Nested scopes and the Singleton pattern

HAL_9000 · 14 January 2004 21:40

Hello all,

This is really two questions.

I’ve been putting together a nested class structure in pieces
since it is a little large and unwieldy.

Here’s a snippet of something I’ve run across.

By the way, my last change was to implement a simple version of the
Singleton pattern (without using singleton.rb).

Note: Reopening existing classes

class ABC
class XYZ
@@instance = nil
def initialize(foo)
# Do some stuff…
# and then:
def XYZ.new(foo)
@@instance
end
end
end
end

This works fine.

First question: Is this a valid way to implement Singleton, or am I
overlooking something?

Then I thought: Well, let’s get rid of the unnecessary verbiage.
(Remember, I’m REOPENING these classes.)

So I did:

Note: Reopening existing classes

class ABC::XYZ
@@instance = nil
def initialize(foo)
# Do some stuff…
# and then:
def ABC::XYZ.new(foo)
@@instance
end
end
end

But it didn’t work.

This does work:

Note: Reopening existing classes

class ABC::XYZ
@@instance = nil
def initialize(foo)
# Do some stuff…
# and then:
klass = ABC::XYZ
def klass.new(foo)
@@instance
end
end
end

Second question: Why should these two be different?

Thanks,
Hal Fulton

Nobuyoshi_Nakada · 15 January 2004 01:49

Hi,

By the way, my last change was to implement a simple version of the
Singleton pattern (without using singleton.rb).

Note: Reopening existing classes

class ABC
class XYZ
@@instance = nil
def initialize(foo)
# Do some stuff…
# and then:
def XYZ.new(foo)
@@instance
end
end
end
end

This works fine.

First question: Is this a valid way to implement Singleton, or am I
overlooking something?

Have you considered about race conditions?

So I did:

Note: Reopening existing classes

class ABC::XYZ
@@instance = nil
def initialize(foo)
# Do some stuff…
# and then:
def ABC::XYZ.new(foo)
@@instance
end
end
end

But it didn’t work.

def ABC::XYZ means singleton method XYZ of ABC by itself, so
trailing . is superfluous. Try

   def (ABC::XYZ).new(foo)

···

At Thu, 15 Jan 2004 06:40:07 +0900, Hal Fulton wrote:

–
Nobu Nakada

HAL_9000 · 15 January 2004 04:24

First question: Is this a valid way to implement Singleton, or am I
overlooking something?

Have you considered about race conditions?

Thank you, Nobu. No, I did not consider race conditions.

I suppose singleton.rb is thread safe?

def ABC::XYZ means singleton method XYZ of ABC by itself, so
trailing . is superfluous. Try
   def (ABC::XYZ).new(foo)

Ahh, that is clear. But I could not see it before.

Thanks very much,
Hal

···

nobu.nokada@softhome.net wrote:

Nobuyoshi_Nakada · 15 January 2004 04:50

Hi,

···

At Thu, 15 Jan 2004 13:24:19 +0900, Hal Fulton wrote:

First question: Is this a valid way to implement Singleton, or am I
overlooking something?

Have you considered about race conditions?

Thank you, Nobu. No, I did not consider race conditions.

I suppose singleton.rb is thread safe?

Just using Thread.critical. And seems close to your idea
excepting for it uses “instance” method rather than “new”.

–
Nobu Nakada

HAL_9000 · 15 January 2004 04:55

Actually I like “new” instead of “instance”. I guess this
(instance) is to emphasize that there can only be one. But
I prefer simply calling new as usual, which was why I did this.

Hal

···

nobu.nokada@softhome.net wrote:

Hi,

At Thu, 15 Jan 2004 13:24:19 +0900, > Hal Fulton wrote:

First question: Is this a valid way to implement Singleton, or am I
overlooking something?

Have you considered about race conditions?

Thank you, Nobu. No, I did not consider race conditions.

I suppose singleton.rb is thread safe?

Just using Thread.critical. And seems close to your idea
excepting for it uses “instance” method rather than “new”.

Nobuyoshi_Nakada · 15 January 2004 09:39

Hi,

I suppose singleton.rb is thread safe?

Just using Thread.critical. And seems close to your idea
excepting for it uses “instance” method rather than “new”.

Not accurate, it uses 3-state; before, during and after
creation, but I feel it should use mutex or something.

