Adding method to existing class

ruby-talk
1

I need to include some methods in class stored in some variable:

some_class = Struct.new( ‘SomeClass’, ‘some_field’ )
blah = some_class.new( ‘anything’ )

Now I want object blah (and other objects of some_class) to respond
to method some_method. I’ve tried to include module with my method
but I don’t like evaling:

module Feature
def some_method
…
end
end
some_class.class_eval { include Feature }
blah = some_class.new( ‘anything’ )

Now blah responds to some_method. Is there way to include methods
without eval?

···

–
Best regards,
Eugene [team Enticla] mailto:Eugene.Scripnik@itgrp.net

2

class ExistingClass
…
end

ExistingClass.new.some_method # → NameError: undefined method
`some_method’ for #ExistingClass:.....

module Feature
def some_method
…
end
end

class ExistingClass
include Feature
end

ExistingClass.new.some_method # → ok

···

On Thu, Aug 15, 2002 at 09:20:24PM +0900, Eugene Scripnik wrote:

Now I want object blah (and other objects of some_class) to respond
to method some_method. I’ve tried to include module with my method
but I don’t like evaling:

module Feature
def some_method
…
end
end
some_class.class_eval { include Feature }
blah = some_class.new( ‘anything’ )

Now blah responds to some_method. Is there way to include methods
without eval?

–
Meisterbohne Söflinger Straße 100 Tel: +49-731-399 499-0
eLösungen 89077 Ulm Fax: +49-731-399 499-9

3

  Now blah responds to some_method. Is there way to include methods
  without eval?

You must understand that there is nothing wrong in #eval, this is just
that some times #eval is evil :slight_smile:

You have, mainly, 2 form for #eval

  * eval "some_string"

    if you *trust completely* (for example you have hardcoded it in the
    script) the string there is no problem.

    It can exist a problem when you try to use #eval with any strings (even
    strings that you don't know the content) and this is where reside the
    evil side of #eval, because eval is very powerfull and can do anything.

    In this case you can always try to protect you with $SAFE, something
    like this

    Thread.new do
       $SAFE = 4
       eval string
    end.join

  * eval { ... }

    If you use this form, you have fatally hardcoded the block and this mean
    that you can, perhaps, trust it.

#eval is powerfull, but it's sometimes too powerfull (its evil side) or
  overkill and in many cases you can just use another form, like __send__

For you example you can write

pigeon% cat b.rb
#!/usr/bin/ruby
some_class = Struct.new( 'SomeClass', 'some_field' )
blah = some_class.new( 'anything' )

module Feature
   def some_method
      puts "some_method"
   end
end

Feature.__send__(:append_features, some_class)
blah.some_method
pigeon%

pigeon% b.rb
some_method
pigeon%

You just want to use include, no need for the evil eval just send the
method #append_features to the module

Use the evil eval only when it don't exist another way to do it and when
it's secure to use this evil eval

Guy Decoux

4

s/some/most/ :slight_smile:

But if it weren’t useful for some purpose, we wouldn’t have it. In this
regard, it is not much different from ‘goto’ in C.

Paul

···

On Thu, Aug 15, 2002 at 10:05:14PM +0900, ts wrote:

Now blah responds to some_method. Is there way to include methods
without eval?

You must understand that there is nothing wrong in #eval, this is just
that some times #eval is evil :slight_smile: