Methods search

With classes structured like this, you can easily imagine the method call process. The m_tbl of the object’s class is searched, and if the method was not found, the m_tbl of super is searched, and so on. If there is no more super, that is to say the method was not found even in Object, then it must not be defined.

The sequential search process in m_tbl is done by search_method().

struct st_table_entry {
    unsigned int hash;
    char *key;
    char *record;
    st_table_entry *next;
};

st.c

struct st_hash_type {
    int (*compare)(); /*compare为函数指针*/
    int (*hash)();  /*hash 为函数指针*/
};

struct st_table {
    struct st_hash_type *type;
    int num_bins;
    int num_entries;
    struct st_table_entry **bins;
};

#define EQUAL(table,x,y) ((x)==(y) || (*table->type->compare)((x),(y)) == 0)
#define PTR_NOT_EQUAL(table, ptr, hash_val, key) \
((ptr) != 0 && (ptr->hash != (hash_val) || !EQUAL((table), (key), (ptr)->key)))

#ifdef HASH_LOG
#define COLLISION collision++
#else
#define COLLISION
#endif

#define FIND_ENTRY(table, ptr, hash_val, bin_pos) do {\
    bin_pos = hash_val%(table)->num_bins;\
    ptr = (table)->bins[bin_pos];\  /*不懂*/
    if (PTR_NOT_EQUAL(table, ptr, hash_val, key)) {\
	COLLISION;\
	while (PTR_NOT_EQUAL(table, ptr->next, hash_val, key)) {\
	    ptr = ptr->next;\
	}\
	ptr = ptr->next;\
    }\
} while (0)

int
st_lookup(table, key, value)
    st_table *table;
    register char *key;
    char **value;
{
    unsigned int hash_val, bin_pos;
    register st_table_entry *ptr;

    hash_val = do_hash(key, table); /*st.c line68   #define do_hash(key,table) (unsigned int)(*(table)->type->hash)((key))   不知hash在何处被赋值 */
    FIND_ENTRY(table, ptr, hash_val, bin_pos);

    if (ptr == 0) {
	return 0;
    }
    else {
	if (value != 0)  *value = ptr->record;
	return 1;
    }

}

▼

search_method()

 256  static NODE*
 257  search_method(klass, id, origin)
 258      VALUE klass, *origin;
 259      ID id;
 260  {
 261      NODE *body;
 262
 263      if (!klass) return 0;
 264      while (!st_lookup(RCLASS(klass)->m_tbl, id, &body)) {
 265          klass = RCLASS(klass)->super;
 266          if (!klass) return 0;
 267      }
 268
 269      if (origin) *origin = klass;
 270      return body;
 271  }

(eval.c)

This function searches the method named id in the class object klass.

RCLASS(value) is the macro doing:

((struct RClass*)(value))

st_lookup() is a function that searches in st_table the value corresponding to a key. If the value is found, the function returns true and puts the found value at the address given in third parameter (&body).

Nevertheless, doing this search each time whatever the circumstances would be too slow. That’s why in reality, once called, a method is cached. So starting from the second time it will be found without following super one by one. This cache and its search will be seen in the 15th chapter “Methods”.