eightbitraptor[wiki]

Ruby Hacking Guide - Ruby Objects

see also: [ruby-object-lifecycle]

In Object Oriented programming sense, an object is a combination of data (state) and behaviour. Preferably combined in such a way that makes sense in the context of the program in which it lives.

1. Identity. The ability to differentiate itself from other objects.
2. Methods. The ability to respond to messages sent to it by other objects.
3. Instance variables. Data local to the object where it can store it’s internal state.

In Ruby the body of an object is always expressed as a struct (one of the members of the as union in an RVALUE that we talked about in [rhg-garbage-collection] and [rvalue]).

A different struct will be used by each class but it will always be pointed to by a VALUE pointer.

Different structs used are:

• struct RObject all things for which none of the following applies
• struct RClass class object
• struct RFloat small numbers
• struct RArray array
• struct RRegexp regular expression
• struct RHash hash table
• struct RFile IO, File, Socket, etc…
• struct RData all the classes defined at C level, except the ones mentioned above
• struct RStruct Ruby’s Struct class
• struct RBignum big integers

Because VALUE is defined as unsigned long it must be cast before being used as a pointer. Macros exist for each object struct to cast value pointers to the correct pointer type.

RSTRING(obj)->len; // ((struct RString*)obj)->len

Note: I was unfamiliar with the -> syntax in C when I started looking at this. it turns out it is broadly equivalent to the . operator, ie it accesses a member or calls a function. But it treats the caller as a pointer and dereferences it first. so obj->bar is the same as (*obj).bar.

Every object struct contains an RBasic as it’s first member (named basic).

RBasic looks pretty much like this (and is defined in internal/core/rbasic.h)

struct RBasic {
  unsigned long flags;
  VALUE klass;
}

• flags are multipurpose flags