### 5.54 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.