Instantiate different class based on variable contents


#1

Lets say I have a two classes, Foo and Bar.

In some code, I have a variable that can either contain the test ‘Foo’
or the text ‘Bar’. If it contains Foo, I would like to call Foo.new
(v1, v2), if it contains Bar, I would like to call Bar.new(v1, v2).

Obviously I could create an IF statement, such as

if var == ‘Foo’ then
Foo.new(…)
elsif var == ‘Bar’ then
Bar.new(…)
end if

Or if I want to get fancy, maybe a dispatch table:

dispatch = {
‘Foo’ => Foo,
‘Bar’ => Bar
}

obj = dispatch[var].new(…)

Is there another Ruby way to do this sort of thing?

Thanks,

Stephen.


#2

On Wed, Apr 8, 2009 at 2:40 PM, stephen O’D
removed_email_address@domain.invalidwrote:

dispatch = {
‘Foo’ => Foo,
‘Bar’ => Bar
}

obj = dispatch[var].new(…)

Is there another Ruby way to do this sort of thing?

Object.const_get(var).new(…)

or with ActiveSupport:

var.constantize.new(…)


#3

On Apr 8, 2009, at 4:40 PM, stephen O’D wrote:

dispatch = {
‘Foo’ => Foo,
‘Bar’ => Bar
}

obj = dispatch[var].new(…)

This can sometimes be the best approach because if you simply
translate arbitrary text to a class name you are allowing arbitrary
classes to be instantiated. It just depends on where your text is
coming from.

Rick DeNatale recently posted in [ruby-talk:332670] this nice solution
to mapping text to class objects:


#4

On Apr 8, 10:01 pm, Tony A. removed_email_address@domain.invalid wrote:

medioh.com
Thanks, works perfectly!


#5

On Apr 8, 10:13 pm, Gary W. removed_email_address@domain.invalid wrote:

translate arbitrary text to a class name you are allowing arbitrary

inject(Object) { |scope, name| scope.const_defined?(name) ?

scope.const_get(name) : scope.const_missing(name) }
end

Thanks for the tip - I think my data is well enough sanitized, but
you just never know so I may well use this approach. Knowing how to
do things with Object.const_get is useful even if I don’t use it.