format std::reference_wrapper

Author: yfeldblum

include/fmt/std.h

@@ -17,6 +17,7 @@
 #  include <complex>
 #  include <cstdlib>
 #  include <exception>
+#  include <functional>
 #  include <memory>
 #  include <thread>
 #  include <type_traits>
@@ -692,4 +693,14 @@ template <typename T, typename Char> struct formatter<std::complex<T>, Char> {
 };
 
 FMT_END_NAMESPACE
+
+namespace std {
+
+template <typename T>
+constexpr auto format_as(std::reference_wrapper<T> ref) -> T& {
+  return ref.get();
+}
+
+}  // namespace std
+
 #endif  // FMT_STD_H_

test/std-test.cc

@@ -395,3 +395,8 @@ TEST(std_test, format_shared_ptr) {
   EXPECT_EQ(fmt::format("{}", fmt::ptr(sp.get())),
             fmt::format("{}", fmt::ptr(sp)));
 }
+
+TEST(std_test, format_reference_wrapper) {
+  int num = 35;
+  EXPECT_EQ("35", fmt::to_string(std::cref(num)));
+}