smallexample"> struct { |
| int a; |
| struct { |
| int a; |
| }; |
| } foo; |
| </pre> |
| <p class="noindent">it is ambiguous which <code>a</code> is being referred to with ‘<samp><span class="samp">foo.a</span></samp>’. |
| The compiler gives errors for such constructs. |
| |
| <p><a name="index-fms_002dextensions-3486"></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. |
| |
| <p><a name="index-fplan9_002dextensions-3487"></a>The option <samp><span class="option">-fplan9-extensions</span></samp> enables |
| <samp><span class="option">-fms-extensions</span></samp> as well as two other extensions. First, a |
| pointer to a structure is automatically converted to a pointer to an |
| anonymous field for assignments and function calls. For example: |
| |
| <pre class="smallexample"> struct s1 { int a; }; |
| struct s2 { struct s1; }; |
| extern void f1 (struct s1 *); |
| void f2 (struct s2 *p) { f1 (p); } |
| </pre> |
| <p class="noindent">In the call to <code>f1</code> inside <code>f2</code>, the pointer <code>p</code> is |
| converted into a pointer to the anonymous field. |
| |
| <p>Second, when the type of an anonymous field is a <code>typedef</code> for a |
| <code>struct</code> or <code>union</code>, code may refer to the field using the |
| name of the <code>typedef</code>. |
| |
| <pre class="smallexample"> typedef struct { int a; } s1; |
| struct s2 { s1; }; |
| s1 f1 (struct s2 *p) { return p->s1; } |
| </pre> |
| <p>These usages are only permitted when they are not ambiguous. |
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