Nested structs and strict aliasing in c

Question

Please consider the following code:

typedef struct {
  int type;
} object_t;

typedef struct {
  object_t object;
  int age;
} person_t;

int age(object_t *o         


        

Answer 1

One acceptable way that is explicitly condoned by the standard is to make a union of structs with identical initial segment, like so:

struct tag  { int value;                };
struct obj1 { int tag;    Foo x; Bar y; };
struct obj2 { int tag;    Zoo z; Car w; };

typedef union object_
{
  struct tag;
  struct obj1;
  struct obj2;
} object_t;

Now you can pass an object_t * p and examine p->tag.value with impunity, and then access the desired union member.