🚧 trying nodiscard attribute #1433

include/nlohmann/detail/macro_scope.hpp

@@ -37,6 +37,15 @@
     #define JSON_DEPRECATED
 #endif
 
+// allow for portable nodiscard warnings
+#if defined(__has_cpp_attribute) && __has_cpp_attribute(nodiscard)
+    #define JSON_NODISCARD [[nodiscard]]
+#elif defined(__has_cpp_attribute) && __has_cpp_attribute(gnu::warn_unused_result)
+    #define JSON_NODISCARD [[gnu::warn_unused_result]]
+#else
+    #define JSON_NODISCARD
+#endif
+
 // allow to disable exceptions
 #if (defined(__cpp_exceptions) || defined(__EXCEPTIONS) || defined(_CPPUNWIND)) && !defined(JSON_NOEXCEPTION)
     #define JSON_THROW(exception) throw exception

include/nlohmann/detail/macro_unscope.hpp

@@ -16,6 +16,7 @@
 #undef JSON_LIKELY
 #undef JSON_UNLIKELY
 #undef JSON_DEPRECATED
+#undef JSON_NODISCARD
 #undef JSON_HAS_CPP_14
 #undef JSON_HAS_CPP_17
 #undef NLOHMANN_BASIC_JSON_TPL_DECLARATION

include/nlohmann/json.hpp

@@ -317,6 +317,7 @@ class basic_json
 
     @since 2.1.0
     */
+    JSON_NODISCARD
     static basic_json meta()
     {
         basic_json result;
@@ -1489,6 +1490,7 @@ class basic_json
 
     @since version 1.0.0
     */
+    JSON_NODISCARD
     static basic_json array(initializer_list_t init = {})
     {
         return basic_json(init, false, value_t::array);
@@ -1532,6 +1534,7 @@ class basic_json
 
     @since version 1.0.0
     */
+    JSON_NODISCARD
     static basic_json object(initializer_list_t init = {})
     {
         return basic_json(init, false, value_t::object);
@@ -6051,6 +6054,7 @@ class basic_json
 
     @since version 2.0.3 (contiguous containers)
     */
+    JSON_NODISCARD
     static basic_json parse(detail::input_adapter&& i,
                             const parser_callback_t cb = nullptr,
                             const bool allow_exceptions = true)
@@ -6826,6 +6830,7 @@ class basic_json
            @a strict parameter since 3.0.0; added @a allow_exceptions parameter
            since 3.2.0
     */
+    JSON_NODISCARD
     static basic_json from_cbor(detail::input_adapter&& i,
                                 const bool strict = true,
                                 const bool allow_exceptions = true)
@@ -6841,6 +6846,7 @@ class basic_json
     */
     template<typename A1, typename A2,
              detail::enable_if_t<std::is_constructible<detail::input_adapter, A1, A2>::value, int> = 0>
+    JSON_NODISCARD
     static basic_json from_cbor(A1 && a1, A2 && a2,
                                 const bool strict = true,
                                 const bool allow_exceptions = true)
@@ -6933,6 +6939,7 @@ class basic_json
            @a strict parameter since 3.0.0; added @a allow_exceptions parameter
            since 3.2.0
     */
+    JSON_NODISCARD
     static basic_json from_msgpack(detail::input_adapter&& i,
                                    const bool strict = true,
                                    const bool allow_exceptions = true)
@@ -6948,6 +6955,7 @@ class basic_json
     */
     template<typename A1, typename A2,
              detail::enable_if_t<std::is_constructible<detail::input_adapter, A1, A2>::value, int> = 0>
+    JSON_NODISCARD
     static basic_json from_msgpack(A1 && a1, A2 && a2,
                                    const bool strict = true,
                                    const bool allow_exceptions = true)
@@ -7019,6 +7027,7 @@ class basic_json
 
     @since version 3.1.0; added @a allow_exceptions parameter since 3.2.0
     */
+    JSON_NODISCARD
     static basic_json from_ubjson(detail::input_adapter&& i,
                                   const bool strict = true,
                                   const bool allow_exceptions = true)
@@ -7034,6 +7043,7 @@ class basic_json
     */
     template<typename A1, typename A2,
              detail::enable_if_t<std::is_constructible<detail::input_adapter, A1, A2>::value, int> = 0>
+    JSON_NODISCARD
     static basic_json from_ubjson(A1 && a1, A2 && a2,
                                   const bool strict = true,
                                   const bool allow_exceptions = true)
@@ -7104,6 +7114,7 @@ class basic_json
     @sa @ref from_ubjson(detail::input_adapter&&, const bool, const bool) for the
         related UBJSON format
     */
+    JSON_NODISCARD
     static basic_json from_bson(detail::input_adapter&& i,
                                 const bool strict = true,
                                 const bool allow_exceptions = true)
@@ -7119,6 +7130,7 @@ class basic_json
     */
     template<typename A1, typename A2,
              detail::enable_if_t<std::is_constructible<detail::input_adapter, A1, A2>::value, int> = 0>
+    JSON_NODISCARD
     static basic_json from_bson(A1 && a1, A2 && a2,
                                 const bool strict = true,
                                 const bool allow_exceptions = true)
@@ -7710,6 +7722,7 @@ class basic_json
 
     @since version 2.0.0
     */
+    JSON_NODISCARD
     static basic_json diff(const basic_json& source, const basic_json& target,
                            const std::string& path = "")
     {

single_include/nlohmann/json.hpp

@@ -152,6 +152,15 @@ using json = basic_json<>;
     #define JSON_DEPRECATED
 #endif
 
