# Class in instance\_eval -- what the namespace?

**URL:** <https://www.ruby-forum.com/t/class-in-instance-eval-what-the-namespace/187909>\
**Category:** Ruby\
**Created:** [May 1, 2010, 9:21pm UTC](https://www.ruby-forum.com/t/class-in-instance-eval-what-the-namespace/187909 "2010-05-01T21:21:12Z")\
**Posts on this page:** 1\
**Page:** 1

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**Author:** ![simon780](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/s/82dd89/32.png) [@simon780](https://www.ruby-forum.com/u/simon780)\
**Post date:** [May 1, 2010, 9:21pm UTC](https://www.ruby-forum.com/t/class-in-instance-eval-what-the-namespace/187909/1 "2010-05-01T21:21:12Z")

</div>

I’m confused as to how the parent constant is determined when you  
define a class within an instance\_eval. It appears that sometimes  
ruby decides to use a nameless, perhaps temporary class as the parent  
constant.

Interestingly enough, the behavior is different when you use a block  
vs a string, and ruby implementation to implementation. For example  
consider this script:

[script.rb]  
obj = Object.new

obj.instance\_eval do  
class A; end  
puts A  
end  
puts A rescue puts $!.message

obj.instance\_eval %q{  
class B; end  
puts B  
}  
puts B rescue puts $!.message

Running this across several implementations produces a variety of  
results:

% rvm ruby script.rb  
jruby-1.4.0: jruby 1.4.0 (ruby 1.8.7 patchlevel 174) (2009-11-02  
69fbfa3) (Java HotSpot™ 64-Bit Server VM 1.6.0\_17) [x86\_64-java]  
A  
A  
#Class:01x3a98350a::B  
uninitialized constant B  
rbx-1.0.0-rc4: rubinius 1.0.0-rc4 (1.8.7 release 2010-03-31 JI)  
[x86\_64-apple-darwin10.3.0]  
A  
A  
B  
B  
ruby-1.8.7-p249: ruby 1.8.7 (2010-01-10 patchlevel 249) [i686- darwin10.2.0]  
A  
A  
#Class:0x10019c3b8::B  
uninitialized constant B  
ruby-1.9.1-p378: ruby 1.9.1p378 (2010-01-10 revision 26273) [i386- darwin10.2.0]  
#Class:0x000001008481e0::A  
uninitialized constant A  
#Class:0x000001008481e0::B  
uninitialized constant B

Any thoughts? And specifically, does anyone know of a way to set the  
default namespace for the class keyword? Specifying the full constant  
name like ‘class ::A’ works to assign the constants and uniformly  
produce a rubinius-like output but I was wondering if there is another  
way.