foo.h

typedef struct {
  Object *items;
  size_t  size;
  size_t  capacity;
} Array;

typedef struct {
  ObjectType type;
  union {
    Array      array;
    Boolean    boolean;
    Buffer     buffer;
    Dictionary dictionary;
    Float      floating;
    Integer    integer;
    String     string;
    Tabpage    tabpage;
    Window     window;
  } data;
} Object;