(gcc.info.gz) Unnamed Fields

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 5.53 Unnamed struct/union fields within structs/unions
 ======================================================
 
 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:
 
      struct {
        int a;
        union {
          int b;
          float c;
        };
        int d;
      } foo;
 
  In this example, the user would be able to access members of the
 unnamed union with code like `foo.b'.  Note that only unnamed structs
 and unions are allowed, you may not have, for example, an unnamed `int'.
 
  You must never create such structures that cause ambiguous field
 definitions.  For example, this structure:
 
      struct {
        int a;
        struct {
          int a;
        };
      } foo;
 
  It is ambiguous which `a' is being referred to with `foo.a'.  Such
 constructs are not supported and must be avoided.  In the future, such
 constructs may be detected and treated as compilation errors.
 
  Unless `-fms-extensions' is used, the unnamed field must be a
 structure or union definition without a tag (for example, `struct { int
 a; };').  If `-fms-extensions' is used, the field may also be a
 definition with a tag such as `struct foo { int a; };', a reference to
 a previously defined structure or union such as `struct foo;', or a
 reference to a `typedef' name for a previously defined structure or
 union type.
 

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