Generate accessors for an hash

Ruby
1

Hello, I need to generate read and write accessors for all the keys in
an hash.
Say I have an attribute named my_attribute, I need “my_attribute” and
“my_attribute=” methods to read and write it to
@attributes[my_attribute].

I tried this code in the class constructor:

for a in all_attributes
  method_name = a.name
  self.class.class_eval do
    define_method method_name do
      @attributes[method_name]
    end
    define_method method_name+"=" do |val|
      @attributes[method_name] = val
    end
  end
end

It generates the method for the class, but all the methods read and
write to the last attribute in the all_attributes list. That’s
strange…
Thank you,

Andrea Gianarro

2 
     @attributes[method_name] = val
   end
 end

end

It generates the method for the class, but all the methods read and
write to the last attribute in the all_attributes list. That’s
strange…

Ah, the strange joy that is closures. Basically, those ‘define_method’
calls
allow the code block to retain access to whatever variables were in
scope
when the methods are defined. So, when those methods actually run, the
variable ‘method_name’ will have the last value in the loop – each
closure
references a single variable, it does not copy its value.

You might be better off with method_missing

class Hash
def method_missing(name, value = nil)
name = name.to_s
if name =~ /=$/
self[name.sub(/=$/, ‘’)] = value
else
return self[name]
end
end
end

3

Andrea Gianarro wrote:

Hello, I need to generate read and write accessors for all the keys in
an hash.

Wouldn’t a simpler solution be to use OpenStruct?

require ‘ostruct’

hash={“my_attribute”=>“value”}
s=OpenStruct.new(hash)
puts s.my_attribute

value
s.my_attribute=“new”
=> “new”
puts s.my_attribute
new
=> nil

And you get the freebie of creating new keys from the setter:

s.new_attribute=“better”
=> “better”
puts s.new_attribute

better

Cheers,
V.-

4

James C. wrote:

You might be better off with method_missing

class Hash
def method_missing(name, value = nil)
name = name.to_s
if name =~ /=$/
self[name.sub(/=$/, ‘’)] = value
else
return self[name]
end
end
end

Thank you very much, that’s exactly what I did. Strange ruby behaviour
anyway. :stuck_out_tongue:
bye,

Andrea Gianarro