Actually I like “new” instead of “instance”. I guess this
(instance) is to emphasize that there can only be one. But
I prefer simply calling new as usual, which was why I did this.

singleton.rb is for generic use, so it gets rid of overriding
instance methods, I guess.

···

At Thu, 15 Jan 2004 13:55:44 +0900, Hal Fulton wrote:

–
Nobu Nakada

Paul_Brannan · 15 January 2004 15:25

I think I agree. It took me an hour to understand the Singleton code
last time I read it, and when I look at it again now, I seem to have
forgotten how it works.

It seems like using a Mutex plus Thread.exclusive around the code that
redefines instance() should be sufficient, but perhaps I’m missing
something.

Paul

···

On Thu, Jan 15, 2004 at 06:39:13PM +0900, nobu.nokada@softhome.net wrote:

At Thu, 15 Jan 2004 13:55:44 +0900, > Hal Fulton wrote:

Just using Thread.critical. And seems close to your idea
excepting for it uses “instance” method rather than “new”.

Not accurate, it uses 3-state; before, during and after
creation, but I feel it should use mutex or something.

Christoph4 · 15 January 2004 22:46

nobu wrote
…

Just using Thread.critical. And seems close to your idea
excepting for it uses “instance” method rather than “new”.

Not accurate, it uses 3-state; before, during and after
creation, but I feel it should use mutex or something.

You probably always need a 3-state if you want a self
modifying first instance call (at least in some implicit way).

Here is a mutexy version of singleton.rb I wrote some time
ago - it is probably more robust then the current code but equally
obscure - sorry …

/Christoph

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end

Nobuyoshi_Nakada · 16 January 2004 03:20

Hi,

···

At Fri, 16 Jan 2004 07:46:39 +0900, Christoph wrote:

Just using Thread.critical. And seems close to your idea
excepting for it uses “instance” method rather than “new”.

Not accurate, it uses 3-state; before, during and after
creation, but I feel it should use mutex or something.

You probably always need a 3-state if you want a self
modifying first instance call (at least in some implicit way).

Here is a mutexy version of singleton.rb I wrote some time
ago - it is probably more robust then the current code but equally
obscure - sorry …

Due to the line ending codes, your patch contains whole files.
Following is a bit modified version.

Index: lib/singleton.rb

RCS file: /cvs/ruby/src/ruby/lib/singleton.rb,v
retrieving revision 1.21
diff -u -2 -p -d -r1.21 singleton.rb
— lib/singleton.rb 22 Aug 2003 08:09:58 -0000 1.21
+++ lib/singleton.rb 16 Jan 2004 01:26:06 -0000
@@ -12,5 +12,5 @@

a,b = Klass.instance, Klass.instance

a == b # => true

-# a.new # NoMethodError - new is private …
+# Klass.new # NoMethodError - new is private …

* ``The instance’’ is created at instanciation time, in other

@@ -31,7 +31,5 @@

Providing (or modifying) the class methods

-# * Klass.inherited(sub_klass) and Klass.clone() -
-# to ensure that the Singleton pattern is properly
-# inherited and cloned.
+# * Klass.inherited(sub_klass) and Klass.initialize_copy()

* Klass.instance() - returning ``the instance’'. After a

@@ -45,20 +43,12 @@

* Klass._load(str) - calling Klass.instance()

-# * Klass._instanciate?() - returning ``the instance’’ or
-# nil. This hook method puts a second (or nth) thread calling
-# Klass.instance() on a waiting loop. The return value
-# signifies the successful completion or premature termination
-# of the first, or more generally, current “instanciation thread”.
-#

The instance method of Singleton are

-# * clone and dup - raising TypeErrors to prevent cloning or duping
+# * clone and dup - raising TypeErrors to prevent cloning

-# * _dump(depth) - returning the empty string. Marshalling strips
-# by default all state information, e.g. instance variables and
-# taint state, from `the instance''. Providing custom _load(str) -# and _dump(depth) hooks allows the (partially) resurrections of -# a previous state of` the instance’'.

+# * _dump(depth) - returning the empty string, in other words
+# marshalling strips all state information. Providing custom
+# _load(str) and _dump(depth) hooks allows the (partially)
+# resurrections of a previous state of ``the instance’'.

@@ -82,60 +72,61 @@ end
class << Singleton

Method body of first instance call.

