Do Ruby variables store value or address?

Question

Since in Ruby everything is an object, do Ruby variables store value or address of immediate types (read primitives)? In contrast to C that stores values in variables, if they are primitives.

Answer 1

NB, all of the following is for the default Ruby, which internally uses YARV aka "Yet another Ruby VM," other rubies like JRuby may use different internal representations…

Good question.

Ruby uses tag pointers for integers, and everything else is stored as a reference to an object.

How do they work? One bit in the pointer is used as tag, if that bit is set the rest of the pointer is interpreted as an integer and otherwise as an address.

This works because some bits in a pointer are not used. The bottom most bits of a memory address are usually not used. Most systems only allow address of aligned memory addresses, for example aligned to 4 bytes and so 2 bit become available to be used as tag. And then if that tag is set, the other 31 bit of the pointer are interpreted as integer.

You can see this when you look at the object_id of integers

20.to_s(2) # => "10100"
20.object_id.to_s(2) # => "101001"

On some systems two tag bits are used and then floating-point numbers are represented using the other tag bit. And there are some special objects like nil, true, false that are represented with reserved numbers that are unlikely to be valid memory addresses. Symbols are also represented as tagged integers internally but with a different bitmask than actual integers.

All other values are represented as pointers.

Fun fact, you can inspect all of that yourself using ObjectSpace class.

(0..100).each { |n| p([n, ObjectSpace._id2ref(n)]) rescue nil }

On my system this prints

[0, false]
[1, 0]
[2, 2.0]
[3, 1]
[5, 2]
[6, -2.0]
[7, 3]
[8, nil]
[9, 4]
[10, 2.0000000000000004]
[11, 5]
[13, 6]
[14, -2.0000000000000004]
[15, 7]
[17, 8]
[18, 2.000000000000001]
[19, 9]
[20, true]
[21, 10]
[22, -2.000000000000001]
[23, 11]
...

Answer 2

tl;dr: it doesn't matter, you can't tell, and since you can't tell, the Ruby Language Specification doesn't say anything about it, which allows the different implementors to make different choices, and they do in fact make different choices.

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It doesn't matter.

The only way you could tell the difference is by modifying the object, but since all immediate objects are immutable, there is no way for you to tell one way or the other.

As it turns out, different Ruby implementations treat them differently, and there is nothing wrong with that. For example, YARV stores Integers as tagged pointers (called fixnums) or object references (called bignums) depending on size, but again, the very same number may be stored as either, because on 64 bit systems, it uses 63 bits for fixnums, and on 32 bit systems, it only uses 31 bit. JRuby OTOH doesn't use tagged pointers (it doesn't use pointers at all, since Java simply doesn't have them), and uses full 64 bits for fixnums regardless of machine word size, instead of YARV which uses 31 or 63 bits.

Likewise, YARV on 64 bit systems uses a 62 bit tagged pointer format for Floats which fit in 62 bits (which they call flonums), but on 32 bit systems and for larger Floats, it uses a different encoding.

Answer 3

Every object in Ruby has an address. You can find it by doing this obj.object_id << 1. Here is were everything gets interesting though

a = 'Hello'
puts a.object_id << 1   # 140608279312760
a += 'World'
puts a.object_id << 1   # 140608279205240
b = 'HelloWorld'
puts b.object_id << 1   # 140608271586720
c = a
puts c.object_id << 1   # 140608279205240

This shows that objects are stored by address in ruby. Additionally, addresses are unique, unless using =. However, note that in our previous example, 140608279205240 only refers to that specific 'HelloWorld'. Any changes made to either a or c will not affect the other, but will change the address of the variable with the change made to it.