Can't change the value of self - II

I’m still not “getting it.”

Consider


class DirectoryString < String
def initialize(arg)
super arg
end

def convert_forward_slash_to_back_slash
self.gsub(///, “\”)
end

def convert_forward_slash_to_back_slash!
self.gsub!(///, “\”)
end

def remove_trailing_backslash_if_any
self.chomp("\")
end

def remove_trailing_backslash_if_any!
self.chomp!("\")
end

def append_trailing_backslash_if_needed
remove_trailing_backslash_if_any + “\”
end

def append_trailing_backslash_if_needed!
# ??? What do I do here ??? … The following is illegal
self = “Help!”
end
end


The only thing I can think of is converting from the standard IS_A
(Inheritance) to a HAS_A (create a member variable).

If I do the latter, above, I will have to create a ton of simple
“forwarding” messages.

What’s the Ruby way of doing this sort of thing?

You can use the #replace method to do that sort of thing:

self.replace "Help! Oh, well, I'm saved!"

This method is defined in classes String, Hash, Array, and Set, and
works the same way in each one.

jeremy

Wednesday, August 11, 2010, 5:52:57 PM, you wrote:

jR> You can use the #replace method to do that sort of thing:

jR> self.replace “Help! Oh, well, I’m saved!”

jR> This method is defined in classes String, Hash, Array, and Set, and
jR> works the same way in each one.

jR> jeremy

jeremy,

That’s great and solves my particular problem.

But what if the class being derived from does not have a replace method?

Ralph

On Thu, 12 Aug 2010, Ralph S. wrote:

jeremy,

That’s great and solves my particular problem.

But what if the class being derived from does not have a replace method?

If the object has state that can be modified, then there will be (by
definition) ways to modify that state. If it doesn’t, then there won’t
be, and the class in question is probably a bad starting point if you
want to create objects with state that can be modified.

That’s one of the advantages of using proxy objects and delegators: you
gain an extra axis along which you can make decisions about things like
object state. Even though you can’t change (say) a Fixnum, you can
create objects with integer attributes that can be changed.

David


David A. Black, Senior Developer, Cyrus Innovation Inc.

The Ruby training with Black/Brown/McAnally
Compleat Philadelphia, PA, October 1-2, 2010
Rubyist http://www.compleatrubyist.com

David,

That’s great and solves my particular problem.

But what if the class being derived from does not have a replace method?

DAB> If the object has state that can be modified, then there will be
(by
DAB> definition) ways to modify that state. If it doesn’t, then there
won’t
DAB> be, and the class in question is probably a bad starting point if
you
DAB> want to create objects with state that can be modified.

DAB> That’s one of the advantages of using proxy objects and delegators:
you
DAB> gain an extra axis along which you can make decisions about things
like
DAB> object state. Even though you can’t change (say) a Fixnum, you can
DAB> create objects with integer attributes that can be changed.

Could you expand on this with an example, please?

On Thu, Aug 12, 2010 at 2:38 AM, Ralph S. [email protected]
wrote:

DAB> That’s one of the advantages of using proxy objects and delegators: you
DAB> gain an extra axis along which you can make decisions about things like
DAB> object state. Even though you can’t change (say) a Fixnum, you can
DAB> create objects with integer attributes that can be changed.

Could you expand on this with an example, please?

irb(main):001:0> require ‘delegate’
=> true
irb(main):002:0> class MutableFixnum < Delegator
irb(main):003:1> def initialize i
irb(main):004:2> super
irb(main):005:2> @value = i
irb(main):006:2> end
irb(main):007:1> def getobj
irb(main):008:2> @value
irb(main):009:2> end
irb(main):014:1> def setobj value
irb(main):015:2> @value = value
irb(main):016:2> end
irb(main):017:1> def add! x
irb(main):018:2> setobj(getobj + x)
irb(main):019:2> end
irb(main):020:1> end
=> nil
irb(main):021:0> a = MutableFixnum.new 1
=> 1
irb(main):022:0> a + 3
=> 4
irb(main):023:0> a.add! 3
=> 4
irb(main):024:0> a
=> 4

Jesus.

Ralph S. wrote:

Could you expand on this with an example, please?

[Just to show that delegation is nothing magic, and you don’t need to
use the Delegator class]

class MyCounter
def initialize(n=0)
@n = n
end
def inc!
@n += 1
end
def to_int
@n
end
def to_s
@n.to_s
end
end

c = MyCounter.new
c.inc!
c.inc!
puts “c is #{c}”

Some more work is required to make c duck-type like an Integer though.

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