Add fmt::underlying for enum classes

doc/api.rst

@@ -321,6 +321,8 @@ Utilities
 .. doxygenfunction:: fmt::ptr(const std::unique_ptr<T> &p) -> const void*
 .. doxygenfunction:: fmt::ptr(const std::shared_ptr<T> &p) -> const void*
 
+.. doxygenfunction:: fmt::underlying(Enum e) -> typename std::underlying_type<Enum>::type
+
 .. doxygenfunction:: fmt::to_string(const T &value) -> std::string
 
 .. doxygenfunction:: fmt::to_string_view(const Char *s) -> basic_string_view<Char>

include/fmt/format.h

@@ -2675,6 +2675,22 @@ template <typename T> auto ptr(const std::shared_ptr<T>& p) -> const void* {
   return p.get();
 }
 
+/**
+  \rst
+  Converts ``e`` to the underlying type.
+
+  **Example**::
+
+    enum class color { red, green, blue };
+    auto s = fmt::format("{}", fmt::underlying(color::red));
+  \endrst
+ */
+template <typename Enum>
+constexpr auto underlying(Enum e) noexcept ->
+    typename std::underlying_type<Enum>::type {
+  return static_cast<typename std::underlying_type<Enum>::type>(e);
+}
+
 class bytes {
  private:
   string_view data_;

test/format-test.cc

@@ -1409,8 +1409,14 @@ TEST(format_test, format_pointer) {
   EXPECT_EQ("0x0", fmt::format("{}", nullptr));
 }
 
+enum class color { red, green, blue };
+
+TEST(format_test, format_enum_class) {
+  EXPECT_EQ(fmt::format("{}", fmt::underlying(color::red)), "0");
+}
+
 TEST(format_test, format_string) {
-  EXPECT_EQ("test", fmt::format("{0}", std::string("test")));
+  EXPECT_EQ(fmt::format("{0}", std::string("test")), "test");
   EXPECT_THROW((void)fmt::format(fmt::runtime("{:x}"), std::string("test")),
                fmt::format_error);
 }