Dynamic object construction

[John Lam]

I'm trying to create a new object where the class of the object is
parameterized. My current solution is to use eval():

def create(class)

@obj = eval("#{class}.new()")

end

Is there a better way of doing this to avoid the hit of eval()?

Thanks,

-John

http://www.iunknown.com

[Bill Atkins]

If class is a a class name (a String):

@obj = Object.const_get(klass).new

Using "class" as a parameter name won't work - the standard is to use
"klass" instead.

Bill

--
Bill Atkins

[John Lam]

Thanks guys! const_get() works like a charm.

My fault for typing class as the parameter name - that's what I get for
typing code frags from scratch without running them first in a language
I've been using for 12 hours :)

-John
http://www.iunknown.com

[David Heinemeier Hansson]

> Thanks guys! const_get() works like a charm.

If you're working with Rails, or just have the Active Support gem
available, this pattern has been boiled down to
klass.constantize.new().
--
David Heinemeier Hansson,
http://www.basecamphq.com/ -- Web-based Project Management
http://www.rubyonrails.org/ -- Web-application framework for Ruby
http://www.loudthinking.com/ -- Broadcasting Brain

[George Moschovitis]

> @obj = Object.const_get(klass).new

does this work if klass == My::Namespace::Klass ?

-g.

[Trans]

George Moschovitis wrote:
> > @obj = Object.const_get(klass).new
>
> does this work if klass == My::Namespace::Klass ?

Assuming you mean the string value

klass = "My::Namespace::Klass"

The answer is No. One has to do:

klass = "Klass"
@obj = My::Namespace.const_get(klass).new

The namespace issue is problematic. One alternative is in Ruby Facets:

require 'facet/object/constant'

klass == "My::Namespace::Klass"

@obj = constant(klass).new

T.

[George Moschovitis]

> The answer is No. One has to do:
> klass = "Klass"
> @obj = My::Namespace.const_get(klass).new

I was expecting the negative answer ;-)

> One alternative is in Ruby Facets

Interesting, I 'll see how that works...

-g.

[Nicholas Seckar]

On 4/20/05, Trans <tran...@gmail.com> wrote:
> Assuming you mean the string value
> klass = "My::Namespace::Klass"

> The answer is No. One has to do...

Or the ever popular

klass.split('::').inject(Object) {|parent, name| parent.const_get(name)}