class_eval

[Derek Chesterfield]

Stealing someone else's recent example code, what is the difference
between this:

class A
def meth2
puts "I'm meth2. I try to access something I ain't
allowed: #{static_var}"
end
static_var = 3
define_methodmeth3) do
puts "I'm meth3. I'm allowed to access #{static_var}"
end
end
a = A.new
a.meth3
a.meth2

and this, which seems to do the same thing, but without the class_eval:

class B
def meth2
puts "I'm meth2. I try to access something I ain't
allowed: #{static_var}"
end
class_eval do
static_var = 3
define_methodmeth3) do
puts "I'm meth3. I'm allowed to access #{static_var}"
end
end
end
b = B.new
b.meth3
b.meth2

In other words, what does class_eval do that I'm not understanding?!

 

[Robert Klemme]

Stealing someone else's recent example code, what is the difference
between this:


and this, which seems to do the same thing, but without the
class_eval:


In other words, what does class_eval do that I'm not understanding?!

In this case it's not really interesting to use class_eval. It's more
interesting for cases like these
10
=> nil

And note the difference to instance_eval:
=> "ban">> Foo.instance_eval "def borg() 'org' end"
=> nilNoMethodError: undefined method `borg' for #<Foo:0x101b4e50>
from (irb):23
from :0=> "org"

HTH

Kind regards

robert