A Ruby struct solves one simple problem:
How can you create a class that is used for storing data & not for its methods?
Point with two coordinates (
You wouldn’t want to create a whole class just to store x & y together!
In this post:
You’ll learn how to create structs, the difference between a Ruby Struct & OpenStruct, and a few things you should watch out for!
A struct is a way to create a new class for the whole purpose of being a data container.
You can create an Struct by passing a list of symbols that’ll become the instance variables of this class.
They will have accessors defined by default, both for reading & writing.
Here’s an example:
person = Struct.new(:name, :age, :gender)
Now you can create new objects of this class with
john = person.new "john", 30, "M" puts john.age puts john.class
There are some differences with a “normal” class that you should be aware of.
For example, you may have noticed that the class of our
john object is just “Class”…
…to change this, you can do one of the following:
# Option 1 - Assign to a constant Person = Struct.new(:name, :age, :gender) # Option 2 - Subclass class Person < Struct.new(:name, :age, :gender) end
Both of these options will cause your new objects to have the class name you want.
Another caveat with Struct-generated classes... they won't enforce the correct number of arguments for the constructor.
With a proper class you would see this error:
ArgumentError: wrong number of arguments (0 for 3)
But if you are using a Struct the missing arguments will be nil:
Person.new("peter") # struct Person name="peter", age=nil, gender=nil
Keep this in mind when working with Struct objects!
Let's say that you're reading lines from a file & each line represents one item.
200 /login 18:00 404 /bacon 18:03 200 /books 18:04
To make it easier to work with this data you want to create objects.
You can do it like this:
LogEntry = Struct.new(:status, :url, :time) LogEntry.new(200, '/books', '18:04')
But to make it extra clear what every argument represents you want to use keyword arguments.
Ruby 2.5 added support for keywords arguments in Struct objects.
Here's how to use it:
LogEntry = Struct.new(:status, :url, :time, keyword_init: true) LogEntry.new(status: 200, url: '/books', time: '18:04')
Now you can parse your file & convert it into
If you just need a one-off object, then you should consider using OpenStruct instead.
Here's an example:
require 'ostruct' cat = OpenStruct.new(color: 'black') puts cat.class puts cat.color
The main difference with Struct is that it only produces objects, in other words, you can't call
cat.new in the example above to get more objects like it.
As long as you are aware of the special characteristics of each of these clases you will be fine.
Now go and start coding!