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comp.lang.ruby

Ruby classes and << Syntax

Christoph Schiessl

5/16/2008 4:55:00 PM

I have written a simple snippet of code and would appreciate any help
with understanding what's really going on.


### Creating Basic Class
class Person # Same as: class Person < Object
attr_accessor :firstname
attr_accessor :lastname

def initialize(firstname, lastname)
@firstname, @lastname = firstname, lastname
end

def fullname
@firstname + ' ' + @lastname
end
end

### Create two instances of class Person
john = Person.new('John', 'Doe')
john.firstname # => "John"
john.lastname # => "Doe"
john.to_s # => "#<Person:0x356678>"

jack = Person.new('Jack', 'Stone')
jack.firstname # => "Jack"
jack.lastname # => "Stone"
jack.to_s # => "#<Person:0x3512f4>"

class Person
def to_s
'Person: ' + fullname
end
end

john.to_s # => "Person: John Doe"
jack.to_s # => "Person: Jack Stone"
### Everything as expected until here...

### Now the interesing part:
class <<Person
def birthday
'2008-05-16'
end
end

### NOT EXPECTED:
john.birthday # => NoMethodError: undefined method `birthday' for
#<Person ...>
jack.birthday # => NoMethodError: undefined method `birthday' for
#<Person ...>

frank = Person.new('Frank', 'New') # => Try with new instance
frank.birthday # => NoMethodError: undefined method `birthday' for
#<Person ...>

### So basically my question is: What does the 'class <<Person'
Statement really do?


Thank you very much!
Christoph Schiessl
3 Answers

Todd Benson

5/16/2008 5:10:00 PM

0

On Fri, May 16, 2008 at 11:54 AM, Christoph Schiessl <c.schiessl@gmx.net> wrote:
> I have written a simple snippet of code and would appreciate any help with
> understanding what's really going on.
>
>
> ### Creating Basic Class
> class Person # Same as: class Person < Object
> attr_accessor :firstname
> attr_accessor :lastname
>
> def initialize(firstname, lastname)
> @firstname, @lastname = firstname, lastname
> end
>
> def fullname
> @firstname + ' ' + @lastname
> end
> end
>
> ### Create two instances of class Person
> john = Person.new('John', 'Doe')
> john.firstname # => "John"
> john.lastname # => "Doe"
> john.to_s # => "#<Person:0x356678>"
>
> jack = Person.new('Jack', 'Stone')
> jack.firstname # => "Jack"
> jack.lastname # => "Stone"
> jack.to_s # => "#<Person:0x3512f4>"
>
> class Person
> def to_s
> 'Person: ' + fullname
> end
> end
>
> john.to_s # => "Person: John Doe"
> jack.to_s # => "Person: Jack Stone"
> ### Everything as expected until here...
>
> ### Now the interesing part:
> class <<Person
> def birthday
> '2008-05-16'
> end
> end

You are defining a class method and not a class instance method. Do this...

class Person
def birthday
'2008-05-16'
end
end

Todd

Christoph Schiessl

5/16/2008 5:18:00 PM

0

Which means:

class <<Person
def birthday
'2008-05-16'
end
end

Is EXACTLY the same as:

class Person
def self.birthday
'2008-05-16'
end
end

Or:

class Person
class <<self
def birthday
'2008-05-16'
end
end
end

Thank you very much!
Christoph Schiessl

On May 16, 2008, at 7:10 PM, Todd Benson wrote:

> On Fri, May 16, 2008 at 11:54 AM, Christoph Schiessl <c.schiessl@gmx.net
> > wrote:
>> I have written a simple snippet of code and would appreciate any
>> help with
>> understanding what's really going on.
>>
>>
>> ### Creating Basic Class
>> class Person # Same as: class Person < Object
>> attr_accessor :firstname
>> attr_accessor :lastname
>>
>> def initialize(firstname, lastname)
>> @firstname, @lastname = firstname, lastname
>> end
>>
>> def fullname
>> @firstname + ' ' + @lastname
>> end
>> end
>>
>> ### Create two instances of class Person
>> john = Person.new('John', 'Doe')
>> john.firstname # => "John"
>> john.lastname # => "Doe"
>> john.to_s # => "#<Person:0x356678>"
>>
>> jack = Person.new('Jack', 'Stone')
>> jack.firstname # => "Jack"
>> jack.lastname # => "Stone"
>> jack.to_s # => "#<Person:0x3512f4>"
>>
>> class Person
>> def to_s
>> 'Person: ' + fullname
>> end
>> end
>>
>> john.to_s # => "Person: John Doe"
>> jack.to_s # => "Person: Jack Stone"
>> ### Everything as expected until here...
>>
>> ### Now the interesing part:
>> class <<Person
>> def birthday
>> '2008-05-16'
>> end
>> end
>
> You are defining a class method and not a class instance method. Do
> this...
>
> class Person
> def birthday
> '2008-05-16'
> end
> end
>
> Todd

Ruby Freak

5/17/2008 2:04:00 PM

0

Which means:

class <<Person
def birthday
'2008-05-16'
end
end

Is EXACTLY the same as:

class Person
def self.birthday
'2008-05-16'
end
end

Or:

class Person
class <<self
def birthday
'2008-05-16'
end
end
end

True, but all 3 are class methods and none will work as you intend.

You would have to call Person.birthday to get a response and everyone
will have the same birthday (what a party!)
Of course you knew that, but some readers might not and I am bored, so
I wrote this.