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comp.lang.ruby

Constants and metaclasses

Andreas Launila

6/11/2007 6:19:00 PM

Is there a way to define a constant in a metaclass and then have it used
in methods defined in the original class?

An example:


class Foo
def foo
p BAR
end
end

f = Foo.new
class <<f
BAR = "Hello"

def bar
p BAR
end
end

f.bar
f.foo


The line "f.bar" will work fine, but "f.foo" will fail with "NameError:
uninitialized constant Foo::BAR". Is there some way to have the method
defined in the original class pick up on the constant in the metaclass
(without altering "p BAR")?

--
Andreas Launila
5 Answers

Joel VanderWerf

6/11/2007 6:38:00 PM

0

Andreas Launila wrote:
> Is there a way to define a constant in a metaclass and then have it used
> in methods defined in the original class?
>
> An example:
>
>
> class Foo
> def foo
> p BAR
> end
> end
>
> f = Foo.new
> class <<f
> BAR = "Hello"
>
> def bar
> p BAR
> end
> end
>
> f.bar
> f.foo
>
>
> The line "f.bar" will work fine, but "f.foo" will fail with "NameError:
> uninitialized constant Foo::BAR". Is there some way to have the method
> defined in the original class pick up on the constant in the metaclass
> (without altering "p BAR")?
>

Yes, with a little wrangling:

class Object
def singleton_class
class << self; self; end
end
end

class Foo
def foo
p self.singleton_class::BAR
end
end

f = Foo.new
class <<f
BAR = "Hello"

def bar
p BAR
end
end

f.bar
f.foo



--
vjoel : Joel VanderWerf : path berkeley edu : 510 665 3407

Andreas Launila

6/11/2007 7:16:00 PM

0

Joel VanderWerf wrote:
> Andreas Launila wrote:
>> The line "f.bar" will work fine, but "f.foo" will fail with "NameError:
>> uninitialized constant Foo::BAR". Is there some way to have the method
>> defined in the original class pick up on the constant in the metaclass
>> (without altering "p BAR")?
>>
>
> Yes, with a little wrangling:
>
> class Object
> def singleton_class
> class << self; self; end
> end
> end
>
> class Foo
> def foo
> p self.singleton_class::BAR
> end
> end
>
>

Yes that works, but I'm specifically looking for something that does not
alter the line "p BAR". To get around that one would have to dynamically
set the constant BAR in the main class before executing "p BAR". A
problem with that comes when one has multiple instances and metaclasses,
one would end up changing the value of a constant each time the method
is invoked. Thanks for the reply though.

--
Andreas Launila

dblack

6/11/2007 7:39:00 PM

0

Joel VanderWerf

6/11/2007 7:48:00 PM

0

Andreas Launila wrote:
> Joel VanderWerf wrote:
>> Andreas Launila wrote:
>>> The line "f.bar" will work fine, but "f.foo" will fail with "NameError:
>>> uninitialized constant Foo::BAR". Is there some way to have the method
>>> defined in the original class pick up on the constant in the metaclass
>>> (without altering "p BAR")?
>>>
>> Yes, with a little wrangling:
>>
>> class Object
>> def singleton_class
>> class << self; self; end
>> end
>> end
>>
>> class Foo
>> def foo
>> p self.singleton_class::BAR
>> end
>> end
>>
>>
>
> Yes that works, but I'm specifically looking for something that does not
> alter the line "p BAR". To get around that one would have to dynamically
> set the constant BAR in the main class before executing "p BAR". A
> problem with that comes when one has multiple instances and metaclasses,
> one would end up changing the value of a constant each time the method
> is invoked. Thanks for the reply though.

Oops, sorry.

It doesn't seem possible to use const_missing to do this, because
const_missing is called on Foo (not on the instance or the singleton
class) and there is no way to identify the instance of Foo in the
const_missing method.

--
vjoel : Joel VanderWerf : path berkeley edu : 510 665 3407

Andreas Launila

6/11/2007 9:01:00 PM

0

Joel VanderWerf wrote:
> It doesn't seem possible to use const_missing to do this, because
> const_missing is called on Foo (not on the instance or the singleton
> class) and there is no way to identify the instance of Foo in the
> const_missing method.
>

I didn't know that const_missing existed, that might be just enough to
not have to redefine constants if one accepts some dirtiness. Here is an
adaptation of your code with that thrown in.


class Object
def singleton_class
class << self; self; end
end
end

class Foo
def foo
p BAR
end

private

def self.const_missing(name)
if !@caller.nil? and @caller.singleton_class.const_defined? name
@caller.singleton_class.const_get(name)
else
super
end
end

def self.next_caller(caller)
@caller = caller
end
end

f = Foo.new
class <<f
BAR = "Hello"

alias_method :old_foo, :foo
def foo
self.class.next_caller(self)
old_foo
self.class.next_caller(nil)
end
end

g = Foo.new
class <<g
BAR = "Goodbye"

alias_method :old_foo, :foo
def foo
self.class.next_caller(self)
old_foo
self.class.next_caller(nil)
end
end

f.foo
g.foo
f.foo


Outputs
"Hello"
"Goodbye"
"Hello"

It isn't pretty, it isn't thread safe, it will not work if Foo is called
directly, it could be shortened/generalized/altered/improved depending
on the purpose.

Thank you :)

--
Andreas Launila