Code snippets:tags typedefs

From C

 /**
  * Tags and typedefs
  */
 
 #define ILLEGAL 0
 
 
 /*
  * Declare a structure type, with 'tag1' in the
  * tag namespace
  */
 struct tag1;
 
 
 #if ILLEGAL
 /*
  * Cannot use the tag again with a union type,
  * because 'tag1' is already in the tag namespace
  * as a structure tag
  */
 union tag1;
 
 #endif
 
 
 /*
  * Redeclare the same structure type as before
  * and complete the definition of the type
  */
 struct tag1 {
     int i;
     double d;
   };
 
 
 /*
  * Declare a structure type, a tag, and an object.
  * (Note that 'obj1' is a pointer, not a 'struct tag2')
  */
 struct tag2 * obj1;
 
 
 #if ILLEGAL
 /*
  * Cannot redeclare the tag, for the same reason
  * as before
  */
 union tag2 * obj2;
 
 #endif
 
 
 /* Redeclare the tag2 structure type and complete it */
 struct tag2 {
     int j;
     double e;
   };
 
 
 /*
  * Declare a union type, its tag, complete the type,
  * declare an object of that type
  */
 union tag3 {
     short k;
     float f;
   } obj3;
 
 
 /*
  * The tag namespace and the ordinary namespace
  * are different, so there's no conflict, below.
  * There is an 'id4' union type tag, and an 'id4'
  * object
  */
 union id4 {
     long l;
     float g;
   } id4;
 
 
 #if ILLEGAL
 /*
  * But 'int' isn't a tag, so it's in the ordinary
  * namespace and this is a conflict
  */
 int int;
 
 #endif
 
 
 /* Declare a new structure type, with a tag */
 struct tag5;
 
 
 /*
  * Redeclare that structure type and define an
  * alias for that type
  */
 typedef struct tag5 alias5;
 
 
 /* These two objects have the same pointer type */
 struct tag5 * obj5a;
 alias5 * obj5b;
 
 
 /* Another structure type, completed */
 struct tag6 {
     int m;
     double h;
   };
 
 
 /* Another typedef */
 typedef struct tag6 alias6;
 
 
 /* These objects have the same type */
 struct tag6 obj6a;
 alias6 obj6b;
 
 
 /* Declare a structure type and a type-name alias */
 typedef struct tag7 alias7;
 
 
 /* These two have the same pointer type */
 struct tag7 * obj7a;
 alias7 * obj7b;
 
 
 /* Complete the structure type */
 struct tag7 {
     int n;
     double d;
   };
 
 
 /* Now we can declare objects of that complete type */
 struct tag7 obj8a;
 alias7 obj8b;
 
 
 /*
  * Type-name aliases are not in the tag namespace, so
  * there is no conflict, below
  */
 struct type9;
 typedef struct type9 type9;
 
 
 /* Complete the type */
 struct type9 {
     int o;
     double e;
   };
 
 
 /* These have the same type */
 struct type9 obj9a;
 type9 obj9b;
 
 
 /*
  * Declare a structure type with its tag, complete it,
  * establish a type-name alias
  */
 typedef struct type10 {
     int p;
     float f;
   } type10;
 
 
 /* These have the same type */
 struct type10 obj10a;
 type10 obj10b;
 
 
 /*
  * Declare a tagless structure type and an object
  * of that type
  */
 struct {
     int q;
     float g;
   } obj11;
 
 
 /*
  * Declare a tagless structure type and establish a
  * type-name alias for that type
  */
 typedef struct {
     int r;
     short h;
   } type12;
 
 
 int main(void) { return 0; }