instance_eval vs. class_eval context

[Andrew Gibson]

Is it the same context when a class is the receiver of instance_eval or
class_eval/module_eval?

 

[Arnaud Bergeron]

Is it the same context when a class is the receiver of instance_eval or
class_eval/module_eval?

No. In case of instance_eval you will be in the class, and in the
case of (class|module)_eval you will be in the class of the class
(which is Class) [Hurray for multiple meanings to a word!]

 

[ts]

A> No. In case of instance_eval you will be in the class, and in the
A> case of (class|module)_eval you will be in the class of the class
A> (which is Class) [Hurray for multiple meanings to a word!]

no, not really. ruby use self and internally ruby_class which give it
where it can define method when it find keyword like 'def', 'alias', ...

With obj.instance_eval ruby will make
* self = obj , ruby_class = obj singleton class

This mean that inside instance_eval it will define singleton method

#instance_eval work with any object, but a class is a little special
because it can be seen as an object or as a class. This is why
#module_eval, #class_eval exist.

With obj.class_eval ruby will make

* self = obj, ruby_class = obj

This mean that inside class_eval ruby will define instance method.

One way to see it

moulon% ruby -e 'class A; end; A.instance_eval{ p self; def a() puts "A::a" end}; A.a'
A
A::a
moulon%

moulon% ruby -e 'class A; end; A.class_eval{ p self; def a() puts "A#a" end}; A.new.a'
A
A#a
moulon%



Guy Decoux

 

[James Edward Gray II]

A> No. In case of instance_eval you will be in the class, and in the
A> case of (class|module)_eval you will be in the class of the class
A> (which is Class) [Hurray for multiple meanings to a word!]

no, not really. ruby use self and internally ruby_class which give it
where it can define method when it find keyword like 'def',
'alias', ...

With obj.instance_eval ruby will make
* self = obj , ruby_class = obj singleton class

This mean that inside instance_eval it will define singleton method

#instance_eval work with any object, but a class is a little special
because it can be seen as an object or as a class. This is why
#module_eval, #class_eval exist.

With obj.class_eval ruby will make

* self = obj, ruby_class = obj

This mean that inside class_eval ruby will define instance method.

One way to see it

moulon% ruby -e 'class A; end; A.instance_eval{ p self; def a()
puts "A::a" end}; A.a'
A
A::a
moulon%

moulon% ruby -e 'class A; end; A.class_eval{ p self; def a() puts
"A#a" end}; A.new.a'
A
A#a
moulon%

I've always had trouble understanding this and for some reason this
is the email that finally clicked all the pieces into place for me.
Thanks Guy!

James Edward Gray II