'pass parameter by reference' in Ruby?

Question

In Ruby, is it possible to pass by reference a parameter with value-type semantics (e.g. a Fixnum)? I'm looking for something similar to C#'s 'ref' keyword.

Example:

def func(x) 
    x += 1
end

a = 5
func(a)  #this should be something like func(ref a)
puts a   #should read '6'

Btw. I know I could just use:

a = func(a)

Answer 1

You can accomplish this by explicitly passing in the current binding:

def func(x, bdg)
  eval "#{x} += 1", bdg
end

a = 5
func(:a, binding)
puts a # => 6

Answer 2

Ruby doesn't support "pass by reference" at all. Everything is an object and the references to those objects are always passed by value. Actually, in your example you are passing a copy of the reference to the Fixnum Object by value.

The problem with the your code is, that x += 1 doesn't modify the passed Fixnum Object but instead creates a completely new and independent object.

I think, Java programmers would call Fixnum objects immutable.

Answer 3

In Ruby you can't pass parameters by reference. For your example, you would have to return the new value and assign it to the variable a or create a new class that contains the value and pass an instance of this class around. Example:

class Container
attr_accessor :value
 def initialize value
   @value = value
 end
end

def func(x)
  x.value += 1
end

a = Container.new(5)
func(a)
puts a.value

Answer 4

You can try following trick:

def func(x) 
    x[0] += 1
end

a = [5]
func(a)  #this should be something like func(ref a)
puts a[0]   #should read '6'

Answer 5

http://ruby-doc.org/core-2.1.5/Fixnum.html

Fixnum objects have immediate value. This means that when they are assigned or passed as parameters, the actual object is passed, rather than a reference to that object.

Also Ruby is pass by value.

Answer 6

However, it seems that composite objects, like hashes, are passed by reference:

fp = {}
def changeit(par)
  par[:abc] = 'cde'
end

changeit(fp)

p fp

gives

{:abc=>"cde"}