FirstInstanceCall = proc do
@instance takes on one of the following values
* nil - before and after a failed creation
* false - during creation
* sub_class instance - after a successful creation
the form makes up for the lack of returns in progs
Thread.critical = true
if @instance.nil?
```
 @__instance__  = false
```
```
 Thread.critical = false
```
```
 begin
```

FirstInstanceCall = proc do ||
critical, Thread.critical = Thread.critical, true
begin
```
 if instanciating = @__instance__.nil?
```
```
   @__instance__ = false
```

   Thread.critical = critical
   @__instance__ = new

```
 ensure
```
```
   if @__instance__
```
```
     class <<self
```
```
       remove_method :instance
```
```
       def instance; @__instance__ end
```
```
     end
```
```
   else
```

     @__instance__ = nil #  failed instance creation

```
   end
```

```
 elsif !@__instance__
```

   @__instanciating_queue__ << Thread.current

```
   Thread.stop
```
```
   Thread.critical = true
```
```
   retry
 end
```

elsif _instanciate?()
```
 Thread.critical = false    
```
else
```
 @__instance__  = false
```
```
 Thread.critical = false
```
```
 begin
```
```
   @__instance__ = new
```
```
 ensure
```

```
 @__instance__
```
ensure
```
 if instanciating
```

   Thread.critical= true
   if @__instance__

```
     class <<self
```

     class << self
       remove_method :instance
       def instance; @__instance__ end
     end

     remove_instance_variable(@__instanciating_queue__).each do |thr|

```
       thr.wakeup
```

     end
   else
     @__instance__ = nil

     if thr = @__instanciating_queue__.shift

```
       thr.wakeup
```
```
     end
   end
 end
```
```
 Thread.critical = critical
```
end

@instance
end
module SingletonClassMethods
properly clone the Singleton pattern - did you know
that duping doesn’t copy class methods?
def clone
```
 Singleton.__init__(super)
```

module SingletonClassMethods
def self.extended(klass)
```
 klass.instance_eval { 
```
```
   @__instance__ = nil
```
```
   @__instanciating_queue__ = [] 
```
```
 }
```
```
 class << klass
```

   define_method(:instance, FirstInstanceCall)

```
 end
```
end

private

ensure that the Singleton pattern is properly inherited

def initialize_copy(orig)
```
 super
```
```
 SingletonClassMethods.extended(self)
```
end
def inherited(sub_klass)
super

```
 Singleton.__init__(sub_klass)
```

 SingletonClassMethods.extended(sub_klass)

end

@@ -143,23 +134,6 @@ class << Singleton
instance
end

waiting-loop hook
def _instanciate?()
```
 while false.equal?(@__instance__)
```
```
   Thread.critical = false
```
```
   sleep(0.08)   # timeout
```
```
   Thread.critical = true
```
```
 end
```
```
 @__instance__
```
end
end
def init(klass)
klass.instance_eval { @instance = nil }
class << klass

 define_method(:instance,FirstInstanceCall)

end
klass
end

private
@@ -177,7 +151,7 @@ class << Singleton
def included(klass)
super
klass.private_class_method :new, :allocate

klass.private_class_method :new,:allocate
klass.extend SingletonClassMethods

Singleton.init(klass)

SingletonClassMethods.extended(klass)
end
end
@@ -186,9 +160,4 @@ end

if FILE == $0

-def num_of_instances(klass)

“#{ObjectSpace.each_object(klass){}} #{klass} instance(s)”
-end

The basic and most important example.

@@ -196,5 +165,7 @@ class SomeSingletonClass
include Singleton
end
-puts “There are #{num_of_instances(SomeSingletonClass)}”
+
+num = ObjectSpace.each_object(SomeSingletonClass) {}
+puts “There are #{num} of SomeSingletonClass instances”

a = SomeSingletonClass.instance
@@ -209,90 +180,81 @@ end

+puts “\nThreaded example with exception”
+$stdout.sync= true

-puts “\nThreaded example with exception and customized #_instanciate?() hook”; p
-Thread.abort_on_exception = false
+class Foo < SomeSingletonClass

@attempts = 0

-class Ups < SomeSingletonClass
def initialize

self.class.__sleep
puts “initialize called by thread ##{Thread.current[:i]}”
end
-end

-class << Ups

def _instanciate?
@enter.push Thread.current[:i]
while false.equal?(@instance)
```
 Thread.critical = false
```
```
 sleep 0.08 
```
```
 Thread.critical = true
```

