Extending object instances with <<

[Tim Becker]

Hi,

I'm trying to write a class with a method to extend instances of
itself to contain additional accessors. I thought using `class <<
self` would be the most elegant way to go about it, but I'm running
into some problems. To illustrate:

class Test
# takes an array of symbols to add to the instance.
def add syms
syms.each { |sym|
@@__tmp = sym
$__tmp = sym
class << self
#attr_accessor sym # this would be my preferance, but sym
isn't in scope here
#attr_accessor $__tmp # this works, but uses globals
attr_accessor @@__tmp # this is nearly as bad as using globals
end # <<
} # each
end # add
end # Test

t = Test.new
t.add [:thingie, :thingie2]

t.thingie="whatever"
t.thingie2="bla"
puts t.thingie
puts t.thingie2



I don't like the idea of using globals to transport the symbol
information and the class members approach is nearly as bad
(synchronization issues mainly, apart from elegance). But I can't
think of another way to transport dynamic data into the `class<<self`
block.

Alternatives would be to handle this using `method_missing` though
that wouldn't just affect a single instance or using `eval` which
would involve executing strings I'm banging together.

Another thing I tried was:

...
self.class.attr_accessor sym
...

but that doesn't work because `attr_accessor` is private (contrary to
what it says in the documentation...)

Any ideas? Am I missing something?
-tim

 

[Robert Klemme]

Hi,

I'm trying to write a class with a method to extend instances of
itself to contain additional accessors. I thought using `class <<
self` would be the most elegant way to go about it, but I'm running
into some problems. To illustrate:

class Test
# takes an array of symbols to add to the instance.
def add syms
syms.each { |sym|
@@__tmp = sym
$__tmp = sym
class << self
#attr_accessor sym # this would be my preferance, but sym
isn't in scope here
#attr_accessor $__tmp # this works, but uses globals
attr_accessor @@__tmp # this is nearly as bad as using globals
end # <<
} # each
end # add
end # Test

t = Test.new
t.add [:thingie, :thingie2]

t.thingie="whatever"
t.thingie2="bla"
puts t.thingie
puts t.thingie2

I don't like the idea of using globals to transport the symbol
information and the class members approach is nearly as bad
(synchronization issues mainly, apart from elegance). But I can't
think of another way to transport dynamic data into the `class<<self`
block.

There is:

irb(main):017:0> class Bar
irb(main):018:1> def add(*syms)
irb(main):019:2> cl = class<<self;self;end
irb(main):020:2> cl.instance_eval { attr_accessor *syms }
irb(main):021:2> end
irb(main):022:1> end
=> nil
irb(main):023:0> f=Bar.new
=> #<Bar:0x3c1a40>
irb(main):024:0> f.add :bar
=> nil
irb(main):025:0> f.bar=10
=> 10
irb(main):026:0> f.bar
=> 10

Alternatives would be to handle this using `method_missing` though
that wouldn't just affect a single instance or using `eval` which
would involve executing strings I'm banging together.

Another thing I tried was:

..
self.class.attr_accessor sym
..

but that doesn't work because `attr_accessor` is private (contrary to
what it says in the documentation...)

Any ideas? Am I missing something?

See above. Apart from that you could simply use OpenStruct or inherit
OpenStruct which does all this for you already automagically:

irb(main):013:0> require 'ostruct'
=> true
irb(main):014:0> f=OpenStruct.new
=> #<OpenStruct>
irb(main):015:0> f.bar=10
=> 10
irb(main):016:0> f.bar
=> 10

Major difference is that you do not explicitly control accessor creation
but automatically get *all* - even spelling errors.

Kind regards

robert

 

[Ken Bloom]

See above. Apart from that you could simply use OpenStruct or inherit
OpenStruct which does all this for you already automagically:

irb(main):013:0> require 'ostruct'
=> true
irb(main):014:0> f=OpenStruct.new
=> #<OpenStruct>
irb(main):015:0> f.bar=10
=> 10
irb(main):016:0> f.bar
=> 10

Major difference is that you do not explicitly control accessor creation
but automatically get *all* - even spelling errors.

Test=Struct.new(*syms)
or
Test=Struct.newfoo,:bar,:baz)

This will guard you against spelling errors.

--Ken

 

[Robert Klemme]

Test=Struct.new(*syms)
or
Test=Struct.newfoo,:bar,:baz)

This will guard you against spelling errors.

That's true. However, I interpreted the OP's posting that he needs to
to the extension on a per instance basis. That does not work with the
approach you presented.

Kind regards

robert

 

[Giles Bowkett]

I think you're correct. The original poster's problem was to add
accessors to instances, splats are handy in the general case but
instance_eval is what you need here.