Rails - Jan 2006 - what this line means exactly? (@@active_connections ||= {})

Hi, 

I have problem to understand this line does exactly?
this is from the class ConnectionSpecficication,
methode "active_connections"

def self.active_connections #:nodoc:
  if allow_concurrency
    Thread.current[''active_connections''] ||= {}
  else
    @@active_connections ||= {}
  end
end

what the line "@@active_connections ||= {}" means
exactly? 
as I undertands is if the class varaible
@@active_connections has a value then return the
value, otherwise return {}, but what is {}, a bloc of
nothing??? or what?

Thanks you very much

Saiho

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On 1/24/06, Saiho Yuen <sayoyo@yahoo.com> wrote:
> Hi,
>
> I have problem to understand this line does exactly?
> this is from the class ConnectionSpecficication,
> methode "active_connections"
>
> def self.active_connections #:nodoc:
>   if allow_concurrency
>     Thread.current[''active_connections''] ||= {}
>   else
>     @@active_connections ||= {}
>   end
> end
>
> what the line "@@active_connections ||= {}" means
> exactly?
> as I undertands is if the class varaible
> @@active_connections has a value then return the
> value, otherwise return {}, but what is {}, a bloc of
> nothing??? or what?
a = 2
a ||= 3
# a is 2

b ||= 3
# b is 3

So, ||= will set a variable to some value if the variable doesn''t have
a value yet.

{} is a hash.  Could''ve used:
    @@active_connections ||= Hash.new
instead.

{} is an anonymous hash

It seems to be saying

"return whatever it is, or initialize it as an empty hash and return
it."



On 1/24/06, Saiho Yuen <sayoyo@yahoo.com> wrote:
>
> Hi,
>
> I have problem to understand this line does exactly?
> this is from the class ConnectionSpecficication,
> methode "active_connections"
>
> def self.active_connections #:nodoc:
>   if allow_concurrency
>     Thread.current[''active_connections''] ||= {}
>   else
>     @@active_connections ||= {}
>   end
> end
>
> what the line "@@active_connections ||= {}" means
> exactly?
> as I undertands is if the class varaible
> @@active_connections has a value then return the
> value, otherwise return {}, but what is {}, a bloc of
> nothing??? or what?
>
> Thanks you very much
>
> Saiho
>
> __________________________________________________
> Do You Yahoo!?
> Tired of spam?  Yahoo! Mail has the best spam protection around
> http://mail.yahoo.com
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> Rails mailing list
> Rails@lists.rubyonrails.org
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>
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Yeah, it will use @@active_connections if it has been set, otherwise it 
will set @@active_connections to an empty hash and return it.

One thing about this feature has caught me out a couple of times. Look 
at this:

x = false
x ||= 4
# x = 4  !

x is set to 4 even though it has already been set to false. It does this 
because ||= is simply a shortcut for x = x || 4, and (false || 4) 
results in 4.

Interesting :)

-Jonny.

Kelly Dwight Felkins wrote:
> {} is an anonymous hash
> 
> It seems to be saying
> 
> "return whatever it is, or initialize it as an empty hash and return 
> it."

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