Cloning object with array members

ruby-talk
1

Consider a simple class like this:
class MyClass
   def initialize
     @var1 = ''
     @arr1 = []
   end
end

Now if I have an instance of MyClass, and want to clone it, what is the 'cleanest' way to do this?

a = MyClass.new
b = a.clone

This method leaves 'b.arr1' pointing to the same data as a.arr1.

Hmmm... I'll try overriding clone, and manually deal with the arrays:

def clone
   rslt = super.clone
end

This blows the stack, I believe because all methods are inherited virtually.

Ok, so now I have to manually copy everything in clone, whatever, I am getting sick of this:

   def clone
     rslt = MyClass.new
     rslt.var1 = @var1
     rslt.arr1 = @arr1.clone
     rslt
   end

Turns out this does not compute either, as there is no method var1= or arr1= ! I want to keep var1 private, so can't go this route either.

One more attempt:
   def clone
     rslt = MyClass.new

     self.instance_variables.each do |member|
       rslt.instance_variable_set( member, self.instance_variable_get(member).clone )
     end
   end

This fails as the methos do not seem to copy over.

There has to be a simple way to do something like this? What is common recipe for something like this? Should this really be that hard in ruby?

~S

2

Hi --

···

On Sat, 6 Jan 2007, Shea Martin wrote:

Consider a simple class like this:
class MyClass
def initialize
   @var1 = ''
   @arr1 =
end
end

Now if I have an instance of MyClass, and want to clone it, what is the 'cleanest' way to do this?

a = MyClass.new
b = a.clone

This method leaves 'b.arr1' pointing to the same data as a.arr1.

I don't see an a.arr1 method. Did you mean to create some accessors
methods?

The most common idiom I've seen for deep copying is:

   b = Marshal.load(Marshal.dump(a))

David

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A. Ruby Power and Light, LLC (http://www.rubypal.com)

3

Warning: I didn't test these...

   def clone
     rslt = super # Use the default #clone instead of Myclass.new

     instance_variables.each do |member|
       rslt.instance_variable_set( member, instance_variable_get(member).clone )
     end

     rslt
   end

For your particular case you could avoid the loop of course:

   def clone
     rslt = super
     rslt.instance_variable_set "@var1", @var1.clone
     rslt.instance_variable_set "@arr1", @arr1.clone
     rslt
   end

Gary Wright

···

On Jan 5, 2007, at 4:40 PM, Shea Martin wrote:

Hmmm... I'll try overriding clone, and manually deal with the arrays:

There has to be a simple way to do something like this? What is common recipe for something like this? Should this really be that hard in ruby?

4

Hmmm... I'll try overriding clone, and manually deal with the arrays:

def clone
   rslt = super.clone
end

This blows the stack, I believe because all methods are inherited
virtually.

The method which must be overriden is initialize_copy

def initialize_copy(from)
  @arr1 = from.instance_eval('@arr1').clone
end

···

--
Olivier Renaud

5

Shea Martin wrote:

Consider a simple class like this:
class MyClass
  def initialize
    @var1 = ''
    @arr1 =
  end
end

Now if I have an instance of MyClass, and want to clone it, what is the 'cleanest' way to do this?

a = MyClass.new
b = a.clone

This method leaves 'b.arr1' pointing to the same data as a.arr1.

Hmmm... I'll try overriding clone, and manually deal with the arrays:

def clone
  rslt = super.clone
end

This blows the stack, I believe because all methods are inherited virtually.

Ok, so now I have to manually copy everything in clone, whatever, I am getting sick of this:

  def clone
    rslt = MyClass.new
    rslt.var1 = @var1
    rslt.arr1 = @arr1.clone
    rslt
  end

Turns out this does not compute either, as there is no method var1= or arr1= ! I want to keep var1 private, so can't go this route either.

One more attempt:
  def clone
    rslt = MyClass.new

    self.instance_variables.each do |member|
      rslt.instance_variable_set( member, self.instance_variable_get(member).clone )
    end
  end

This fails as the methos do not seem to copy over.

There has to be a simple way to do something like this? What is common recipe for something like this? Should this really be that hard in ruby?

~S

Thanks for all the help. My last attempt at clone turns out to be fine, I just forgot to return 'rslt'. That fixed everything.

~S

6

Shea, you found your answer in the meantime, but I wanted to show you what was wrong with your other attempts:

a = MyClass.new
b = a.clone

This method leaves 'b.arr1' pointing to the same data as a.arr1.

Yes, the default #dup and #clone create a shallow copy.

def clone
  rslt = super.clone
end

This blows the stack, I believe because all methods are inherited virtually.

"super" isn't the same as in Java. super calls the same method of the superclass, so what you are doing here is:

   temp = super
   rslt = temp.clone

Since temp is an instance of MyClass, you're calling clone again, which gives you and endless recursion. The correct way would have been:

   def clone
     super
   end

But this obviously doen't help you.

  def clone
    rslt = MyClass.new
    rslt.var1 = @var1
    rslt.arr1 = @arr1.clone
    rslt
  end

Turns out this does not compute either, as there is no method var1= or arr1= ! I want to keep var1 private, so can't go this route either.

The solution to this problem was:

  def clone
    rslt = MyClass.new

    self.instance_variables.each do |member|
      rslt.instance_variable_set( member, self.instance_variable_get(member).clone )
    end
  end

This fails as the methos do not seem to copy over.

And here, as you've noted yourself, you just forgot to return your new instance.

Regards,
Pit

7

def clone
     rslt = BuildConfig.new

     self.instance_variables.each do |member|
           rslt.instance_variable_set( member,
                        self.instance_variable_get(member).clone )
     end

     return rslt
   end

This clone method bombs if any of the instance variables are FixNums, Floats, boolean, etc. Is there a way in my loop to know if an instance variable supports 'clone'?

~S

8

Alle 22:35, martedì 9 gennaio 2007, Shea Martin ha scritto:

   def clone
     rslt = BuildConfig.new

     self.instance_variables.each do |member|
           rslt.instance_variable_set( member,
                        self.instance_variable_get(member).clone )
     end

     return rslt
   end

This clone method bombs if any of the instance variables are FixNums,
Floats, boolean, etc. Is there a way in my loop to know if an instance
variable supports 'clone'?

~S

I don't know whether there is a way to know in advance if a variable supports
clone, so I'd say: try it. If it works, good. If it doesn't, it'll raise an
exception, which you can rescue:

def clone
  rslt = BuildConfig.new

  self.instance_variables.each do |member|
    begin rslt.instance_variable_set( member, self.instance_variable_get(member).clone )
    rescue Exception: rslt.instance_variable_set(member, self.instance_variable_get(member))
    end
  end

return rslt
end

I'm not an expert, so there may be better ways to do this I don't know, but this should work.

Stefano