# Relationship operator for classes

**URL:** <https://www.ruby-forum.com/t/relationship-operator-for-classes/112210>\
**Category:** Ruby\
**Created:** [August 16, 2007, 12:47pm UTC](https://www.ruby-forum.com/t/relationship-operator-for-classes/112210 "2007-08-16T12:47:46Z")\
**Posts on this page:** 5\
**Page:** 1

<div class="post-metadata">

**Author:** ![ronaldfischer](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/r/67e7ee/32.png) [@ronaldfischer](https://www.ruby-forum.com/u/ronaldfischer)\
**Post date:** [August 16, 2007, 12:47pm UTC](https://www.ruby-forum.com/t/relationship-operator-for-classes/112210/1 "2007-08-16T12:47:46Z")

</div>

I wanted to distinguish between cases, where a variable  
is of type Symbol, or Hash, or something other. My first  
attempt failed:

case v.class  
when Symbol  
…  
when Hash  
…  
else  
…  
end

Even if v was a Hash, processing endet up in the “else” part.  
Indeed, a quick check with irb showed:

```
Hash === Hash # => false

```

I solved my problem by changing my code to

case v.class.to\_s  
when ‘Symbol’  
…  
when ‘Hash’  
…  
else  
…  
end

But now I wonder why === is defined for classes that way - it seems to  
_always_ return false.

Ronald

---

<div class="post-metadata">

**Author:** ![ronaldfischer](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/r/67e7ee/32.png) [@ronaldfischer](https://www.ruby-forum.com/u/ronaldfischer)\
**Post date:** [August 16, 2007, 1:01pm UTC](https://www.ruby-forum.com/t/relationship-operator-for-classes/112210/2 "2007-08-16T13:01:47Z")

</div>

> But now I wonder why === is defined for classes that way - it seems to  
> _always_ return false.

This is just by my own experimenting, but it seems === is for testing  
instance variables.

irb(main):001:0\> Hash === {}  
=\> true  
irb(main):002:0\> Hash === 3  
=\> false  
irb(main):003:0\> Hash === Class  
=\> false  
irb(main):004:0\> Class === Hash  
=\> true  
irb(main):005:0\> Fixnum === 3  
=\> true  
irb(main):006:0\> Object === Class  
=\> true  
irb(main):007:0\> Class === Object  
=\> true  
irb(main):008:0\>

Cheers,  
Arlen.

---

<div class="post-metadata">

**Author:** ![ronaldfischer](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/r/67e7ee/32.png) [@ronaldfischer](https://www.ruby-forum.com/u/ronaldfischer)\
**Post date:** [August 16, 2007, 1:02pm UTC](https://www.ruby-forum.com/t/relationship-operator-for-classes/112210/3 "2007-08-16T13:02:47Z")

</div>

Alle giovedì 16 agosto 2007, Ronald F. ha scritto:

> ```
> ...
> 
> ```
> 
> when ‘Symbol’  
> Ronald  
> You need

case v  
when Symbol  
…  
when Hash  
…  
else  
…  
end

The reason is that Class#=== returns true if the argument is an instance  
of  
the class or of its descendents. In other words, it works like  
Object#instance\_of?:

a = []  
a.instance\_of? Array  
=\> true  
a.instance\_of? Object  
=\> true  
a.instance\_of? String  
=\> false

Array === a  
=\> true  
Object === a  
=\> true  
String === a  
=\> false

Notice that you need to be careful if you need to check whether the  
class of  
an object is _exactly_ a given class. If you need to distinguish between  
a  
class and a descendent, you have to put them in the correct order in the  
case  
statement: first the derived class, then the base class:

class A;end

class B \< A; end

case obj  
when B  
…  
when A  
…  
end

I hope this helps

Stefano

---

<div class="post-metadata">

**Author:** ![ronaldfischer](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/r/67e7ee/32.png) [@ronaldfischer](https://www.ruby-forum.com/u/ronaldfischer)\
**Post date:** [August 16, 2007, 1:17pm UTC](https://www.ruby-forum.com/t/relationship-operator-for-classes/112210/4 "2007-08-16T13:17:50Z")

</div>

> The reason is that Class#=== returns true if the argument is  
> an instance of  
> the class or of its descendents.

Thanks a lot, that was it!!

Ronald

---

<div class="post-metadata">

**Author:** ![ronaldfischer](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/r/67e7ee/32.png) [@ronaldfischer](https://www.ruby-forum.com/u/ronaldfischer)\
**Post date:** [August 16, 2007, 1:05pm UTC](https://www.ruby-forum.com/t/relationship-operator-for-classes/112210/5 "2007-08-16T13:05:48Z")

</div>

> case v.class  
> when Symbol  
> …  
> when Hash  
> …  
> else  
> …  
> end

So on second thoughts, try just removing the “.class” from that code and  
try again. 🙂

- Arlen