Inheritance (sub class from Unit) no longer works

Fixes #339

.rubocop.yml

@@ -24,3 +24,5 @@ Style/FormatString:
   Enabled: false
 Style/DateTime:
   Enabled: false
+Metrics/ClassLength:
+  Enabled: false

Gemfile.lock

@@ -175,6 +175,7 @@ GEM
 PLATFORMS
   arm64-darwin-21
   arm64-darwin-22
+  arm64-darwin-23
   java
   universal-java-11
   universal-java-18
@@ -203,4 +204,4 @@ DEPENDENCIES
   yard
 
 BUNDLED WITH
-   2.4.19
+   2.5.3

lib/ruby_units/unit.rb

@@ -1,6 +1,6 @@
 require 'date'
 module RubyUnits
-  # Copyright 2006-2023
+  # Copyright 2006-2024
   # @author Kevin C. Olbrich, Ph.D.
   # @see https://github.com/olbrich/ruby-units
   #
@@ -165,6 +165,17 @@ class << self
 
     # Class Methods
 
+    # Callback triggered when a subclass is created. This properly sets up the internal variables, and copies
+    # definitions from the parent class.
+    #
+    # @param [Class] subclass
+    def self.inherited(subclass)
+      super
+      subclass.definitions = definitions.dup
+      subclass.instance_variable_set(:@kinds, @kinds.dup)
+      subclass.setup
+    end
+
     # setup internal arrays and hashes
     # @return [Boolean]
     def self.setup

spec/ruby_units/subclass_spec.rb

@@ -0,0 +1,31 @@
+RSpec.describe 'Subclass' do
+  subject(:subclass) { Class.new(RubyUnits::Unit) }
+
+  it 'can be subclassed' do
+    expect(subclass).to be < RubyUnits::Unit
+  end
+
+  it 'can be instantiated' do
+    expect(subclass.new('1 m')).to be_a(RubyUnits::Unit)
+  end
+
+  it 'compares to the parent class' do
+    expect(subclass.new('1 m')).to eq(RubyUnits::Unit.new('1 m'))
+  end
+
+  it 'can be added to another subclass instance' do
+    expect(subclass.new('1 m') + subclass.new('1 m')).to eq(RubyUnits::Unit.new('2 m'))
+  end
+
+  it 'returns a subclass object when added to another instance of a subclass' do
+    expect(subclass.new('1 m') + subclass.new('1 m')).to be_an_instance_of(subclass)
+  end
+
+  it 'returns an instance of the parent class when added to another instance of a subclass' do
+    expect(RubyUnits::Unit.new('1 m') + subclass.new('1 m')).to be_an_instance_of(RubyUnits::Unit)
+  end
+
+  it 'returns an instance of the subclass when added to an instance of the parent class' do
+    expect(subclass.new('1 m') + RubyUnits::Unit.new('1 m')).to be_an_instance_of(subclass)
+  end
+end