Class behaviour

Just curious, can I write a class that can check values as they are
assigned to objects?

foo = Klass.new
foo.legal_values = (4..10)

foo.value = 5 # OK
foo.value = 11 # ERROR - out of bounds, raise exception or something

Also, do I have to have an explicit method for the 'value', or can it
operate like a "builtin" class, like this :

foo = Klass.new
foo.legal_values = (4..10)

foo = 5 # OK
foo = 11 # ERROR - out of bounds, raise exception or something

In Perl, I would probably use tiescalar for something like this, where I
can intercept an assignment...

Also, it may not be numbers, but enums as well.

Thanks.

···

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Posted via http://www.ruby-forum.com/.

Geoff Barnes wrote:

Just curious, can I write a class that can check values as they are
assigned to objects?

Sure, just declare accessors in the usual way, then write your own methods
to check the values:

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#!/usr/bin/ruby

class Suspicious
   attr_accessor :a
   def a=(v)
      raise "Number Range Error" if v < 0 || v > 5
      @a = v
   end
end

s = Suspicious.new

s.a = 10 # number range error

puts s.a # never gets here

------------------------------------

--
Paul Lutus
http://www.arachnoid.com

Geoff Barnes wrote:

Just curious, can I write a class that can check values as they are assigned to objects?

foo = Klass.new
foo.legal_values = (4..10)

foo.value = 5 # OK
foo.value = 11 # ERROR - out of bounds, raise exception or something

You can define a special class method as a replacement for attr_accessor:

class Module
   def checked_attr(name,&test)
     define_method("#{name}=") do |val|
       test[val] or raise ArgumentError
       instance_variable_set "@#{name}", val
     end

     attr_reader name
   end
end

class T
   checked_attr :foo do |x|
     x >= 0
   end
end

irb(main):018:0* t=T.new
=> #<T:0x392088>
irb(main):019:0> t.foo = 10
=> 10
irb(main):020:0> t.foo = -10
ArgumentError: ArgumentError
         from (irb):4:in `foo='
         from (irb):20
irb(main):021:0>

Also, do I have to have an explicit method for the 'value', or can it operate like a "builtin" class, like this :

foo = Klass.new
foo.legal_values = (4..10)

foo = 5 # OK
foo = 11 # ERROR - out of bounds, raise exception or something

You cannot interfere here so there is no way to check these assignments.

In Perl, I would probably use tiescalar for something like this, where I can intercept an assignment...

Also, it may not be numbers, but enums as well.

I'm not sure I understand you here.

Kind regards

  robert

···

from :0

Geoff Barnes wrote:

foo = Klass.new
foo.legal_values = (4..10)

foo.value = 5 # OK
foo.value = 11 # ERROR - out of bounds, raise exception or something

Do you want the legal values to be different per instance, or set for
the whole class?
Here are two options:

class Klass
  class << self
    attr_accessor :legal_values
  end

  attr_reader :value
  def value=( new_value )
    if value_range = self.class.legal_values
      raise "Out of Range" unless value_range.include?( new_value )
    end
    @value = new_value
  end
end

foo = Klass.new
bar = Klass.new
Klass.legal_values = 4..10

foo.value = 8
puts foo.value #=> 8

bar.value = 17 #=> RuntimeError: Out of Range

class Klass
  attr_accessor :legal_values
  attr_reader :value

  def value=( new_value )
    if @legal_values
      raise "Out of Range" unless @legal_values.include?( new_value )
    end
    @value = new_value
  end

  def legal_values=( new_range )
    raise "Value outside Range" if @value && !new_range.include?( value
)
    @legal_values = new_range
  end
end

foo = Klass.new
foo.legal_values = 4..10

foo.value = 8
puts foo.value #=> 8

bar = Klass.new
bar.value = 200
puts bar.value #=> 200

bar.legal_values = 1..10 #=> RuntimeError: Value outside Range

Also, do I have to have an explicit method for the 'value', or can it
operate like a "builtin" class, like this :

foo = Klass.new
foo.legal_values = (4..10)

foo = 5 # OK
foo = 11 # ERROR - out of bounds, raise exception or something

No, you can't do this - there is no assignment method (=) that you can
override to do something different. "foo = 5" will always change the
local variable 'foo' to point to a Fixnum of 5.

Perfect, thanks for the ideas. Makes sense...

···

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