Can I 'extend' a struct in C?

Question

typedef struct foo_s {
    int a;
} foo;

typedef struct bar_s {
    foo;
    int b;
} bar;

Essentially I want to do:

bar b;
b.a;
<         


        

Answer 1

You can, using pointers, because a pointer to a structure object is guaranteed to point its first member. See e.g. this article.

#include 
#include 

typedef struct foo_s {
    int a;
} foo;

typedef struct bar_s {
    foo super;
    int b;
} bar;

int fooGetA(foo *x) {
  return x->a;
}

void fooSetA(foo *x, int a) {
  x->a = a;
}

int main() {
  bar* derived = (bar*) calloc(1, sizeof(bar));
  fooSetA((foo*) derived, 5);
  derived->b = 3;
  printf("result: %d\n", fooGetA((foo*) derived));
  return 0;
}