allow to stream user defined types in a MemoryWriter (#456)

* allow to stream user defined types in a MemoryWriter

* fix indent

* follow Google C++ Style

* make code c++98 compatible

* fix macro usage

* disable ability to stream user defined types if not at least c++11

* fix for disable ability to stream user defined types if not at least c++11

* use FMT_STATIC_ASSERT

test/ostream-test.cc

@@ -111,6 +111,14 @@ std::ostream &operator<<(std::ostream &os, EmptyTest) {
   return os << "";
 }
 
+#if __cplusplus >= 201103L
+struct UserDefinedTest { int i = 42; };
+
+std::ostream &operator<<(std::ostream &os, const UserDefinedTest &u) {
+  return os << u.i;
+}
+#endif
+
 TEST(OStreamTest, EmptyCustomOutput) {
   EXPECT_EQ("", fmt::format("{}", EmptyTest()));
 }
@@ -129,6 +137,17 @@ TEST(OStreamTest, WriteToOStream) {
   EXPECT_EQ("foo", os.str());
 }
 
+#if __cplusplus >= 201103L
+TEST(OStreamTest, WriteUserDefinedTypeToOStream) {
+  std::ostringstream os;
+  fmt::MemoryWriter w;
+  UserDefinedTest u;
+  w << "The answer is " << u;
+  fmt::internal::write(os, w);
+  EXPECT_EQ("The answer is 42", os.str());
+}
+#endif
+
 TEST(OStreamTest, WriteToOStreamMaxSize) {
   std::size_t max_size = std::numeric_limits<std::size_t>::max();
   std::streamsize max_streamsize = std::numeric_limits<std::streamsize>::max();