class_eval, instance_eval, eval

A couple of days ago I attended an interesting discussion of metaprogramming by Arild Shirazi at a meeting of the Northern Virginia Ruby User Group. Arild showed how he used metaprogramming (class_eval in particular) to generate functions whose names would only be known at runtime. His talk was very effective at reminding me that I don’t know as much about metaprogramming as I thought!

(Feel free to offer suggestions and corrections, and I’ll try to update the article accordingly.)

Dave Thomas, in his excellent Advanced Ruby training, emphasizes the value of knowing just who self is at any point in the code. (For a good time, bounce around an rspec source file and try to guess what self is in various places…).

class_eval provides an alternate way to define characteristics of a class. It should be used only when absolutely necessary. The only legitimate use I can think of is when the necessary code cannot be known until runtime.

Knowing very little about class_eval, I assumed that it changed self to be the class of the current value of self. I was wrong. class_eval doesn’t change self at all; in fact, in this respect it functions identically to eval:

> class ClassEvalExample
>   class_eval "def foo; puts 'foo'; end"
> end
> ClassEvalExample.new.foo
foo

eval appears to do the exact same thing:

> class EvalExample
>   eval "def foo; puts 'foo'; end"
> end
> EvalExample.new.foo
foo

There is a difference, though, when you call them outside the class definition. For a class C, you can call C.class_eval, but not C.eval:

> class C1; end
> C1.class_eval "def foo; puts 'foo'; end"
> C1.new.foo
foo

> class C2; end
> C2.eval "def foo; puts 'foo'; end"
NoMethodError: private method `eval' called for C2:Class
	from (irb):2
	from :0

If class_eval could be used to define an instance method on a class in a class definition outside a function, what would happen if it were used inside a function, where self is no longer the class, but the instance of the class? Would it define a method on the singleton class (a.k.a. eigenclass)? Let’s try it:

 :001 > class D
 :002?>     def initialize
 :003?>         puts "In initialize"
 :004?>         class_eval "def foo; puts 'foo'; end"
 :005?>       end
 :006?>   end
 => nil
 :007 >
 :008 >   D.new.foo
In initialize
NoMethodError: undefined method `class_eval' for #
	from (irb):4:in `initialize'
	from (irb):8:in `new'
	from (irb):8
	from :0

No, this didn’t work…but wait a minute, isn’t class_eval a Kernel method? Let’s find out:

> Kernel.methods.include? 'class_eval'
=> true

Alas, I was asking the wrong question. I should have asked if Kernel had an instance method named class_eval:

> Kernel.instance_methods.include? 'class_eval'
=> false

It doesn’t, but Class does:

> Class.instance_methods.include? 'class_eval'
=> true

…which is why the Kernel.methods.include? above worked.

Although class_eval didn’t work, instance_eval will work:

> class F
>   def initialize
>     instance_eval 'def foo; puts "object id is #{object_id}"; end'
>   end
> end
> F.new.foo
object id is 2149391220
> F.new.foo
object id is 2149362060

To illustrate that foo has not been created as a class or member function on class F, but only on object f:

> F.methods(false).include? 'foo'
 => false
> F.instance_methods(false).include? 'foo'
 => false
> f = F.new
> f.methods(false).include? 'foo'
 => true

Could eval be substituted for instance_eval in the same way as it was for class_eval? Let’s find out…

>   class F2
>     def initialize
>         eval 'def foo; puts "object id is #{object_id}"; end'
>     end
> end
> F2.new.foo
object id is 2149180440

Apparently, yes. However, similarly to class_eval, instance_eval can be called outside of a class definition, but eval cannot:

> class C; end
> c = C.new
> c.instance_eval 'def foo; puts "object id is #{object_id}"; end'
> c.foo
object id is 2149446940

> class D; end
> d = D.new
> d.eval 'def foo; puts "object id is #{object_id}"; end'
NoMethodError: private method `eval' called for #
	from (irb):7
	from :0

Hmmm, I wonder, if we can define a function using the eval methods, can we also declare an instance variable?:

# First, class_eval:
> class E
>   class_eval "@@foo = 123"
>   def initialize; puts "@@foo = #{@@foo}"; end
>   end
> E.new
@@foo = 123

# Next, instance_eval:
> o = Object.new
> o.instance_eval '@var = 456'
> o.instance_eval 'def foo; puts "@var = #{@var}"; end'
> o.foo
@var = 456

What’s interesting is that we created instance variable var in instance o, but its class Object knows nothing about this new variable. In the data storage world, this would be analogous to using a document store such as MongoDB and adding a variable to a single document, unlike in an RDBMS where you would have to add it to the table definition and include it in all rows of the table.

Techniques such as these are cool and powerful, but are not without cost. If your code accesses a function or variable that is not defined in a standard class definition, the reader may have a hard time tracking down the creation and meaning of that function or variable. We should be kind to our fellow developers and use these techniques only when absolutely necessary.

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