How to override Numeric class


#1

Hi all,
I am new to Ruby and am trying to overload “+” operator on Integer
class, without success.

class TestNum < Integer
def +(other)
self.value - other
end
end

puts TestNum.new(5)+5; # should return 0

I know that it should be simple, but… any help appreciated.


#2

Bojan M. wrote:

puts TestNum.new(5)+5; # should return 0

I know that it should be simple, but… any help appreciated.

Did you look at the error?

?> TestNum.new(5)+5
NoMethodError: undefined method `new’ for TestNum:Class
from (irb):7
from :0

Your problem starts already with the creation… Integer is an abstract
class and I suspect it does something to #new.

You should also look at #coerce, see
http://www.rubygarden.org/ruby?CoerceExplanation for example if you want
to implement operators properly.

Kind regards

robert

#3

On Jan 6, 2006, at 10:35 AM, Bojan M. wrote:

puts TestNum.new(5)+5; # should return 0

I know that it should be simple, but… any help appreciated.

Ruby provides the coerce method, for hooking into its math routines:

class TestNum
def initialize( value )
@value = value
end
attr_reader :value
def +( other )
if other.is_a? self.class
value + other.value
else
?> value + other

end

end
def coerce( other )
[self.class.new(other), self]
end
end
=> nil

tn = TestNum.new(5)
=> #<TestNum:0x31d2b8 @value=5>

tn + 5
=> 10

tn + tn
=> 10

5 + tn
=> 10

Hope that helps.

James Edward G. II


#4

Thanks James. As I understand coerce method in Ruby classes is to assure
‘other’ is of right type and that operation could be performed. In this
example coerce is called from FixNum class, and ‘+’ method is always
preformed in TestNum class, right?


#5

On Jan 6, 2006, at 12:57 PM, Bojan M. wrote:

Thanks James. As I understand coerce method in Ruby classes is to
assure ‘other’ is of right type and that operation could be
performed. In this example coerce is called from FixNum class, and
‘+’ method is always preformed in TestNum class, right?

Yes, that’s correct.

James Edward G. II