I read the ‘native machine word minus 1 bit’ before, but I never really understood what that bit is used for.

So in reality there are no Fixnum instances, it’s just a big charade :) When the interpreter encounters an odd object reference it knows it really is a Fixnum with value (oid-1)/2.

irb> 0.object_id
=>1
irb> 100.object_id
=>201

Only when it overflows does it become an actual Bignum instance, where different instances with the same value have different oids.

irb> (2**5000).object_id
=> 1073082710
irb> (2**5000).object_id
=> 1073078250

I feel humbled by this learning experience.

Where ‘x’ has a reference to any other object its object ID will be even. I guess that it is the address of the object in memory. Where x ‘is’ a fixnum, its object ID is (2 * value + 1). That is – the value is stored in ‘x’ directly. Any object ID that is odd has the value stored in its upper 31 or 63 bits.

This is done for efficiency. The object stuff is just fakery.

I think I heard that this may change in a later version of Ruby.

I made my claim based on the ruby documentation : “Fixnum objects have immediate value. This means that when they are assigned or passed as parameters, the actual object is passed, rather than a reference to that object.” (http://dev.rubycentral.com/ref/ref_c_fixnum.html)

So how I understand it, is that you always work with the actual object, never with a reference to it.

This doesn’t appear to me to be entirely correct, Fixnum objects are unique and immutable, so every ‘5′ in the system is the same Fixnum instance with the same oid. However, variables can still contain references to this instance, just like every ruby variable contains just a reference to an object.

So when you write x=5, some space for x is allocated and this space contains a reference to the Fixnum instance 5, but no new Fixnum instance is created (as you first assumed in your previous post). It’s just a reference to the exisiting Fixnum instance.

Very nice blog by the way!