From: Stefano Crocco <stefano.crocco@...>
Date: 2011-01-28T04:59:22+09:00
Subject: Re: why is overloading invalid in ruby.

On Friday 28 January 2011 04:22:47 Ted Flethuseo wrote:
> I don't understand why when I try to overload I get an error. Can I
> overload somehow?
> 
> #!/usr/bin/env ruby
> 
> class Summer
>   def sum(x)
>     return x + 2
>   end
> 
>   def sum(x,y)
>     return x + y
>   end
> 
>   def sum(x,y,z)
>     return x + y + z
>   end
> end
> 
> s = Summer.new
> puts s.sum(3)
> 
> ERROR:
> ArgumentError: wrong number of arguments (1 for 3)
> 
> method sum  in summer.rb at line 18
> at top level  in summer.rb at line 18
> 
> 
> Ted

Ruby doesn't support method overloading. What your code does is giving three 
definition of the sum method, with each definition overwriting the previous 
one. This means that the first two definitions have no effect, since they're 
overwritten by the third. At the end of your code, the sum method is defined 
to take three arguments, so you obviously get an error when you try to call it 
with only one argument.

You can achieve the same result giving some of the arguments default values:

class Summer

  def sum x, y = 2, z = 0
    x + y + z
  end

end

Now the sum method can be called with one, with two or with three arguments. 
In the first case, y will be 2 and z will be three, so the result will be 
computed as

x + 2 + 0

which is what you wanted. If it's called with two arguments, x and y will be 
given the values passed as argument and z will be 0.

Another approach is to give sum a variable number of arguments, like this:

def sum  x, *args
  args << 2 if args.empty?
  res = x
  args.each{|i| res += i}
  res
end

The * in front of args in the method definition tells ruby that args is 
special: it stands in place of any number of arguments, from 0 upwards. When 
the method is called, the first argument is assigned to x. Any other argument 
(if any) is put in an array which is assigned to the variable args. In 
particular, if sum is called with only one argument, args will be empty, so we 
insert the value 2 in it (by the way, you could use Array#inject here, which 
is much more elegant than what I wrote).

I hope this helps

Stefano