From: Gavin Kistner <gavin@...>
Date: 2005-09-14T21:36:51+09:00
Subject: Re: Assigning to dynamic class attributes

On Sep 13, 2005, at 2:36 PM, daz wrote:
> Allen wrote:
>> Also, I don't completely understand how symbols work or
>> why they are needed.
>
> If you use strings to identify fields, each time you write the
> field name, a new string is created.  Symbols are unique and
> memory-efficient.
>
> p 'cremote'.object_id  #-> 23356052
> p 'cremote'.object_id  #-> 23356016
> p 'cremote'.object_id  #-> 23355992
> puts
> p :@cremote.object_id  #-> 3504654
> p :@cremote.object_id  #-> 3504654
> p :@cremote.object_id  #-> 3504654

While :@cremote is a valid symbol, it sort of implies that the @  
symbol is somewhow special for symbols. The corresponding symbol to  
"cremote" is :cremote.

irb(main):001:0> a = 'cremote'
=> "cremote"

irb(main):002:0> a.to_sym
=> :cremote

irb(main):003:0> a.to_sym.to_s
=> "cremote"


Note also that symbols are immutable (like a large integer), while  
strings are mutable (like an Array). Think of symbols as a unique  
identifier that you can pass around in a lightweight and unambiguous  
way. For example, symbols are often used for the value of constants:

class Person
     MALE = :male
     FEMALE = :female
     attr_reader :sex
     def initialize( sex )
         @sex = sex
     end
end

pat = Person.new( Person::MALE )
jim = Person.new( Person::MALE )
if jim.sex == Person::FEMALE
     # This won't occur
     raise "Something went bad!"
end


Compare to this pathological case:

class Person
     MALE = 'male'
     FEMALE = 'female'
     attr_reader :sex
     def initialize( sex )
         @sex = sex
     end
end

pat = Person.new( Person::MALE )
jim = Person.new( Person::MALE )

# Trying to give JUST pat a sex change, the wrong way
pat.sex.sub!( /^/, 'fe' )

if jim.sex == Person::FEMALE
     # This will occur!
     raise "Something went bad!"
end