instance_eval

Instance evaluation

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

object = Object.new

object.instance_eval do
  @variable = :value
end

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
end

Implementing with

Many languages feature a with statement that allows programmers to omit the receiver of method calls.

with can be easily emulated in Ruby using instance_eval:

def with(object, &block)
  object.instance_eval &block
end

The with method can be used to seamlessly execute methods on objects:

hash = Hash.new

with hash do
  store :key, :value
  has_key? :key       # => true
  values              # => [:value]
end

Syntax

• object.instance_eval ‘code’
• object.instance_eval ‘code’, ‘filename’
• object.instance_eval ‘code’, ‘filename’, ‘line number’
• object.instance_eval { code }
• object.instance_eval { |receiver| code }

Parameters

|Parameter|Details |---|---|---|---|---|---|---|---|---|--- |string|Contains the Ruby source code to be evaluated. |filename|File name to use for error reporting. |lineno|Line number to use for error reporting. |block|The block of code to be evaluated. |obj|The receiver is passed to the block as its only argument.