subclassing Structs

subclassing Structs

[Eugene Scripnik]

I'm trying to create class which behaves as struct (almost) and has some 
other useful methods. But subclassing Struct seems to work differently 
from other builtin classes. Here is the sample:

class SubStruct < Struct
     def initialize
         super( nil, :name, :surname )
         puts 'in initialize'
     end
end

class SubArray < Array
     def initialize
         super( 10 ) { 'default value' }
     end
end

a = SubArray.new
p a.size
p a[5]

s = SubStruct.new

SubArray works as I expected but SubStruct fails in new method. Is this 
a bug or I should reorganize my code?

-- 
Eugene Scripnik
IT Group
Software Architect
Tel./Fax +380 (372) 58-43-10
email: Eugene.Scripnik@itgrp.net
http://www.itgrp.net/

Re: subclassing Structs

[Yukihiro Matsumoto]

Hi,

In message "subclassing Structs"
    on 03/08/14, Eugene Scripnik <Eugene.Scripnik@itgrp.net> writes:

|I'm trying to create class which behaves as struct (almost) and has some 
|other useful methods. But subclassing Struct seems to work differently 
|from other builtin classes. Here is the sample:

That's because Struct class is a abstract factory for each Struct
(e.g. Struct.new does not return instance of Struct, but subclass of
it).  I'm not sure what you want to achieve by subclassing, though.

							matz.

Re: subclassing Structs

[Eugene Scripnik]

Yukihiro Matsumoto wrote:

> |I'm trying to create class which behaves as struct (almost) and has some 
> |other useful methods. But subclassing Struct seems to work differently 
> |from other builtin classes. Here is the sample:
> 
> That's because Struct class is a abstract factory for each Struct
> (e.g. Struct.new does not return instance of Struct, but subclass of
> it).  I'm not sure what you want to achieve by subclassing, though.
My example didn't show that but I wanted to create struct-class with 
dynamic attributes and additional constructors.

class SubStruct < Struct
   def new2
     new( 'attr1_value', 'attr2_value' )
   end
end

new2 - instance method of SubStruct (Struct) and I thought it has to be 
class method of SubStruct.new( ... ) class.

-- 
Eugene Scripnik
IT Group
Software Architect
Tel./Fax +380 (372) 58-43-10
email: Eugene.Scripnik@itgrp.net
http://www.itgrp.net/

Re: subclassing Structs

[Yukihiro Matsumoto]

Hi,

In message "Re: subclassing Structs"
    on 03/08/14, Eugene Scripnik <Eugene.Scripnik@itgrp.net> writes:

|My example didn't show that but I wanted to create struct-class with 
|dynamic attributes and additional constructors.
|
|class SubStruct < Struct
|   def new2
|     new( 'attr1_value', 'attr2_value' )
|   end
|end
|
|new2 - instance method of SubStruct (Struct) and I thought it has to be 
|class method of SubStruct.new( ... ) class.

Unlike other classes, Struct.new returns its subclass, not instance.
This is very exceptional behavior.

new2 is an instance method of SubStruct.

  class SubStruct < Struct
    def new2
       new(:foo, :bar)
    end
  end

  C = MyStruct.new(:foo)
  p C.ancestors         # => [C, MyStruct, Struct, Enumerable, Object, Kernel]
  p C.new.new2          # "new2" here.  but not "new"

							matz.

Re: subclassing Structs

[Eugene Scripnik]

Yukihiro Matsumoto wrote:
> Hi,
> 
> In message "Re: subclassing Structs"
>     on 03/08/14, Eugene Scripnik <Eugene.Scripnik@itgrp.net> writes:
> 
> |My example didn't show that but I wanted to create struct-class with 
> |dynamic attributes and additional constructors.
> |
> |class SubStruct < Struct
> |   def new2
> |     new( 'attr1_value', 'attr2_value' )
> |   end
> |end
> |
> |new2 - instance method of SubStruct (Struct) and I thought it has to be 
> |class method of SubStruct.new( ... ) class.
> 
> Unlike other classes, Struct.new returns its subclass, not instance.
> This is very exceptional behavior.
> 
> new2 is an instance method of SubStruct.
> 
>   class SubStruct < Struct
>     def new2
>        new(:foo, :bar)
>     end
>   end
> 
>   C = MyStruct.new(:foo)
>   p C.ancestors         # => [C, MyStruct, Struct, Enumerable, Object, Kernel]
>   p C.new.new2          # "new2" here.  but not "new"
OK, thank you for explanations. Problem solved by changing def new2 to 
def self.new2

-- 
Eugene Scripnik
IT Group
Software Architect
Tel./Fax +380 (372) 58-43-10
email: Eugene.Scripnik@itgrp.net
http://www.itgrp.net/