Dynamic array with nested structs

Question

I am trying to build dynamic array with nested structs. When insert an item I am getting the following. error: incompatible types when assigning to type 'struct B' from type 'struct B *'

What is the issue and where i am doing the mistake. Please help.

typedef struct {
   size_t used;
   size_t size;

   struct B {
      int *record1;
      int *record2; 
   } *b;

} A;


void insertArray(A *a, int element) {
  struct B* b =  (struct B*)malloc(sizeof(struct B));
  A->b[element] =  b;
}

Answer 1

The problem is that Ab is not an array of struct s, it is a pointer . You can make a pointer point to an array of struct s, but it does not do so by default.

The simplest approach is to malloc the correct number of struct B s into Ab at the beginning, and then put copies of the struct B s right into that array.

void initWithSize(struct A *a, size_t size) {
    a->size = size;
    a->b = malloc(sizeof(struct B) * size);
}

void insertArray(A *a, int index, struct B element) {
    assert(index < a->size); // Otherwise it's an error
    // There's no need to malloc b, because the required memory
    // has already been put in place by the malloc of initWithSize
    a->b[index] = element;
}

A slightly more complex approach would be using flexible array members of C99, but then the task of organizing struct A s into an array of their own would be a lot more complex.

Answer 2

void insertArray(A *a, int element) {
        struct B b ;
        b.record1 = 100;
        b.record2 = 200;
        A->b[element] =  b;
}