Object address in Ruby

Question

Short version: The default inspect method for a class displays the object's address.* How can I do this in a custom inspect method of my own?

*(To be clear, I want the 8-digit hex number you would normally get from inspect. I don't care about the actual memory address. I'm just calling it a memory address because it looks like one. I know Ruby is memory-safe.)

Long version: I have two classes, Thing and ThingList. ThingList is a subclass of Array specifically designed to hold Things. Due to the nature of Things and the way they are used in my program, Things have an instance variable @container that points back to the ThingList that holds the Thing.

It is possible for two Things to have exactly the same data. Therefore, when I'm debugging the application, the only way I can reliably differentiate between two Things is to use inspect, which displays their address. When I inspect a Thing, however, I get pages upon pages of output because inspect will recursively inspect @container, causing every Thing in the list to be inspected as well!

All I need is the first part of that output. How can I write a custom inspect method on Thing that will just display this?

#<Thing:0xb7727704>

EDIT: I just realized that the default to_s does exactly this. I didn't notice this earlier because I have a custom to_s that provides human-readable details about the object.

Assume that I cannot use to_s, and that I must write a custom inspect.

Answer 1

You can get the address using object_id and multiplying it by 2^* and display it in hex using sprintf (aka %):

"#<Thing:0x%08x>" % (object_id * 2)

Of course, as long as you only need the number to be unique and don't care that it's the actual address, you can just leave out the * 2.

^* For reasons that you don't need to understand (meaning: I don't understand them), object_id returns half the object's memory address, so you need to multiply by 2 to get the actual address.

Answer 2

This is impossible. There is no way in Ruby to get the memory address of an object, since Ruby is a memory-safe language which has (by design) no methods for accessing memory directly. In fact, in many implementations of Ruby, objects don't even have a memory address. And in most of the implementations that do map objects directly to memory, the memory address potentially changes after every garbage collection.

The reason why using the memory address as an identifier in current versions of MRI and YARV accidentally works, is because they have a crappy garbage collector implementation that never defragments memory. All other implementations have garbage collectors which do defragment memory, and thus move objects around in memory, thereby changing their address.

If you tie your implementation to the memory address, your code will only ever work on slow implementations with crappy garbage collectors. And it isn't even guaranteed that MRI and YARV will always have crappy garbage collectors, in fact, in both implementations the garbage collector has been identified as one of the major performance bottlenecks and it is safe to assume that there will be changes to the garbage collectors. There are already some major changes to YARV's garbage collector in the SVN, which will be part of YARV 1.9.3 and YARV 2.0.

If you want an ID for objects, use Object#object_id.