Attr_reader explained

Ruby
1

can someone explain how attr_reader works?

i can’t find a good explanation anywhere.

please help!

2

On 3/3/07, libsfan01 [email protected] wrote:

can someone explain how attr_reader works?

i can’t find a good explanation anywhere.

please help!

It’s built in to the language, so it ‘works’ by magic for all
practical purposes, it’s implemented in object.c if you want to take
a look at the implementation. If you wanted to implement it in Ruby,
you do something like this:

def new_attr_reader cl, sym
str = “def #{sym.to_s}; @#{sym.to_s}; end”
cl.class_eval str
end

class Test; end
new_attr_reader Test, :my_new_reader
Test.new().my_new_reader

The built in attr_reader takes an array of sym’s, but you get the
idea.
-tim

3

libsfan01 wrote:

can someone explain how attr_reader works?

i can’t find a good explanation anywhere.

please help!

attr_reader, in it’s common usage, creates instance variables and
defines a method by which you can read them.

Class Test
def initialize(num)
@test=num
end

def test
   @test
end

end

If you were to use attr_reader instead, your class definition would be

Class Test
attr_reader :test
def initialize(num)
@test = num
end
end

4

On 3/3/07, Tim B. [email protected] wrote:

On 3/3/07, libsfan01 [email protected] wrote:

can someone explain how attr_reader works?
i can’t find a good explanation anywhere.
It’s built in to the language, so it ‘works’ by magic for all
practical purposes, it’s implemented in object.c if you want to take
a look at the implementation. If you wanted to implement it in Ruby,
you do something like this:

Actually, that’s not true. It’s written in C, but that’s not the same
as being built into the language. It’s a method on Module, so it can
be overridden.

class Module
alias old_attr_reader attr_reader
def attr_reader(*names)
puts “Making attribute readers for #{names.join(”, “)}”
old_attr_reader *names
end
end
=> nil
class Foo
attr_reader :bar
attr_reader :baz, :quux
end
Making attribute readers for bar
Making attribute readers for baz, quux

-austin

5

Alle sabato 3 marzo 2007, Raj S. ha scritto:

attr_reader, in it’s common usage, creates instance variables and
defines a method by which you can read them.

Actually, attr_reader doesn’t create the variable, just the accessor
method.
To see this, do the following in irb:

class C
attr_reader :var
def var_defined?
defined?(@var)
end
end
=> nil
c=C.new
=> #<C:0xb7a4c650>
c.var_defined?
=> nil
class C
def initialize
@var=nil
end
end
=> nil
c1=C.new
=> #<C:0xb7cf1a04 @var=nil>
c1.var_defined?
=> “instance-variable”

As you can see, in the first case (the variable called c) doesn’t have
the
@var instance variable defined. Even after you call the var method
defined
using attr_reader, the variable is still not defined. To define it, you
need
to explicitly assign it to a value (using @var= something inside an
instance
method of the class or using instance_variable_set)

Stefano

6

On Mar 3, 2007, at 15:25, Austin Z. wrote:

Actually, that’s not true. It’s written in C, but that’s not the same
as being built into the language. It’s a method on Module, so it can
be overridden.

attr* also cheats, so methods defined by attr* are slightly faster
than methods defined with def:

http://blog.segment7.net/articles/2006/03/06/attr-vs-method-vs-
define_method