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)
Was it helpful?

Solution

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

OTHER TIPS

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.

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

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'

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.

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"}
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