Refactor Currency#new method

lib/money/currency.rb

@@ -18,6 +18,7 @@ class Currency
 
     # Keeping cached instances in sync between threads
     @@mutex = Mutex.new
+    @@instances = {}
 
     # Thrown when a Currency has been registered without all the attributes
     # which are required for the current action.
@@ -34,18 +35,17 @@ def initialize(method, currency, attribute)
     class UnknownCurrency < ArgumentError; end
 
     class << self
-      alias_method :original_new, :new
       def new(id)
         id = id.to_s.downcase
         unless stringified_keys.include?(id)
           raise UnknownCurrency, "Unknown currency '#{id}'"
         end
 
-        @@mutex.synchronize { instances[id] } || super
+        _instances[id] || @@mutex.synchronize { _instances[id] ||= super }
       end
 
-      def instances
-        @instances ||= Hash.new { |h, k| h[k] = original_new(k) }
+      def _instances
+        @@instances
       end
 
       # Lookup a currency with given +id+ an returns a +Currency+ instance on
@@ -168,7 +168,7 @@ def stringified_keys
       # @option delimiter [String] character between each thousands place
       def register(curr)
         key = curr.fetch(:iso_code).downcase.to_sym
-        @@mutex.synchronize { instances.delete(key.to_s) }
+        @@mutex.synchronize { _instances.delete(key.to_s) }
         @table[key] = curr
         @stringified_keys = stringify_keys
       end

spec/currency_spec.rb

@@ -170,7 +170,8 @@ def to_s
 
 
     describe "#initialize" do
-      before { Currency.instances.clear }
+      before { Currency._instances.clear }
+
       it "lookups data from loaded config" do
         currency = Currency.new("USD")
         expect(currency.id).to                    eq :usd
@@ -185,6 +186,13 @@ def to_s
         expect(currency.smallest_denomination).to eq 1
       end
 
+      it 'caches instances' do
+        currency = Currency.new("USD")
+
+        expect(Currency._instances.length).to           eq 1
+        expect(Currency._instances["usd"].object_id).to eq currency.object_id
+      end
+
       it "raises UnknownCurrency with unknown currency" do
         expect { Currency.new("xxx") }.to raise_error(Currency::UnknownCurrency, /xxx/)
       end