Constants and metaclasses

Ruby
1

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”)?

2

Andreas L. wrote:

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

3

Joel VanderWerf wrote:

def singleton_class

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.

4

Hi –

On Tue, 12 Jun 2007, Andreas L. wrote:

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”)?

I don’t think so (and I hope not, as it would be kind of strange :slight_smile:
but you could define BAR in the original class:

class << f
Foo::BAR = “Hello”

etc.

David

5

Andreas L. wrote:

def singleton_class

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.

6

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 :slight_smile: