ObjectSpace.each_object(Foo).count

Question

I'am trying to figure out ObjectSpace.each_object
In console:

class Foo; end  
Foo.new  
ObjectSpace.each_object(Foo).count  
=> 1  
GC.start  
ObjectSpace.each_object(Foo).count  
=> 1 

I've seen examples and I know that the second count should be 0.
Any ideas what is going on here?
Thanks.

Answer 1

It depends on your console.

IRB

The last result is saved as _, even if it hasn't been explicitely assigned. Running GC.start won't remove the last object :

irb(main):001:0> class Foo; end
=> nil
irb(main):002:0>
irb(main):003:0* Foo.new
=> #<Foo:0x007fca7a309f98>
irb(main):004:0> p ObjectSpace.each_object(Foo).count; GC.start; p ObjectSpace.each_object(Foo).count
1
1
=> 1
irb(main):005:0> p ObjectSpace.each_object(Foo).count; GC.start; p ObjectSpace.each_object(Foo).count
1
0
=> 0

Pry

You can access the last result and the second to last result with _ and __ :

[1] pry(main)> 'a'
=> "a"
[2] pry(main)> 'b'
=> "b"
[3] pry(main)> p _, __
"b"
"a"
=> ["b", "a"]

Pry saves all the 100 last results in _out_ Pry::HistoryArray:

[1] pry(main)> class Foo; end
=> nil
[2] pry(main)> Foo.new
=> #<Foo:0x007fd093102118>
[3] pry(main)> ObjectSpace.each_object(Foo).count
=> 1
[4] pry(main)> GC.start
=> nil
[5] pry(main)> ObjectSpace.each_object(Foo).count
=> 1
[6] pry(main)> _out_[2]
=> #<Foo:0x007fd093102118>

You can use _out_.pop! to remove its last element :

[1] pry(main)> class Foo; end
=> nil
[2] pry(main)> Foo.new
=> #<Foo:0x007fa90b1ad360>
[3] pry(main)> ObjectSpace.each_object(Foo).count
=> 1
[4] pry(main)> GC.start
=> nil
[5] pry(main)> ObjectSpace.each_object(Foo).count
=> 1
[6] pry(main)> 5.times{_out_.pop!}
=> 5
[7] pry(main)> GC.start
=> nil
[8] pry(main)> ObjectSpace.each_object(Foo).count
=> 0

Inside a script

If you execute :

class Foo; end

Foo.new
p ObjectSpace.each_object(Foo).count

GC.start
p ObjectSpace.each_object(Foo).count

inside a script, you get :

1
0

Answer 2

GC.start does not force the garbage collector to start.

It is slightly unclear from the documentation, but it just instructs the engine to schedule a garbage collection. That said, one can not rely on GC.start would immediately remove objects from the heap.