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
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...)
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
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:
Test=Struct.new(*syms)
or
Test=Struct.new(:foo,:bar,:baz)
This will guard you against spelling errors.
--Ken
···
On Sat, 24 Feb 2007 14:08:19 +0100, Robert Klemme wrote:
On 24.02.2007 13:52, Tim Becker wrote:
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
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:
Major difference is that you do not explicitly control accessor creation
but automatically get *all* - even spelling errors.
--
Ken Bloom. PhD candidate. Linguistic Cognition Laboratory.
Department of Computer Science. Illinois Institute of Technology. http://www.iit.edu/~kbloom1/
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
···
On 26.02.2007 02:38, Ken Bloom wrote:
On Sat, 24 Feb 2007 14:08:19 +0100, Robert Klemme wrote:
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.new(:foo,:bar,:baz)
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.
···
On 2/26/07, Robert Klemme <shortcutter@googlemail.com> wrote:
On 26.02.2007 02:38, Ken Bloom wrote:
> On Sat, 24 Feb 2007 14:08:19 +0100, Robert Klemme wrote:
>> 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.new(:foo,: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.