miceda

if defined? “what does it mean?!”

Plowing through various Rails and Ruby source code, I keep running across the method defined?. However, a quick look though the API turned up nothing on it. Where’s it coming from? I can’t find it in Module, Kernel, or Object…time to explore! A quick Google search shows RedHanded solved this mystery way back in 2004.

As it turns out defined? lives in eval.c. The logic responsible for defined? actually has the identity is_defined starting around line 2249. As Redhanded explains: “defined? takes its argument and simply queries the symbol table to see if it is defined. If it is, you get a simple string of identification”:

$ x = 10
=> 10
$ defined? x
=> "local-variable"
$ def hello; puts "Hello" end
$ defined? hello
=> "method"
$ class Test; end
$ defined? Test
=>"constant"

Examples of use

Since I haven’t had the opportunity to use it yet, Here’s a couple examples I’ve found:

In Rails initializer.rb uses it to test for the existence of a constant:

   def initialize_logger
     return if defined?(RAILS_DEFAULT_LOGGER)

The method real_connect in mysql.rb uses it to check UNIXSocket:

 if (host == nil or host == "localhost")
    and defined? UNIXSocket then ...

Cliff notes version

1. defined? is classified as an operator not a method
2. defined? returns nil if the argument passed to it is NOT defined
3. defined? returns a string identification of the argument passed as defined in the symbol table

Now, I wonder where and how the method is_defined is translated into the operator defined?