Add Support.class_of for extracting class of any object

Changelog.md

@@ -5,6 +5,8 @@ Enhancements:
 
 * Improve compatibility with `--enable-frozen-string-literal` option
   on Ruby 2.3+. (Pat Allan, #320)
+* Add `Support.class_of` for extracting class of any object.
+  (Yuji Nakayama, #325)
 
 Bug Fixes:
 

lib/rspec/support.rb

@@ -73,6 +73,16 @@ def self.method_handle_for(object, method_name)
       end
     end
 
+    # @api private
+    #
+    # Used internally to get a class of a given object, even if it does not respond to #class.
+    def self.class_of(object)
+      object.class
+    rescue NoMethodError
+      singleton_class = class << object; self; end
+      singleton_class.ancestors.find { |ancestor| !ancestor.equal?(singleton_class) }
+    end
+
     # A single thread local variable so we don't excessively pollute that namespace.
     def self.thread_local_data
       Thread.current[:__rspec] ||= {}

lib/rspec/support/object_formatter.rb

@@ -199,8 +199,7 @@ def inspect
         end
 
         def klass
-          singleton_class = class << object; self; end
-          singleton_class.ancestors.find { |ancestor| !ancestor.equal?(singleton_class) }
+          Support.class_of(object)
         end
 
         # http://stackoverflow.com/a/2818916

spec/rspec/support/object_formatter_spec.rb

@@ -178,7 +178,7 @@ def with_delegate_loaded
         end
       end
 
-      context 'with an object that does not respond to #inspect such as BasicObject' do
+      context 'with an object that does not respond to #class and #inspect such as BasicObject' do
         subject(:output) do
           ObjectFormatter.format(input)
         end
@@ -197,7 +197,7 @@ def self.to_s
               'BasicObject'
             end
 
-            undef inspect, respond_to?
+            undef class, inspect, respond_to?
           end
         end
 

spec/rspec/support_spec.rb

@@ -118,6 +118,73 @@ def method_missing(name, *args, &block)
       end
     end
 
+    describe '.class_of' do
+      subject(:klass) do
+        Support.class_of(object)
+      end
+
+      context 'with a String instance' do
+        let(:object) do
+          'foo'
+        end
+
+        it { should equal(String) }
+      end
+
+      context 'with a BasicObject instance' do
+        let(:object) do
+          basic_object_class.new
+        end
+
+        let(:basic_object_class) do
+          defined?(BasicObject) ? BasicObject : fake_basic_object_class
+        end
+
+        let(:fake_basic_object_class) do
+          Class.new do
+            def self.to_s
+              'BasicObject'
+            end
+
+            undef class, inspect, respond_to?
+          end
+        end
+
+        it { should equal(basic_object_class) }
+      end
+
+      context 'with nil' do
+        let(:object) do
+          nil
+        end
+
+        it { should equal(NilClass) }
+      end
+
+      context 'with an object having a singleton class' do
+        let(:object) do
+          object = 'foo'
+
+          def object.some_method
+          end
+
+          object
+        end
+
+        it 'returns its non-singleton ancestor class' do
+          expect(klass).to equal(String)
+        end
+      end
+
+      context 'with a Class instance' do
+        let(:object) do
+          String
+        end
+
+        it { should equal(Class) }
+      end
+    end
+
     describe "failure notification" do
       before { @failure_notifier = RSpec::Support.failure_notifier }
       after  { RSpec::Support.failure_notifier = @failure_notifier }