Structures and casting in C

Question

I was wondering:

If I have structure definitions, for example, like this:

struct Base {
  int foo;
};

struct Derived {
  int foo; // int foo is comm         


        

Answer 1

The great/bad thing about C is that you can cast just about anything -- the problem is, it might not work. :) However, in your case, it will*, since you have two structs whose first members are both of the same type; see this program for an example. Now, if struct derived had a different type as its first element -- for example, char *bar -- then no, you'd get weird behavior.

* I should qualitfy that with "almost always", I suppose; there're a lot of different C compilers out there, so some may have different behavior. However, I know it'll work in GCC.

Answer 2

There is another little thing that might be helpful or related to what you are doing ..

#define SHARED_DATA int id;

typedef union base_t {
    SHARED_DATA;
    window_t win;
    list_t   list;
    button_t button;         
}

typedef struct window_t {
    SHARED_DATA;
    int something;
    void* blah;
}

typedef struct window_t {
    SHARED_DATA;
    int size;
 }

typedef struct button_t {
    SHARED_DATA;
    int clicked;
 }

Now you can put the shared properties into SHARED_DATA and handle the different types via the "superclass" packed into the union.. You could use SHARED_DATA to store just a 'class identifier' or store a pointer.. Either way it turned out handy for generic handling of event types for me at some point. Hope i'm not going too much off-topic with this