User-defined literals for format and named arguments

format.h

@@ -177,6 +177,16 @@ inline uint32_t clzll(uint64_t x) {
     TypeName& operator=(const TypeName&)
 #endif
 
+#ifndef FMT_USE_USER_DEFINED_LITERALS
+// All compilers which support UDLs also support variadic templates. This
+// makes the fmt::literals implementation easier. However, an explicit check
+// for variadic templates is added here just in case.
+# define FMT_USE_USER_DEFINED_LITERALS \
+   FMT_USE_VARIADIC_TEMPLATES && \
+   (FMT_HAS_FEATURE(cxx_user_literals) || \
+       (FMT_GCC_VERSION >= 407 && FMT_HAS_GXX_CXX11) || _MSC_VER >= 1900)
+#endif
+
 #ifndef FMT_ASSERT
 # define FMT_ASSERT(condition, message) assert((condition) && message)
 #endif
@@ -3005,6 +3015,49 @@ FMT_VARIADIC(int, printf, CStringRef)
 FMT_VARIADIC(int, fprintf, std::FILE *, CStringRef)
 }
 
+#if FMT_USE_USER_DEFINED_LITERALS
+namespace fmt {
+namespace internal {
+
+template <typename Char>
+struct UdlFormat {
+  const Char *str;
+
+  template <typename... Args>
+  auto operator()(Args && ... args) const
+                  -> decltype(format(str, std::forward<Args>(args)...)) {
+    return format(str, std::forward<Args>(args)...);
+  }
+};
+
+template <typename Char>
+struct UdlArg {
+  const Char *str;
+
+  template <typename T>
+  NamedArg<Char> operator=(T &&value) const {
+    return {str, std::forward<T>(value)};
+  }
+};
+
+} // namespace internal
+
+inline namespace literals {
+
+inline internal::UdlFormat<char>
+operator"" _format(const char *s, std::size_t) { return {s}; }
+inline internal::UdlFormat<wchar_t>
+operator"" _format(const wchar_t *s, std::size_t) { return {s}; }
+
+inline internal::UdlArg<char>
+operator"" _a(const char *s, std::size_t) { return {s}; }
+inline internal::UdlArg<wchar_t>
+operator"" _a(const wchar_t *s, std::size_t) { return {s}; }
+
+} // inline namespace literals
+} // namespace fmt
+#endif // FMT_USE_USER_DEFINED_LITERALS
+
 // Restore warnings.
 #if FMT_GCC_VERSION >= 406
 # pragma GCC diagnostic pop

test/format-test.cc

@@ -1602,3 +1602,25 @@ TEST(FormatTest, MaxArgs) {
             fmt::format("{}{}{}{}{}{}{}{}{}{}{}{}{}{}{}",
                         0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 'a', 'b', 'c', 'd', 'e'));
 }
+
+#if FMT_USE_USER_DEFINED_LITERALS
+using namespace fmt::literals;
+
+TEST(LiteralsTest, Format) {
+  EXPECT_EQ(format("{}c{}", "ab", 1), "{}c{}"_format("ab", 1));
+  EXPECT_EQ(format(L"{}c{}", L"ab", 1), L"{}c{}"_format(L"ab", 1));
+}
+
+TEST(LiteralsTest, NamedArg) {
+  EXPECT_EQ(format("{first}{second}{first}{third}",
+                   fmt::arg("first", "abra"), fmt::arg("second", "cad"),
+                   fmt::arg("third", 99)),
+            format("{first}{second}{first}{third}",
+                   "first"_a="abra", "second"_a="cad", "third"_a=99));
+  EXPECT_EQ(format(L"{first}{second}{first}{third}",
+                   fmt::arg(L"first", L"abra"), fmt::arg(L"second", L"cad"),
+                   fmt::arg(L"third", 99)),
+            format(L"{first}{second}{first}{third}",
+                   L"first"_a=L"abra", L"second"_a=L"cad", L"third"_a=99));
+}
+#endif // FMT_USE_USER_DEFINED_LITERALS