Asp Forum - Noobie ... Simple Inheriting from Hash Question ..

Neville Burnell

5/17/2005 11:59:00 PM

Hi,

I have a class inheriting from Hash which has some specific methods
operating on the hash elements:

Class MyHash < Hash

def foo
...
end

End

....

Now, with Hash class, its really easy to create a new hash, eg

h = {:key1 => "val1", :key2 => "val2"}

What I would like to do is create a new instance of the class with the
same simplicity, but I'd like to avoid creating a redundant Hash and
tranferring the contents one by one to MyHash which happens if I code:

h = MyHash.new(:key1 => "val1", :key2 => "val2")

and then define initialize(h={})

Whats the "ruby way" tm for something like this?

Thanks

Nev

6 Answers

David Mitchell

5/18/2005 12:11:00 AM

You could just extend the hash class, rather than inheriting from it.
That is, instead of this:

class myHash < Hash
def foo
...
end
end

Do this:

class Hash
def foo
...
end
end

Then, all your hash objects will be given your 'foo' method and you can
do things like this:

{:key => "value"}.foo

David

Neville Burnell wrote:
> Hi,
>
> I have a class inheriting from Hash which has some specific methods
> operating on the hash elements:
>
> Class MyHash < Hash
>
> def foo
> ...
> end
>
> End
>
> ....
>
> Now, with Hash class, its really easy to create a new hash, eg
>
> h = {:key1 => "val1", :key2 => "val2"}
>
> What I would like to do is create a new instance of the class with the
> same simplicity, but I'd like to avoid creating a redundant Hash and
> tranferring the contents one by one to MyHash which happens if I code:
>
> h = MyHash.new(:key1 => "val1", :key2 => "val2")
>
> and then define initialize(h={})
>
> Whats the "ruby way" tm for something like this?
>
> Thanks
>
> Nev
>
>
>
>
>
>
>
>
>
>
>
>
>
>

--
David Mitchell
Software Engineer
Telogis

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Joel VanderWerf

5/18/2005 12:23:00 AM

Neville Burnell wrote:
> Hi,
>
> I have a class inheriting from Hash which has some specific methods
> operating on the hash elements:
>
> Class MyHash < Hash
>
> def foo
> ...
> end
>
> End
>
> ....
>
> Now, with Hash class, its really easy to create a new hash, eg
>
> h = {:key1 => "val1", :key2 => "val2"}
>
> What I would like to do is create a new instance of the class with the
> same simplicity, but I'd like to avoid creating a redundant Hash and
> tranferring the contents one by one to MyHash which happens if I code:
>
> h = MyHash.new(:key1 => "val1", :key2 => "val2")
>
> and then define initialize(h={})
>
> Whats the "ruby way" tm for something like this?

irb(main):001:0> class H < Hash
irb(main):002:1> def foo; "foo"; end
irb(main):003:1> end
=> nil
irb(main):004:0> h = H[1,2,3,4]
=> {1=>2, 3=>4}
irb(main):005:0> h.class
=> H

dblack

5/18/2005 2:01:00 AM

Mark Hubbart

5/18/2005 3:13:00 AM

On 5/17/05, David A. Black <dblack@wobblini.net> wrote:
> Hi --
>
> On Wed, 18 May 2005, David Mitchell wrote:
>
> > You could just extend the hash class, rather than inheriting from it. That
> > is, instead of this:
> >
> > class myHash < Hash
> > def foo
> > ...
> > end
> > end
> >
> > Do this:
> >
> > class Hash
> > def foo
> > ...
> > end
> > end
> >
> > Then, all your hash objects will be given your 'foo' method and you can do
> > things like this:
> >
> > {:key => "value"}.foo
>
> This will work but also suffers from the usual problem with extending
> core classes -- namely, it's unsafe to do unless you're sure
> that your code will run in isolation.
>
> Another possibility is to add the behavior on a per-object basis:
>
> module MyHashStuff
> def foo
> # ...
> end
> end
>
> h = {1,2,3,4}
> h.extend(MyHashStuff)
> h.foo # h now has the food method

Another option is to add a to_myhash method to Hash:

class MyHash < Hash
def foo
...
end
end

class Hash
def to_myhash
MyHash.new.update self
end
end

Now you can create myhashes like this:

{1=>2,3=>4}.to_myhash

cheers,
Mark

Robert Klemme

5/18/2005 7:30:00 AM

Mark Hubbart wrote:
> On 5/17/05, David A. Black <dblack@wobblini.net> wrote:
>> Hi --
>>
>> On Wed, 18 May 2005, David Mitchell wrote:
>>
>>> You could just extend the hash class, rather than inheriting from
>>> it. That is, instead of this:
>>>
>>> class myHash < Hash
>>> def foo
>>> ...
>>> end
>>> end
>>>
>>> Do this:
>>>
>>> class Hash
>>> def foo
>>> ...
>>> end
>>> end
>>>
>>> Then, all your hash objects will be given your 'foo' method and you
>>> can do things like this:
>>>
>>> {:key => "value"}.foo
>>
>> This will work but also suffers from the usual problem with extending
>> core classes -- namely, it's unsafe to do unless you're sure
>> that your code will run in isolation.
>>
>> Another possibility is to add the behavior on a per-object basis:
>>
>> module MyHashStuff
>> def foo
>> # ...
>> end
>> end
>>
>> h = {1,2,3,4}
>> h.extend(MyHashStuff)
>> h.foo # h now has the food method
>
> Another option is to add a to_myhash method to Hash:
>
> class MyHash < Hash
> def foo
> ...
> end
> end
>
> class Hash
> def to_myhash
> MyHash.new.update self
> end
> end
>
> Now you can create myhashes like this:
>
> {1=>2,3=>4}.to_myhash

Note though, that this is exactly what the original poster tried to
avoid - the intermediate hash.

Another option that hasn't been mentioned yet is to use delegation. You
can have a wrapper around a hash that contains all your additional methods
and references a Hash instance.

Kind regards

robert

Christoph R.

5/18/2005 12:23:00 PM

David A. Black schrieb:

>
> This will work but also suffers from the usual problem with extending
> core classes -- namely, it's unsafe to do unless you're sure
> that your code will run in isolation.
>
> Another possibility is to add the behavior on a per-object basis:
>
> module MyHashStuff
> def foo
> # ...
> end
> end
>
> h = {1,2,3,4}
> h.extend(MyHashStuff)
> h.foo # h now has the food method

Jet another possibility (it's a pretty rare animal, I've never seen in
the wild)
is to clone a core class and alter the clone - For example, it should be
straight
forward to reimplement the Set class as a clone of the Hash class.

---
Set = Hash.clone

class Set
# aliasing stuff is half the rent ...
alias :each , :each_key
alias :include? , :is_key?

private
# a couple of original method are be private.
alias :_implement_add, , :store

# throw out superfluous original methods
['[]', '[]=',:each_key,:each_value, :store,:is_key? ... ].each {|m|
remove_method(m) }

public
# build the methods
def add(o)
_implement_add(o,true)
end
...
end

---
Note I am not advocating a rewrite of the Set class (if at all it
it should be a c-extension), it just makes a poster child example
for this idiom.

/Christoph

comp.lang.ruby

Noobie ... Simple Inheriting from Hash Question ..