Casting one struct pointer to another - C

Question

Please consider the following code.

enum type {CONS, ATOM, FUNC, LAMBDA};

typedef struct{
  enum type type;
} object;

typedef struct {
  enum type type;
           


        

Answer 1

To add to Dean's answer, here's something about pointer conversions in general. I forgot what the term is for this, but a pointer to pointer cast performs no conversion (in the same way int to float is). It is simply a reinterpretation of the bits that they point to (all for the compiler's benefit). "Non-destructive conversion" I think it was. The data doesn't change, only how the compiler interprets what is being pointed at.

e.g.,
If ptr is a pointer to an object, the compiler knows that there is a field with a particular offset named type of type enum type. On the other hand if ptr is cast to a pointer to a different type, cons_object, again it will know how to access fields of the cons_object each with their own offsets in a similar fashion.

To illustrate imagine the memory layout for a cons_object:

                    +---+---+---+---+
cons_object *ptr -> | t | y | p | e | enum type
                    +---+---+---+---+
                    | c | a | r |   | object *
                    +---+---+---+---+
                    | c | d | r |   | object *
                    +---+---+---+---+

The type field has offset 0, car is 4, cdr is 8. To access the car field, all the compiler needs to do is add 4 to the pointer to the structure.

If the pointer was cast to a pointer to an object:

                    +---+---+---+---+
((object *)ptr)  -> | t | y | p | e | enum type
                    +---+---+---+---+
                    | c | a | r |   |
                    +---+---+---+---+
                    | c | d | r |   |
                    +---+---+---+---+

All the compiler needs to know is that there is a field called type with offset 0. Whatever is in memory is in memory.

The pointers don't even have to be related whatsoever. You can have a pointer to an int and cast it to a pointer to an cons_object. If you were to access the car field, it's just like any ordinary memory access. It has a certain offset from the beginning of the structure. In this case, what is in that memory location is unknown but that's unimportant. To access a field, only the offset is needed and that information is found in the type's definition.

A pointer to an int points to a block of memory:

                        +---+---+---+---+
int             *ptr -> | i | n | t |   | int
                        +---+---+---+---+

Casted to a cons_object pointer:

                        +---+---+---+---+
((cons_object *)ptr) -> | i | n | t |   | enum type
                        +---+---+---+---+
                        | X | X | X | X | object *
                        +---+---+---+---+
                        | X | X | X | X | object *
                        +---+---+---+---+