How to change the default value of a Struct attribute? - ruby

Question

According to the documentation unset attributes of Struct are set to nil:
unset parameters default to nil.
Is it possible to specify the default value for particular attributes?
For example, for the following Struct
Struct.new("Person", :name, :happy)
I would like the attribute happy to default to true rather than nil. How can I do this? If I do as follows
Struct.new("Person", :name, :happy = true)
I get
-:1: syntax error, unexpected '=', expecting ')'
Struct.new("Person", :name, :happy = true)
^
-:1: warning: possibly useless use of true in void context

Answer 1

This can also be accomplished by creating your Struct as a subclass, and overriding
initialize with default values as in the following example:
class Person < Struct.new(:name, :happy)
def initialize(name, happy=true); super end
end
On one hand, this method does lead to a little bit of boilerplate; on the other, it does what you're looking for nice and succinctly.
One side-effect (which may be either a benefit or an annoyance depending on your preferences/use case) is that you lose the default Struct behavior of all attributes defaulting to nil -- unless you explicitly set them to be so. In effect, the above example would make name a required parameter unless you declare it as name=nil

Answer 2

Following #rintaun's example you can also do this with keyword arguments in Ruby 2+
A = Struct.new(:a, :b, :c) do
def initialize(a:, b: 2, c: 3); super end
end
A.new
# ArgumentError: missing keyword: a
A.new a: 1
# => #<struct A a=1, b=2, c=3>
A.new a: 1, c: 6
# => #<struct A a=1, b=2, c=6>
UPDATE
The code now needs to be written as follows to work.
A = Struct.new(:a, :b, :c) do
def initialize(a:, b: 2, c: 3)
super(a, b, c)
end
end

Answer 3

I also found this:
Person = Struct.new "Person", :name, :happy do
def initialize(*)
super
self.location ||= true
end
end

Answer 4

#Linuxios gave an answer that overrides member lookup. This has a couple problems: you can't explicitly set a member to nil and there's extra overhead on every member reference. It seems to me you really just want to supply the defaults when initializing a new struct object with partial member values supplied to ::new or ::[].
Here's a module to extend Struct with an additional factory method that lets you describe your desired structure with a hash, where the keys are the member names and the values the defaults to fill in when not supplied at initialization:
# Extend stdlib Struct with a factory method Struct::with_defaults
# to allow StructClasses to be defined so omitted members of new structs
# are initialized to a default instead of nil
module StructWithDefaults
# makes a new StructClass specified by spec hash.
# keys are member names, values are defaults when not supplied to new
#
# examples:
# MyStruct = Struct.with_defaults( a: 1, b: 2, c: 'xyz' )
# MyStruct.new #=> #<struct MyStruct a=1, b=2, c="xyz"
# MyStruct.new(99) #=> #<struct MyStruct a=99, b=2, c="xyz">
# MyStruct[-10, 3.5] #=> #<struct MyStruct a=-10, b=3.5, c="xyz">
def with_defaults(*spec)
new_args = []
new_args << spec.shift if spec.size > 1
spec = spec.first
raise ArgumentError, "expected Hash, got #{spec.class}" unless spec.is_a? Hash
new_args.concat spec.keys
new(*new_args) do
class << self
attr_reader :defaults
end
def initialize(*args)
super
self.class.defaults.drop(args.size).each {|k,v| self[k] = v }
end
end.tap {|s| s.instance_variable_set(:#defaults, spec.dup.freeze) }
end
end
Struct.extend StructWithDefaults

Answer 5

Just add another variation:
class Result < Struct.new(:success, :errors)
def initialize(*)
super
self.errors ||= []
end
end

Answer 6

I think that the override of the #initialize method is the best way, with call to #super(*required_args).
This has an additional advantage of being able to use hash-style arguments. Please see the following complete and compiling example:
Hash-Style Arguments, Default Values, and Ruby Struct
# This example demonstrates how to create Ruby Structs that use
# newer hash-style parameters, as well as the default values for
# some of the parameters, without loosing the benefits of struct's
# implementation of #eql? #hash, #to_s, #inspect, and other
# useful instance methods.
#
# Run this file as follows
#
# > gem install rspec
# > rspec struct_optional_arguments.rb --format documentation
#
class StructWithOptionals < Struct.new(
:encrypted_data,
:cipher_name,
:iv,
:salt,
:version
)
VERSION = '1.0.1'
def initialize(
encrypted_data:,
cipher_name:,
iv: nil,
salt: 'salty',
version: VERSION
)
super(encrypted_data, cipher_name, iv, salt, version)
end
end
require 'rspec'
RSpec.describe StructWithOptionals do
let(:struct) { StructWithOptionals.new(encrypted_data: 'data', cipher_name: 'AES-256-CBC', iv: 'intravenous') }
it 'should be initialized with default values' do
expect(struct.version).to be(StructWithOptionals::VERSION)
end
context 'all fields must be not null' do
%i(encrypted_data cipher_name salt iv version).each do |field|
subject { struct.send(field) }
it field do
expect(subject).to_not be_nil
end
end
end
end