Metaprogramming Ruby 2

instance_eval()

Where you learn another way to mix code and bindings at will.

The following program demonstrates BasicObject#instance_eval, which evaluates a block in the context of an object:

blocks/instance_eval.rb

class MyClass
  def initialize
    @v = 1
  end
end

obj = MyClass.new

obj.instance_eval do
  self        # => #<MyClass:0x3340dc @v=1>
  @v          # => 1
end

The block is evaluated with the receiver as self, so it can access the receiver’s private methods and instance variables, such as @v. Even if instance_eval changes self, the block that you pass to instance_eval can still see the bindings from the place where it’s defined, like any other block:

v = 2
obj.instance_eval { @v = v }
obj.instance_eval { @v }      # => ...