1 files changed, 16 insertions, 5 deletions
diff --git a/spec/ruby/core/kernel/printf_spec.rb b/spec/ruby/core/kernel/printf_spec.rb
index 4592c08aaf..50939b3794 100644
--- a/spec/ruby/core/kernel/printf_spec.rb
+++ b/spec/ruby/core/kernel/printf_spec.rb
@@ -1,11 +1,10 @@
 require_relative '../../spec_helper'
 require_relative 'fixtures/classes'
 require_relative 'shared/sprintf'
-require "stringio"
 
 describe "Kernel#printf" do
   it "is a private method" do
-    Kernel.should have_private_instance_method(:printf)
+    Kernel.private_instance_methods(false).should.include?(:printf)
   end
 end
 
@@ -33,10 +32,23 @@ describe "Kernel.printf" do
     Kernel.printf(object, "%s", "string")
   end
 
+  it "calls #to_str to convert the format object to a String" do
+    object = mock('format string')
+    object.should_receive(:to_str).and_return("to_str: %i")
+    $stdout.should_receive(:write).with("to_str: 42")
+    Kernel.printf($stdout, object, 42)
+  end
+end
+
+describe "Kernel.printf" do
   describe "formatting" do
+    before :each do
+      require "stringio"
+    end
+
     context "io is specified" do
       it_behaves_like :kernel_sprintf, -> format, *args {
-        io = StringIO.new
+        io = StringIO.new(+"")
         Kernel.printf(io, format, *args)
         io.string
       }
@@ -45,9 +57,8 @@ describe "Kernel.printf" do
     context "io is not specified" do
       it_behaves_like :kernel_sprintf, -> format, *args {
         stdout = $stdout
-
         begin
-          $stdout = io = StringIO.new
+          $stdout = io = StringIO.new(+"")
           Kernel.printf(format, *args)
           io.string
         ensure