Ruby For Rails class_eval question

Rails
1

When I run the following example on page 349, I get the error message:

NoMethodError: undefined method `some_method’ for #<C:0x2e3d698 @x=1>
from (irb):94
from ?:0

c = Class.new
c.class_eval do
def some_method
puts “Created in class_eval”
end
end
c = C.new
c.some_method

How to run this example? I followed the suggestion on the Manning’s
errata page with no luck. TIA.
A.

2

It works now. Why does it have to be constant?

3

Bala P. <bparanj@…> writes:

c = Class.new
Your non-capitalization above is incorrect. The new class should be a
constant.
Try C = Class.new
c.class_eval do

def some_method
puts “Created in class_eval”
end
end
c = C.new
Well, you haven’t defined C because of the capitalization issue above.
c.some_method

4

Bala P. wrote:

It works now. Why does it have to be constant?

It doesn’t really have to be. It’s just a convention that classes are
defined into constants.

You can just as easily do:

x = Class.new
x.class_eval do
def some_method
puts “Created in class_eval”
end
end
c = x.new
c.some_method

The reason your first example failed was b/c you were inconsistently
mixing ‘c’ and ‘C’.

5

Hi –

On Tue, 10 Oct 2006, Bala P. wrote:

puts “Created in class_eval”
end
end
c = C.new
c.some_method

How to run this example? I followed the suggestion on the Manning’s errata page with no luck. TIA.

The erratum has an erratum… The two lines I provided are run
together into one. They should be split, as below.

The whole thing should be:

c = Class.new
c.class_eval do
def some_method
puts “Created in class_eval”
end
end

c_instance = c.new
c_instance.some_method

David

–
David A. Black | [email protected]
Author of “Ruby for Rails” [1] | Ruby/Rails training & consultancy [3]
DABlog (DAB’s Weblog) [2] | Co-director, Ruby Central, Inc. [4]
[1] Ruby for Rails | [3] http://www.rubypowerandlight.com
[2] http://dablog.rubypal.com | [4] http://www.rubycentral.org

6

Hi –

On Wed, 11 Oct 2006, Kian Wright wrote:

x.class_eval do
def some_method
puts “Created in class_eval”
end
end
c = x.new
c.some_method

The reason your first example failed was b/c you were inconsistently
mixing ‘c’ and ‘C’.

My fault – it was wrong in the first printing, and the erratum has
two lines run together as one. (See my other reply on this thread for
the working version.)

David

–
David A. Black | [email protected]
Author of “Ruby for Rails” [1] | Ruby/Rails training & consultancy [3]
DABlog (DAB’s Weblog) [2] | Co-director, Ruby Central, Inc. [4]
[1] Ruby for Rails | [3] http://www.rubypowerandlight.com
[2] http://dablog.rubypal.com | [4] http://www.rubycentral.org