Cloning object with array members

Ruby
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 M. wrote:

‘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

3

On Jan 5, 2007, at 4:40 PM, Shea M. 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?

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 W.

4

Shea M. wrote:

‘cleanest’ way to do this?
end
rslt.arr1 = @arr1.clone

~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

5

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

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

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

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

8

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