Automatic instantiation of subclasses

Ruby
1

Hi!

First sorry I am a beginner ;-( What I want to do is: automatically
instantiate subclasses of a given class, say, A.

class A
end

class A1 < A
end

class A2 <A
end

So I want to know the names of A subclasses: A1 and A2. Then instantiate
via reflection. Is it possible?

Using ObjectSpace it is possible to know all the instantiated objects of
a class. But here no objects yet.

Thanks a lot.

2

On Tue, Aug 08, 2006 at 06:19:12PM +0900, Kilivor K. wrote:

end

So I want to know the names of A subclasses: A1 and A2. Then instantiate
via reflection. Is it possible?

class A
class << self; attr_reader :subclasses end
def self.inherited(x); (@subclasses ||= []) << x end
end

class A1 < A; end
class A2 < A; end

A.subclasses # => [A1, A2]
A.subclasses.map{|x| x.new} # => [#A1:0xa7d73064, #A2:0xa7d73050]

class A1b < A1; end
A1.subclasses # => [A1b]

You can use @@subclasses if you want A.subclasses to include A1b:

class A
@@subclasses = []
def self.subclasses; @@subclasses end
def self.inherited(x); @@subclasses << x end
end

class A1 < A; end
class A2 < A; end

A.subclasses # => [A1, A2]
A.subclasses.map{|x| x.new} # => [#A1:0xa7d3c03c, #A2:0xa7d3c028]

class A1b < A1; end
A.subclasses # => [A1, A2, A1b]

3

On Tue, 08 Aug 2006 18:19:12 +0900, Kilivor K. [email protected]
wrote:

end

So I want to know the names of A subclasses: A1 and A2. Then instantiate
via reflection. Is it possible?

Using ObjectSpace it is possible to know all the instantiated objects of
a class. But here no objects yet.

This is a fun pattern that I use a lot.
Here is a simple code snippet using ObjectSpace:

subclasses = []
ObjectSpace.each_object(Class) do |klass|
if klass != A && klass.ancestors.include?(A)
subclasses << klass
end
end

Then you can create new instances by calling new on the class

subclases.each do |klass|
klass.new
end

Cheers,
Zev

4

Mauricio F. wrote:

[…]
You can use @@subclasses if you want A.subclasses to include A1b:
[…]

Thanks / gracias!!!

5

Kilivor K. wrote:

class A2 <A
end

So I want to know the names of A subclasses: A1 and A2. Then instantiate
via reflection. Is it possible?

class Class
def inherited(subclass)
subclasses.push(subclass)
end

 def subclasses
   @subclasses ||= []
 end

end

class A; end
class B < A; end
class C < A; end

A.subclasses => [B, C]
instances = A.subclasses.map{|subclass| subclass.new }

Cheers,
Daniel

6

Daniel S. wrote:

class Class
def inherited(subclass)
subclasses.push(subclass)
end

def subclasses
  @subclasses ||= []
end

end

Now that I think about it, this may be better:

class Class
def inherited(subclass)
(@subclasses ||= []).push(subclass)
end

 def subclasses
   (@subclasses ||= []).dup
 end

end

That way you can somewhat avoid the problems given by

A.subclasses.push(UnrelatedClass)

Cheers,
Daniel

7

On Tue, 08 Aug 2006 18:32:24 +0900, Mauricio F. [email protected]
wrote:

On Tue, Aug 08, 2006 at 06:19:12PM +0900, Kilivor K. wrote:

So I want to know the names of A subclasses: A1 and A2. Then instantiate
via reflection. Is it possible?

class A
class << self; attr_reader :subclasses end
def self.inherited(x); (@subclasses ||= []) << x end
end

I am sure Mauricio knows this, but just in case others don’t. If you
use this particular solution a lot you might also want to call super,
because your class might inherit from another class that also wants to
use the inherited method. A simple change to fix this is:

def self.inherited(x); (@subclasses ||= []) << x; super; end

Cheers,
Zev

8

Sorry for answering late, I’ve been away for a week.

Kilivor K. schrieb:

end

So I want to know the names of A subclasses: A1 and A2. Then instantiate
via reflection. Is it possible?

Using ObjectSpace it is possible to know all the instantiated objects of
a class. But here no objects yet.

In Ruby, classes are first-class objects :wink:

ObjectSpace.each_object( class << A; self; end ) do |c|
p c
end

Regards,
Pit