# Constant lookup starting in superclass, not derived class

**URL:** <https://www.ruby-forum.com/t/constant-lookup-starting-in-superclass-not-derived-class/68295>\
**Category:** Ruby\
**Created:** [September 11, 2006, 3:00pm UTC](https://www.ruby-forum.com/t/constant-lookup-starting-in-superclass-not-derived-class/68295 "2006-09-11T15:00:12Z")\
**Posts on this page:** 8\
**Page:** 1

<div class="post-metadata">

**Author:** ![crenner](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/c/278dde/32.png) [@crenner](https://www.ruby-forum.com/u/crenner)\
**Post date:** [September 11, 2006, 3:00pm UTC](https://www.ruby-forum.com/t/constant-lookup-starting-in-superclass-not-derived-class/68295/1 "2006-09-11T15:00:12Z")

</div>

I’m no doubt missing something obvious but I found this a little  
confusing. I would have expected the constant lookup to start in Bar  
and therefore succeed. Instead, it appears to start in Base where Foo  
is not defined.

class Base  
def foo  
p Foo  
end  
end

class Bar \< Base  
class Foo  
end  
end

Bar.new.foo

# ~\> -:3:in `foo’: uninitialized constant Base::Foo (NameError)

# ~\> from -:12

Can anyone explain what is happening please?

Cheers,

Chris

---

<div class="post-metadata">

**Author:** ![crenner](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/c/278dde/32.png) [@crenner](https://www.ruby-forum.com/u/crenner)\
**Post date:** [September 11, 2006, 4:14pm UTC](https://www.ruby-forum.com/t/constant-lookup-starting-in-superclass-not-derived-class/68295/2 "2006-09-11T16:14:18Z")

</div>

Hi,

In message “Re: Constant lookup starting in superclass, not derived  
class”  
on Mon, 11 Sep 2006 21:59:21 +0900, “Chris R.”  
[removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) writes:

|I’m no doubt missing something obvious but I found this a little  
|confusing. I would have expected the constant lookup to start in Bar  
|and therefore succeed. Instead, it appears to start in Base where Foo  
|is not defined.  
|  
|class Base  
| def foo  
| p Foo  
| end  
|end

use “self.class::Foo” instead of plain “Foo”.

```
						matz.

```

---

<div class="post-metadata">

**Author:** ![crenner](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/c/278dde/32.png) [@crenner](https://www.ruby-forum.com/u/crenner)\
**Post date:** [September 11, 2006, 4:26pm UTC](https://www.ruby-forum.com/t/constant-lookup-starting-in-superclass-not-derived-class/68295/3 "2006-09-11T16:26:19Z")

</div>

On Mon, Sep 11, 2006 at 09:59:21PM +0900, Chris R. wrote:

> class Bar \< Base  
> class Foo  
> end  
> end
> 
> Bar.new.foo
> 
> # ~\> -:3:in `foo’: uninitialized constant Base::Foo (NameError)
> 
> # ~\> from -:12
> 
> Can anyone explain what is happening please?  
> Constants, unlike methods and instance variables, are “quasi”-lexicaly  
> scoped. (I say “quasi” because a sub class can assign to a constant  
> without munging its parent class’s constant) One way of getting around  
> this particular situation is to use #const\_get. e.g.:

class Base  
def foo  
p self.class.const\_get(‘Foo’)  
end  
end

---

<div class="post-metadata">

**Author:** ![crenner](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/c/278dde/32.png) [@crenner](https://www.ruby-forum.com/u/crenner)\
**Post date:** [September 11, 2006, 4:30pm UTC](https://www.ruby-forum.com/t/constant-lookup-starting-in-superclass-not-derived-class/68295/4 "2006-09-11T16:30:19Z")

</div>

Chris R. wrote:

> class Bar \< Base  
> class Foo  
> end  
> end
> 
> Bar.new.foo
> 
> # ~\> -:3:in `foo’: uninitialized constant Base::Foo (NameError)
> 
> # ~\> from -:12
> 
> Can anyone explain what is happening please?

