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

If you ment

typedef struct foo_s {
    int a;
  } foo;

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

so you can do:

bar b; b.my_foo.a = 3;

Otherwise, There's no way of doing it in C since the sizeof(bar_s) is detriment on compile time. It's not a good practice but you can save a void * ptr; pointer within bar_s, and another enum which describes the ptr type, and cast by the type.

i.e:

typedef enum internalType{
  INTERNAL_TYPE_FOO = 0,
}internalType_t;

typedef struct bar_s {
 internalType_t ptrType;
 void* ptr;
int b;
} bar;

and then:

bar b;  foo f;
b.ptrType = INTERNAL_TYPE_FOO;
b.ptr = &f;

and some where else in the code:

 if (b.ptrType == INTERNAL_TYPE_FOO) {
    foo* myFooPtr = (foo *)b.ptr;
 }