Ruby's class_eval & instance_eval. Is it that confusing?


We can do class_eval & instance_eval using following syntax,

In this example, class_eval? method can be accessed my any objects of the String class. Where as instance_eval? method can be accessed by only String class. 

More clearly, class_eval creates instance methods, but instance_eval creates class methods or it can be applied to only one object at a time.

 [3] pry(main)> String.class_eval?
NoMethodError: undefined method `class_eval?' for String:Class
Did you mean?  class_eval
from (pry):3:in `<main>'
[4] pry(main)> String.instance_eval?
=> true
[5] pry(main)> String.new.class_eval?
=> true

 my_string = "This is a string"

 For example you can simply do like below as any string is an object of String Class
[5] pry(main)> my_string.class_eval?
=> true

 But you can not do 
[7] pry(main)> my_string.instance_eval?
NoMethodError: undefined method `instance_eval?' for "This is a string":String


Comments

Post a Comment

Popular posts from this blog

Rails Kaminari - Ajax pagination

Here is a simple way to implement AJAX pagination using  Kaminari gem. Suppose, We are having a Product Model with fields id and name. So, In the ProductsController , def index   @products =  Product.all.page(params[:page]).per(params[:per])    respond_to do |format|       format.js       format.html     end end in view file index.html.haml   #products     = render 'products'   #paginator     = paginate @products, :remote => true And create _products.html.haml (this will be rendered from index) with content   %table     @products.each do |product|      %tr        %td= product.name Finally, We have to create one more  file _index.js.haml with content $('#products').html("#{escape_javascript(render 'products')}"); $('#paginator').html("#{escape_javascript(paginate(@products, :remote => true)) }"); This post may help you. Please comment on the same, If you guys have any queries.
Read more

Rails mongoid has field model validation

This can be done only with a custom method validation, Such as           class Comment        include Mongoid::Document        include Mongoid::Timestamps         include ActiveModel::Validations              validate :must_be_friends              def must_be_friends           errors.add(:base, 'Must be friends to leave a comment') if hash[:firstname].nil?        end     end
Read more

Rails Upgrading a project

When changing Rails versions, it's best to move slowly, one minor version at a time The process should go as follows: Write tests and make sure they pass. Move to the latest patch version after your current version. Fix tests and deprecated features. Move to the latest patch version of the next minor version. Repeat this process until you reach your target Rails version Rails 6 requires Ruby 2.5.0 or newer. Rails 5 requires Ruby 2.2.2 or newer. Rails 4 prefers Ruby 2.0 and requires 1.9.3 or newer. Rails 3.2.x is the last branch to support Ruby 1.8.7. Rails 3 and above require Ruby 1.8.7 or higher. Support for all of the previous Ruby versions has been dropped officially. You should upgrade as early as possible.   Rails provides the app:update command ( rake rails:update on 4.2 and earlier). After updating the Rails version in the Gemfile , run this command. This will help you with the creation of new files and changes of old files in an interactive session.  
Read more