Dynamic object creation in ruby

[Clash Fasn]

I'm trying to do dynamic object creation in ruby.

This works:

john = eval("Master:erson").new("John")

This fails:

frank = Object.const_get("Master:erson").new("Frank")

Does anyone know why the second way fails?

 

[David Masover]

This works:

john = eval("Master:erson").new("John")

This fails:

frank = Object.const_get("Master:erson").new("Frank")

Does anyone know why the second way fails?

Because there is no constant named Master:erson. There's a constant
named Person inside the constant named Master. This is going to make a
lot more sense by example:

Object.const_get('Master').const_get('Person').new('Frank')

For what it's worth, unless there's a reason to use strings, you may as
well use symbols there:

Object.const_getMaster).const_getPerson).new('Frank')

 

[lasitha]

I'm trying to do dynamic object creation in ruby.

This works:

john = eval("Master:erson").new("John")

This fails:

frank = Object.const_get("Master:erson").new("Frank")

Does anyone know why the second way fails?

This will work:
Object.const_get("Master"):erson.new("Frank")

Or:
Object.const_get("Master").const_get("Person").new("Frank")

Or:
"Master:erson".split('::').reduce(Object){|cls, c| cls.const_get(c) }

But your question was 'why?'... I guess the short answer is #const_get
just doesn't interpret the scope operator, '::'. Actually, thinking
of it as an operator (hence it works in eval) and not a naming
convention is perhaps helpful.

cheers,
lasitha

 

[Brian Candler]

frank = Object.const_get("Master:erson").new("Frank")

Does anyone know why the second way fails?

frank = Master.const_get("Person").new("Frank")

 

[Robert Klemme]

frank = Master.const_get("Person").new("Frank")

Erm, that's a hybrid solution. If you know the names beforehand, you
can just do

frank = Master:erson.new "Frank"

I would assume that the OP receives the full qualified name as a String
and wants to get the class from there which can only be achieved with
any of the other solutions shown.

Cheers

robert

 

[Pascal J. Bourguignon]

Erm, that's a hybrid solution. If you know the names beforehand, you
can just do

frank = Master:erson.new "Frank"

Then the answer for the OP would be:

frank = Object.const_get("Master").const_get("Person").new("Frank")

const_get takes only a simple constant name, not a qualified name.


irb(main):247:0> (module Master
(class Person
(def initialize(name)
@name=name
end)
end)
end)
nil
irb(main):254:0> frank = Object.const_get("Master").const_get("Person").new("Frank")
#<Master:erson:0x3465ac @name="Frank">

I would assume that the OP receives the full qualified name as a
String and wants to get the class from there which can only be
achieved with any of the other solutions shown.

Obviously, he wants to replace the literals by variables.

 

[Robert Klemme]

Then the answer for the OP would be:

frank = Object.const_get("Master").const_get("Person").new("Frank")

Why would anyone want to use Strings when he knows the names (and
consequently their number) beforehand? That does not make sense.

const_get takes only a simple constant name, not a qualified name.

Yes, and that's why the solutions that were provided use some form of
String#split to cut a fully qualified name down to individual constants.

Obviously, he wants to replace the literals by variables.

Apart from the plurals:

This works:

john = eval("Master:erson").new("John")

This fails:

frank = Object.const_get("Master:erson").new("Frank")

Regards

robert