the difference between class Foo, Foo.class_eval, and Foo.instance_eval

code.rb

module TopLevel

  class Foo
    puts "class Foo"
    puts "self: #{self}"
    puts "nesting: #{Module.nesting.inspect}"
    puts "Method definition context: #{self}"
  end

  puts

  Foo.class_eval do
    puts "Foo.class_eval"
    puts "self: #{self}"
    puts "nesting: #{Module.nesting.inspect}"
    puts "Method definition context: #{self}"
  end

  puts

  class << Foo
    puts "class << Foo"
    puts "self: #{self}"
    puts "nesting: #{Module.nesting.inspect}"
    puts "Method definition context: #{self}"
  end

  puts

  Foo.instance_eval do
    puts "class << Foo"
    puts "self: #{self}"
    puts "nesting: #{Module.nesting.inspect}"
    puts "Method definition context: #{class << self; self; end}"
  end

  puts

  Foo.new.instance_eval do
    puts "Foo.new"
    puts "self: #{self}"
    puts "nesting: #{Module.nesting.inspect}"
    puts "Method definition context: #{class << self; self; end}"
  end

end

output.txt

class Foo
self: TopLevel::Foo
nesting: [TopLevel::Foo, TopLevel]
Method definition context: TopLevel::Foo

Foo.class_eval
self: TopLevel::Foo
nesting: [TopLevel]
Method definition context: TopLevel::Foo

class << Foo
self: #<Class:TopLevel::Foo>
nesting: [#<Class:TopLevel::Foo>, TopLevel]
Method definition context: #<Class:TopLevel::Foo>

class << Foo
self: TopLevel::Foo
nesting: [TopLevel]
Method definition context: #<Class:TopLevel::Foo>

Foo.new
self: #<TopLevel::Foo:0x10012be38>
nesting: [TopLevel]
Method definition context: #<Class:#<TopLevel::Foo:0x10012be38>>