Noobie ... Simple Inheriting from Hash Question ..

[Neville Burnell]

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 =3D {:key1 =3D> "val1", :key2 =3D> "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 =3D MyHash.newkey1 =3D> "val1", :key2 =3D> "val2")

and then define initialize(h=3D{})

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

Thanks

Nev







=20

 

[David Mitchell]

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

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.newkey1 => "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]

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.newkey1 => "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

 

[David A. Black]

Hi --

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


David

 

[Mark Hubbart]

Hi --
=20
On Wed, 18 May 2005, David Mitchell wrote:
=20
=20
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.
=20
Another possibility is to add the behavior on a per-object basis:
=20
module MyHashStuff
def foo
# ...
end
end
=20
h =3D {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=3D>2,3=3D>4}.to_myhash

cheers,
Mark

 

[Robert Klemme]

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]

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