@valid = false
sleep(rand(0.1))
if self.class.attempts < 3

 raise  "boom - initialize failed for thread ##{Thread.current[:i]}"

else
```
 @valid = true
```

 puts "yes! - initialize succceeded for thread ##{Thread.current[:i]}"

end

@leave.push Thread.current[:i]
@instance

ensure
self.class.attempts+= 1
end

def __sleep
sleep(rand(0.08))
end

def valid?
@valid
end
+end

def new
begin
```
 __sleep
```

 raise  "boom - thread ##{Thread.current[:i]} failed to create instance"

ensure
```
 # simple flip-flop
```
```
 class << self
```
```
   remove_method :new
```
```
 end
```
end
end
+class << Foo

attr_accessor :attempts

def instanciate_all

@enter =
@leave =
1.upto(9) {|i|
```
 Thread.new { 
```

Thread.current.priority = 101
thrs= Array.new(101) {|i|
```
 curr= Thread.new { 
   begin
```

     sleep(rand(0.1))
     Thread.current[:i] = i

```
     __sleep
     instance
```
```
   rescue RuntimeError => mes
```

   rescue => mes
     puts mes
   end
 }

```
 curr.priority = rand(101)
```
```
 curr
```
}

puts “Before there were #{num_of_instances(self)}”

puts “Before there existed #{num} valid #{self} instance(s)”
sleep 3

puts “Now there is #{num_of_instances(self)}”
puts “#{@enter.join '; '} was the order of threads entering the waiting loop”
puts “#{@leave.join '; '} was the order of threads leaving the waiting loop”

thrs.each {|t| t.join }
puts “Now there exist(s) #{num} valid #{self} instance(s)”
end
-end
def num
cnt = 0
ObjectSpace.each_object(self) {|o| cnt+=1 if o.valid? }
cnt
end
private
def initialize_copy(orig)
super
@attempts = 0
end
+end

-Ups.instanciate_all
-# results in message like
-# Before there were 0 Ups instance(s)
-# boom - thread #6 failed to create instance
-# initialize called by thread #3
-# Now there is 1 Ups instance(s)
-# 3; 2; 1; 8; 4; 7; 5 was the order of threads entering the waiting loop
-# 3; 2; 1; 7; 4; 8; 5 was the order of threads leaving the waiting loop
+Foo.instanciate_all
+# results in something like:
+# Before there were 0 valid Foo instance(s)
+# boom - initialize failed for thread #78
+# boom - initialize failed for thread #93
+# boom - initialize failed for thread #8
+# yes - initialize succceeded for thread #27
+# Now there are 1 valid Foo instance(s)
+# Now there are 1 valid Foo instance(s)

puts “\nLets see if class level cloning really works”
-Yup = Ups.clone
-def Yup.new

begin
__sleep
raise “boom - thread ##{Thread.current[:i]} failed to create instance”
ensure
simple flip-flop
class << self
```
 remove_method :new
```
end
end
-end
-Yup.instanciate_all
+Baz = Foo.clone
+Baz.instanciate_all

–
Nobu Nakada

Topic		Replies	Views
Initialization using the Singleton design pattern ruby-talk	3	116	5 January 2006
Singleton or full static class ruby-talk	4	115	15 August 2007
Singleton classes ruby-talk	3	88	7 January 2009
Class Method & Singletons - newbie question ruby-talk	2	139	4 June 2009
Creating a second instance of a singleton class? ruby-talk	3	113	15 February 2007

Nested scopes and the Singleton pattern

Note: Reopening existing classes

Note: Reopening existing classes

Note: Reopening existing classes

Note: Reopening existing classes

Note: Reopening existing classes

Index: lib/singleton.rb

a,b = Klass.instance, Klass.instance

a == b # => true

* ``The instance’’ is created at instanciation time, in other

Providing (or modifying) the class methods

* Klass.instance() - returning ``the instance’'. After a

* Klass._load(str) - calling Klass.instance()

The instance method of Singleton are

Method body of first instance call.

@instance takes on one of the following values

* nil - before and after a failed creation

* false - during creation

* sub_class instance - after a successful creation

the form makes up for the lack of returns in progs

properly clone the Singleton pattern - did you know

that duping doesn’t copy class methods?

ensure that the Singleton pattern is properly inherited

@@ -143,23 +134,6 @@ class << Singleton instance end

waiting-loop hook

if FILE == $0

The basic and most important example.

simple flip-flop

Related Topics

@@ -143,23 +134,6 @@ class << Singleton
instance
end