strobj.c

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

char*
itos(int value) {
  char* result = malloc(sizeof(char)*100);
  sprintf(result, "%d", value);
  return result;
}

typedef struct {
  char*   value;
  int     length;
} String;

String*
new_string(char* value) {
  String* result = malloc(sizeof(String));
  result->value = value;
  result->length = strlen(value);
  return result;
}

String*
set_string(String* obj, char* value) {
  obj->value = value;
  obj->length = strlen(value);
  return obj;
}

void
inspect_string(String* obj) {
  puts(obj->value);
  puts(itos(obj->length));
}

int main() {
  String* hello = new_string("hello, world!");
  inspect_string(hello);

  set_string(hello, "I am Shelling!");
  inspect_string(hello);

  free(hello);
}

treeobj.c

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

typedef struct tree Tree;

struct tree {
  char* data;
  Tree* left;
  Tree* right;
  Tree* parent;
};

Tree*
tree_new(char* value) {
  Tree* obj = malloc(sizeof(Tree));
  obj->data = value;
  return obj;
}

void
tree_left_set(Tree* base, Tree* left) {
  base->left = left;
  left->parent = base;
}

void
tree_right_set(Tree* base, Tree* right) {
  base->right = right;
  right->parent = base;
}

void
tree_traverse(Tree* obj, int (*function)(const char*)) {
  if (obj) {
    (*function)(obj->data);
    tree_traverse(obj->left, function);
    tree_traverse(obj->right, function);
  }
}

void
tree_inspect(Tree* obj) {
  tree_traverse(obj, puts);
}

int main() {
  Tree* root = tree_new("root");

  tree_left_set(root, tree_new("left"));
  tree_right_set(root, tree_new("right"));

  tree_inspect(root);
  tree_inspect(root->left->parent);
}