Use fmt::formatter specialization for std::reference_wrapper to avoid…

… undefined behavior Signed-off-by: Vladislav Shchapov <vladislav@shchapov.ru>
fmtlib · Sep 18, 2024 · 0d8da58 · 0d8da58
1 parent ed8f8be
commit 0d8da58
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Showing 2 changed files with 15 additions and 10 deletions.
diff --git a/include/fmt/std.h b/include/fmt/std.h
@@ -692,15 +692,17 @@ 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
+FMT_EXPORT
+template <typename T, typename Char>
+struct formatter<std::reference_wrapper<T>, Char,
+                 enable_if_t<is_formattable<remove_cvref_t<T>, Char>::value>>
+    : formatter<remove_cvref_t<T>, Char> {
+  template <typename FormatContext>
+  auto format(const std::reference_wrapper<T>& ref, FormatContext& ctx) const
+      -> decltype(ctx.out()) {
+    return formatter<remove_cvref_t<T>, Char>::format(ref.get(), ctx);
+  }
+};
 
+FMT_END_NAMESPACE
 #endif  // FMT_STD_H_
diff --git a/test/std-test.cc b/test/std-test.cc
@@ -399,4 +399,7 @@ TEST(std_test, format_shared_ptr) {
 TEST(std_test, format_reference_wrapper) {
   int num = 35;
   EXPECT_EQ("35", fmt::to_string(std::cref(num)));
+
+  std::atomic<bool> b(true);
+  EXPECT_EQ(fmt::format("{}", std::cref(b)), "true");
 }