Foo is not in the scope of Base so it cannot be looked up the way you  
did it. See Matz’s explanation how to access it nevertheless.  
Generally this form of dependency is not desirable, because the base  
class should be self contained and not depend on any sub classes. In  
Java you could make Base abstract and add an abstract method that will  
return a Class instance - but in Ruby there are no abstract classes.

Kind regards

```
robert
```

---

<div class="post-metadata">

**Author:** ![crenner](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/c/278dde/32.png) [@crenner](https://www.ruby-forum.com/u/crenner)\
**Post date:** [September 11, 2006, 4:58pm UTC](https://www.ruby-forum.com/t/constant-lookup-starting-in-superclass-not-derived-class/68295/5 "2006-09-11T16:58:28Z")

</div>

Do you have any suggestions of alternative implementation?

I’d rather not use the self.class.const\_get method as it clutters the  
code somewhat.

I wonder if I’m trying to refactor two seemingly identical methods  
that are in fact different.

class Main1  
def Foo; end  
def foo  
Foo.new  
end  
end

class Main2  
def Foo; end  
def foo  
Foo.new  
end  
end

At this point, I created a superclass to hold the common foo method  
definition. However, if I had the fully qualified namespace for Foo  
(Main1::Foo and Main2::Foo) in each method they would be different and  
I wouldn’t have moved them. Maybe that is part of my problem.

I understand the point about the base class depending on child classes  
not being desirable. What about a module relying on constants defined  
elsewhere (does the same apply)?

module Base  
def foo  
p Foo.new  
end  
end

class Foo  
include Base  
class Foo  
end  
end

Thanks again,

Chris

---

<div class="post-metadata">

**Author:** ![crenner](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/c/278dde/32.png) [@crenner](https://www.ruby-forum.com/u/crenner)\
**Post date:** [September 11, 2006, 5:26pm UTC](https://www.ruby-forum.com/t/constant-lookup-starting-in-superclass-not-derived-class/68295/6 "2006-09-11T17:26:31Z")

</div>

Chris R. wrote:

> Do you have any suggestions of alternative implementation?

Depends on what you want to do. If you just want to create instances  
then you can just use a method.

class Base  
def something  
f = create\_foo  
# use f  
end  
end

class Derived \< Base  
class Foo  
end

def create\_foo(\*a,&b)  
Foo.new(\*a,&b)  
end  
end

Or, if you want to do it more fancy you can define create\_foo in the  
base class and have it return nil or raise an exception.

Kind regards

```
robert

```

PS: Please don’t top post and trim quotings - that way others can easier  
follow threads here.

---

<div class="post-metadata">

**Author:** ![crenner](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/c/278dde/32.png) [@crenner](https://www.ruby-forum.com/u/crenner)\
**Post date:** [September 11, 2006, 6:16pm UTC](https://www.ruby-forum.com/t/constant-lookup-starting-in-superclass-not-derived-class/68295/7 "2006-09-11T18:16:40Z")

</div>

On 11/09/06, Robert K. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote:

> PS: Please don’t top post and trim quotings - that way others can easier  
> follow threads here.

Or to put it more clearly, ‘please trim quotations and don’t top post.’

Drifting wildly away from the topic, the ambiguity in the original  
request reminds me of the signs posted by Southwark Council on some of  
their properties: ‘Do not exercise or allow your dog to foul the  
estate.’ (People seem to follow the first half better than the  
second.)

Paul.

---

<div class="post-metadata">

**Author:** ![crenner](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/c/278dde/32.png) [@crenner](https://www.ruby-forum.com/u/crenner)\
**Post date:** [September 12, 2006, 6:49am UTC](https://www.ruby-forum.com/t/constant-lookup-starting-in-superclass-not-derived-class/68295/8 "2006-09-12T06:49:59Z")

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I was bumping against the same wall you are for a while. Later, i  
learned how to use meta classes and then realized that they gave me  
exactly what i was looking for.

[http://whytheluckystiff.net/articles/seeingMetaclassesClearly.html](http://whytheluckystiff.net/articles/seeingMetaclassesClearly.html)

Bret