Forum: Ruby What's mean? global variables

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F799c3864ee7ba420b3c429e3029104e?d=identicon&s=25 BlueFox (Guest)
on 2006-02-23 16:05
(Received via mailing list)
in test.rb
class Test
   def greeting
        "hello world!"
   end
end

def test
   $test ||= Test.new
end

and somewhere

puts test.greeting

what is mean of(outside of the class define)

def test
   $test ||= Test.new
end

and where can find any document?

thanks.
Cff9eed5d8099e4c2d34eae663aae87e?d=identicon&s=25 Jacob Fugal (Guest)
on 2006-02-23 17:39
(Received via mailing list)
On 2/23/06, BlueFox <blue_fox@ynmail.com> wrote:
> what is mean of(outside of the class define)
>
> def test
>    $test ||= Test.new
> end

The keyword def, as you already know, begins a method definition. When
the method definition occurs outside the scope of any class, the
current 'self' is the 'main' object. Defining a method on 'main'
through some black magic, actually creates the method as an instance
method of Object. As such, the method is available to all instances of
Object -- that is, everything. This way, the method can be called
without a receiver in any context and generally have the same behavior
(modulo instance variables, but if you're using instance variables in
top-level methods/procedures, you're asking for trouble). This makes
the method, in effect, a procedure.

$test is a global variable. Thats important in this case for scope --
$test continues to exist and retain its value between invocations of
test. The ||= idiom is one commonly found in ruby. It's similar to +=
in that it is simply short hand for "$test = $test || Test.new". $test
starts with a value of nil, and the first time test is run (unless you
manually set $test beforehand), $test is initialized to a new instance
of Test, since "(nil || anything) == anything". On subsequent runs,
$test retains its value. From this behavior, ||= can be thought of as
a "default initializer" operator.

Does that answer your questions?

Jacob Fugal
D63c268960051bc17a310aa29fffd979?d=identicon&s=25 Dave Cantrell (Guest)
on 2006-02-24 04:41
(Received via mailing list)
Jacob Fugal wrote:
> $test is a global variable. Thats important in this case for scope --
> $test continues to exist and retain its value between invocations of
> test. The ||= idiom is one commonly found in ruby. It's similar to +=
> in that it is simply short hand for "$test = $test || Test.new". $test
> starts with a value of nil, and the first time test is run (unless you
> manually set $test beforehand), $test is initialized to a new instance
> of Test, since "(nil || anything) == anything". On subsequent runs,
> $test retains its value. From this behavior, ||= can be thought of as
> a "default initializer" operator.

Jacob,

So is this a shortcut for creating a singleton in Ruby? Albeit a
slightly different one I would guess, using a global variable to store a
single instance rather than using a class method to instantiate a new or
return the existing instance.

Thanks...
-dave
F799c3864ee7ba420b3c429e3029104e?d=identicon&s=25 BlueFox (Guest)
on 2006-02-24 07:49
(Received via mailing list)
>
>
Oh! the test function just an another way to access the global variable
$test?

yesterday i forgot  that we can define a fuction outside classes(unlike
java) and function's return value is the last statement's return value?



Thanks very much.



leo.
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