How to Use Struct & OpenStruct in Ruby

What is a Struct in Ruby?

A struct is a built-in Ruby class, it’s used to create new classes which produce value objects. A value object is used to store related attributes together.

Here’s an example:

A Point with two coordinates (x & y).

You can represent this data in many different ways.

Like:

  • An array [10, 20]
  • A hash { x: 10, y: 10 }
  • An object Point.new(10, 20)

If you’re going to have more than one Point, it’s often a good practice to use the object approach.

But…

You don’t want to create a whole class just to store these two values together!

Using a Struct solves this problem.

Let’s do this!

How to Create Structs in Ruby

You can create a Struct by calling new & passing in a list of symbols that’ll become the instance variables of this class.

They’ll have accessors defined by default, both for reading & writing.

Here’s an example:

Person = Struct.new(:name, :age, :gender)

Now you can create a objects like this:

john  = Person.new "john", 30, "M"
david = Person.new "david", 25, "M"

Alternative Way

You may find this other way of creating structs in the wild.

It looks like this:

class Person < Struct.new(:name, :age, :gender)
end

I don't recommend this.

But if you find it out there now you know what it does.

Big Benefit

A good thing about structs if that you can compare them directly, based on their attributes.

Example:

john == john
# true

If this was a regular Ruby object you would have to define the == method yourself.

This is what we call a "value object".

How to Use Ruby Structs

One of the major benefits from using a struct over an array, or a hash, is that you get to access the struct members using methods.

For example:

puts john.age
# 30

puts david.gender
# "M"

This is helpful because if you have an array of objects, you can use methods like max, select, sum, etc.

Example:

[john, david].max_by(&:age)

Nice!

WARNING: Structs Can Be Tricky

Another caveat with struct-generated classes is that...

They won't enforce the correct number of arguments for the constructor!

Let me explain.

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!

Another Weird Thing

Take a look at this...

Struct.new(:a).ancestors
[#<Class:0x29b1040>, Struct, Enumerable, Object, Kernel, BasicObject]

This #<Class:0x29b1040> is an anonymous class added to structs, also notice the enumerable module, which allows you to call methods like each & map.

Named Parameters in Ruby 2.5

Let's say that you're reading lines from a file & each line represents one item.

Example:

200 /login 18:00
404 /bacon 18:03
200 /books 18:04

You work more easily with this data if you create a custom class.

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 may want to use keyword arguments.

Good news!

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 LogEntry objects!

How to Use OpenStruct

If you just need a one-off object, then you should consider using OpenStruct instead.

Code example:

require 'ostruct'

cat = OpenStruct.new(color: 'black')

puts cat.class
puts cat.color

Notice how you have to require ostruct to have access to this class.

Warning: OpenStruct is slow and you shouldn't use it on production apps, according to schneems on this reddit comment. Also I found this blog post that has some benchmarks supporting this.

Struct vs OpenStruct

The difference between Struct & OpenStruct:

  • Struct creates a new class with predefined attributes, equality method (==) & enumerable
  • OpenStruct creates a new object with the given attributes

An OpenStruct is a fancy Hash object, while a Struct is like creating a new class from a template.

Watch Video Tutorial

Conclusion

You've learned about Ruby Struct & OpenStruct! As long as you are aware of the special characteristics of each of these classes you'll be fine.

Ruby Struct Mindmap

Now it's your turn to practice 🙂