Question

I confused when i want to take single pointer and when should i take double pointer?
In following structure what exactly did?

struct objc_class {
    Class isa;
    Class super_class;
    const char *name;
    long version;
    long info;
    long instance_size;
    struct objc_ivar_list *ivars;
    struct objc_method_list **methodLists;
    struct objc_cache *cache;
    struct objc_protocol_list *protocols;
};

Why we use the methodLists double pointer?

Edited

int sqlite3_get_table(
      sqlite3 *db,         
      const char *zSql,    
      char ***pazResult,   
      int *pnRow,          
      int *pnColumn,       
      char **pzErrmsg      
    );

In above scenario what will be meaning of triple pointer char ***pazResult?

Answer 1

Well, in C at least, double-pointers are commonly used for 2D arrays. The most common 2D array is probably an array of C strings (char*‘s). Double pointers are also sometimes employed to pass pointers to functions by reference, but this is unlikely to be the use in the code sample you posted.

According to the name methodLists I would guess that this is an array of lists. A (linked) list in C is commonly represented by a pointer to a node, which objc_method_list could be. An array of such lists is then implemented with a double pointer.