Dynamically Allocating a Struct within a Struct

Question

I am dynamically allocating a struct which has a different struct as a member:

struct a {
   // other members
   struct b;
}

struct b

Answer 1

First, let's get some real definitions in place to make this concrete.

struct b {
    int x;
};

struct a_with_b {
    struct b b;
}

struct a_with_b_ptr {
    struct b *bp;
}

When you encapsulate a struct, you just need to allocate the outer struct (and since the inner struct is not a pointer, you use . to reference members of the innert struct):

struct a_with_b *a1 = malloc(sizeof(struct a_with_b));
a1->b.x = 3;

But when you encapsulate a pointer, you have to allocate each independently and use -> when referencing members of the inner struct:

struct a_with_b_ptr *a2 = malloc(sizeof(struct a_with_b_ptr));
a1->b = malloc(sizeof(struct b));
a1->b->x = 3;