Ruby Struct.new doesn't work as expected, it needs extra .new

Question

I am working on a project and I found an interesting problem. I wanted to use Struct . I try to use it like this:

> e = Struct.new(:message, :whateve)
=> #<Class:0x007f881dd98b98>
e.message = "something"
NoMethodError: undefined method `message=' for #<Class:0x007f881dd98b98>
from (pry):5:in `__pry__'

It works fine if I add it like this: e = Struct.new(:message, :whateve).new so I am wondering what is the problem and needs the extra new? I am using ruby 2.2.1p85.

Answer 1

It happens, because Struct.new(:message, :whateve) just defines a new class:

e = Struct.new(:message, :whateve) # define new class
e.class
#=> Class
e.new #define this class instance
#=> #<struct message=nil, whateve=nil>