Add support for C++20 format API

.github/workflows/build.yml

@@ -35,7 +35,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        os: [ubuntu-22.04, ubuntu-20.04]
+        os: [ubuntu-24.04, ubuntu-22.04]
         cxx: [20]
         build_type: ["RelWithDebInfo"]
         llvm_version:
@@ -56,8 +56,6 @@ jobs:
       #     max-size: 256M
       - name: "update APT database"
         run: sudo apt -q update
-
-
       - name: Install clang
         run: |
           wget https://apt.llvm.org/llvm.sh
@@ -78,6 +76,5 @@ jobs:
               -DCMAKE_BUILD_TYPE=${{matrix.build_type}}
       - name: "build"
         run: cmake --build build  --parallel 3
-
       - name: "run test"
         run: ./build/test-boxed-cpp

include/boxed-cpp/boxed.hpp

@@ -207,19 +207,30 @@ struct hash<boxed::detail::boxed<T, U>>
 } // namespace std
 // {{{ fmtlib integration
 // clang-format off
-#if __has_include(<fmt/format.h>)
+#if __has_include(<format>)
+#include <format>
+// clang-format on
 
+template <typename Type, typename Tag>
+struct std::formatter<boxed::detail::boxed<Type, Tag>>: std::formatter<Type>
+{
+    auto format(boxed::detail::boxed<Type, Tag> const& val, auto& ctx) const
+    {
+        return std::formatter<Type>::format(val.value, ctx);
+    }
+};
+#elif __has_include(<fmt/format.h>)
+
+// clang-format off
 #include <fmt/format.h>
 // clang-format on
 
 template <typename Type, typename Tag>
-struct fmt::formatter<boxed::detail::boxed<Type, Tag>>
+struct fmt::formatter<boxed::detail::boxed<Type, Tag>>: fmt::formatter<Type>
 {
-    constexpr auto parse(fmt::format_parse_context& ctx) { return ctx.begin(); }
-
     auto format(boxed::detail::boxed<Type, Tag> const& val, fmt::format_context& ctx) const
     {
-        return fmt::format_to(ctx.out(), "{}", val.value);
+        return fmt::formatter<Type>::format(val.value, ctx);
     }
 };
 

test-boxed-cpp.cpp

@@ -189,7 +189,7 @@ struct Wrap
 {
     overload<Ts...> func_wrap;
 
-    Wrap(Ts... funcs): func_wrap(funcs...) {}
+    Wrap(Ts... funcs): func_wrap { funcs... } {}
 
     template <typename... Args>
     auto operator()(Args... args)
@@ -215,3 +215,24 @@ TEST_CASE("advanced")
     REQUIRE(x_coord(rho, theta, phi) == x_coord(theta, phi, rho));
     REQUIRE(x_coord(phi, theta, rho) == x_coord(phi, theta, rho));
 }
+
+#ifdef __has_include
+    #if __has_include(<version>)
+        #include <version>
+    #endif
+#endif
+
+#if defined(__cpp_lib_format) // && __cpp_lib_format >= 202207LL
+TEST_CASE("formatter")
+{
+    auto constexpr l = Length { 3 };
+    auto constexpr f = From { 2 };
+    auto constexpr t = To { 4 };
+    auto constexpr bd = BoxedDouble { 3.14 };
+
+    REQUIRE(std::format("{}", l) == "3");
+    REQUIRE(std::format("{}", f) == "2");
+    REQUIRE(std::format("{}", t) == "4");
+    REQUIRE(std::format("{}", bd) == "3.14");
+}
+#endif