Ruby Language Dynamic Evaluation Instance evaluation


The instance_eval method is available on all objects. It evaluates code in the context of the receiver:

object =

object.instance_eval do
  @variable = :value

object.instance_variable_get :@variable  # => :value

instance_eval sets self to object for the duration of the code block:

object.instance_eval { self == object }  # => true

The receiver is also passed to the block as its only argument:

object.instance_eval { |argument| argument == object }  # => true

The instance_exec method differs in this regard: it passes its arguments to the block instead.

object.instance_exec :@variable do |name|
  instance_variable_get name  # => :value