Forum: Ruby const_defined? behavior

Is this the intended behavior of const_defined?

>> IO::SYNC
=> 4096
>> IO.const_defined? :SYNC
=> false

It appears that const_defined? only returns true if the class is the
originator of the constant. If the constant was borrowed from
somewhere else (in this case, File::Constants), it returns false.

That's on purpose, right? If that's the case, I must resort to
IO.constants.member?('SYNC') for my intended behavior, I suppose.
Seems a little strange.

Dan Amelang

Hi

At Mon, 16 Jan 2006 17:05:29 +0900,
Daniel Amelang wrote in [ruby-talk:175899]:
> Is this the intended behavior of const_defined?
> 
> >> IO::SYNC
> => 4096
> >> IO.const_defined? :SYNC
> => false
> 
> It appears that const_defined? only returns true if the class is the
> originator of the constant. If the constant was borrowed from
> somewhere else (in this case, File::Constants), it returns false.

Yes.

> That's on purpose, right? If that's the case, I must resort to
> IO.constants.member?('SYNC') for my intended behavior, I suppose.
> Seems a little strange.

defined?(IO::SYNC)

Hi Nobu,

> defined?(IO::SYNC)

In my case, the constant's name is stored in a variable. So, I'd have
to resort to doing an 'eval' if I were to follow your suggestion. So
at least I have two options now:

# Option 1 (Nobu's suggestion)
found = eval("defined?(obj::#{const})")

# Option 2 (My original idea)
found = obj.respond_to?(:constants) && obj.constants.member?(const)

Any better ideas? I usually try to stay clear of 'eval' if I can.

Dan Amelang

On Jan 16, 2006, at 5:52 PM, Daniel Amelang wrote:

>
> # Option 2 (My original idea)
> found = obj.respond_to?(:constants) && obj.constants.member?(const)
>
> Any better ideas? I usually try to stay clear of 'eval' if I can.
>
> Dan Amelang
>

def const_defed(var)
       var.split(/::/).inject(Object) do |left, right|
                begin
                   left.const_get(right)
                rescue NameError
                     break nil
                end
       end
end

Pretty cool!

Not what I was looking for, but may prove useful later.

Dan