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Ruby Struct at Depth

Struct.new builds a small class from a list of field names. This lesson uses keyword_init so every object is built with named fields.

Building a point with named fields

The Point class is built from the symbols x and y. keyword_init: true makes the new call name each field.

Point = Struct.new(:x, :y, keyword_init: true)
point = Point.new(x: 3, y: 4)
puts point.x
puts point.y

Listing the fields with members

members answers the field names as symbols. The order matches the order given to Struct.new.

Point = Struct.new(:x, :y, keyword_init: true)
p Point.members

Comparing structs by value

Two points with the same field values compare as equal. A point with a different x value does not.

Point = Struct.new(:x, :y, keyword_init: true)
first = Point.new(x: 3, y: 4)
second = Point.new(x: 3, y: 4)
third = Point.new(x: 5, y: 4)
puts first == second
puts first == third

Reading the values as a hash

to_h pairs each field name with its current value. The keys are the same symbols passed to Struct.new.

Point = Struct.new(:x, :y, keyword_init: true)
point = Point.new(x: 3, y: 4)
p point.to_h

Example

City = Struct.new(:name, :population, keyword_init: true)
city = City.new(name: "Oslo", population: 700)
city.population = 710
puts city.name
puts city.population
p city.to_h

Key points

  • Struct.new takes field names and returns a new class.
  • keyword_init: true makes construction use named fields.
  • members lists the field names in order.
  • Each field gets a reader with the same name.
  • Two structs are == when their fields hold equal values.
  • to_h builds a hash from the field names and values.
  • A struct is mutable, so a field can be set after construction.

In this example, City is a struct with name and population fields built with keyword_init. The population field is set to a new value after construction, which shows that a struct stays mutable. The final line reads the updated values back as a hash.

Try it Yourself

Edit the code, then press Run.

Running is simulated

Exercise

Two structs from the same class hold the same field values. What does == between them give?