From: Xeno Campanoli <xeno.campanoli@...>
Date: 2010-01-21T04:05:39+09:00
Subject: Re: Looking for object.inherits?(Classname)

Robert Klemme wrote:
> 2010/1/20 Xeno Campanoli <xeno.campanoli@gmail.com>:
>> I'm looking for a way to make sure an object being passed is in some class
>> family, or to say it otherwise, inherits some specific class, but not
>> necessarily directly.  In fact, it could be a great-great-grandparent class,
>> and that would be okay, and needs to be okay.  Is there something where I
>> can take an object and go
>>
>> unless object.inherits?(MyClass)
>>        raise SyntaxError, "etc."
>> end
> 
> First of all that would not be a SyntaxError because the syntax is not
> affected.  Using this error type will create misleading error
> messages.

If I want a method that only takes objects of family Xclass, and I get one 
passed that is not in that family, isn't that a syntax error?  Bad type passed 
in that argument place?  What's the difference between that and passing a string 
when it needs say an array?

> 
> Here are all the ways that I am aware of (trying to summarize what
> others have posted so far):
> 
> irb(main):010:0> a = []
> => []
> irb(main):011:0> a.class.ancestors
> => [Array, Enumerable, Object, Kernel, BasicObject]
> irb(main):012:0> a.class.ancestors.include? Enumerable
> => true
> irb(main):013:0> a.kind_of? Enumerable
> => true
> irb(main):014:0> a.is_a? Enumerable
> => true
> irb(main):015:0> Enumerable === a
> => true
> irb(main):016:0>
> 
> Note that I do not recommend using the test from line 12 as it is
> likely much more inefficient than the other ones.  The test from line
> 15 is especially suited for use with "case" statements as in

Thank you.  That is the kind of answer I really needed.

Sincerely, Xeno
> 
> case a
> when Enumerable
>   puts "it's enumerable"
> when Array, Hash
>   puts "It's a special enumerable"
> when Integer
>   puts "You can count on me."
> else
>   puts "No idea about a."
> end
> 
> Another note: more often than not it is not necessary to test for the
> type of an object.  You rather invoke those methods on it that you
> expect it to implement.  You will get an exception anyway if the
> object does not implement the method (NoMethodError).  You can find
> more details about this concept under the label "duck typing", e.g.
> http://en.wikipedia.org/wiki/Duck_typing
> 
> Kind regards
> 
> robert
> 


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