+// allow for portable nodiscard warnings
+#if defined(__has_cpp_attribute) && __has_cpp_attribute(nodiscard)
+    #define JSON_NODISCARD [[nodiscard]]
+#elif defined(__has_cpp_attribute) && __has_cpp_attribute(gnu::warn_unused_result)
+    #define JSON_NODISCARD [[gnu::warn_unused_result]]
+#else
+    #define JSON_NODISCARD
+#endif
+
 // allow to disable exceptions
 #if (defined(__cpp_exceptions) || defined(__EXCEPTIONS) || defined(_CPPUNWIND)) && !defined(JSON_NOEXCEPTION)
     #define JSON_THROW(exception) throw exception
@@ -12793,6 +12802,7 @@ class basic_json
 
     @since 2.1.0
     */
+    JSON_NODISCARD
     static basic_json meta()
     {
         basic_json result;
@@ -13965,6 +13975,7 @@ class basic_json
 
     @since version 1.0.0
     */
+    JSON_NODISCARD
     static basic_json array(initializer_list_t init = {})
     {
         return basic_json(init, false, value_t::array);
@@ -14008,6 +14019,7 @@ class basic_json
 
     @since version 1.0.0
     */
+    JSON_NODISCARD
     static basic_json object(initializer_list_t init = {})
     {
         return basic_json(init, false, value_t::object);
@@ -18527,6 +18539,7 @@ class basic_json
 
     @since version 2.0.3 (contiguous containers)
     */
+    JSON_NODISCARD
     static basic_json parse(detail::input_adapter&& i,
                             const parser_callback_t cb = nullptr,
                             const bool allow_exceptions = true)
@@ -19302,6 +19315,7 @@ class basic_json
            @a strict parameter since 3.0.0; added @a allow_exceptions parameter
            since 3.2.0
     */
+    JSON_NODISCARD
     static basic_json from_cbor(detail::input_adapter&& i,
                                 const bool strict = true,
                                 const bool allow_exceptions = true)
@@ -19317,6 +19331,7 @@ class basic_json
     */
     template<typename A1, typename A2,
              detail::enable_if_t<std::is_constructible<detail::input_adapter, A1, A2>::value, int> = 0>
+    JSON_NODISCARD
     static basic_json from_cbor(A1 && a1, A2 && a2,
                                 const bool strict = true,
                                 const bool allow_exceptions = true)
@@ -19409,6 +19424,7 @@ class basic_json
            @a strict parameter since 3.0.0; added @a allow_exceptions parameter
            since 3.2.0
     */
+    JSON_NODISCARD
     static basic_json from_msgpack(detail::input_adapter&& i,
                                    const bool strict = true,
                                    const bool allow_exceptions = true)
@@ -19424,6 +19440,7 @@ class basic_json
     */
     template<typename A1, typename A2,
              detail::enable_if_t<std::is_constructible<detail::input_adapter, A1, A2>::value, int> = 0>
+    JSON_NODISCARD
     static basic_json from_msgpack(A1 && a1, A2 && a2,
                                    const bool strict = true,
                                    const bool allow_exceptions = true)
@@ -19495,6 +19512,7 @@ class basic_json
 
     @since version 3.1.0; added @a allow_exceptions parameter since 3.2.0
     */
+    JSON_NODISCARD
     static basic_json from_ubjson(detail::input_adapter&& i,
                                   const bool strict = true,
                                   const bool allow_exceptions = true)
@@ -19510,6 +19528,7 @@ class basic_json
     */
     template<typename A1, typename A2,
              detail::enable_if_t<std::is_constructible<detail::input_adapter, A1, A2>::value, int> = 0>
+    JSON_NODISCARD
     static basic_json from_ubjson(A1 && a1, A2 && a2,
                                   const bool strict = true,
                                   const bool allow_exceptions = true)
@@ -19580,6 +19599,7 @@ class basic_json
     @sa @ref from_ubjson(detail::input_adapter&&, const bool, const bool) for the
         related UBJSON format
     */
+    JSON_NODISCARD
     static basic_json from_bson(detail::input_adapter&& i,
                                 const bool strict = true,
                                 const bool allow_exceptions = true)
@@ -19595,6 +19615,7 @@ class basic_json
     */
     template<typename A1, typename A2,
              detail::enable_if_t<std::is_constructible<detail::input_adapter, A1, A2>::value, int> = 0>
+    JSON_NODISCARD
     static basic_json from_bson(A1 && a1, A2 && a2,
                                 const bool strict = true,
                                 const bool allow_exceptions = true)
@@ -20186,6 +20207,7 @@ class basic_json
 
     @since version 2.0.0
     */
+    JSON_NODISCARD
     static basic_json diff(const basic_json& source, const basic_json& target,
                            const std::string& path = "")
     {
@@ -20505,6 +20527,7 @@ inline nlohmann::json::json_pointer operator "" _json_pointer(const char* s, std
 #undef JSON_LIKELY
 #undef JSON_UNLIKELY
 #undef JSON_DEPRECATED
+#undef JSON_NODISCARD
 #undef JSON_HAS_CPP_14
 #undef JSON_HAS_CPP_17
 #undef NLOHMANN_BASIC_JSON_TPL_DECLARATION

test/src/unit-cbor.cpp

@@ -1962,7 +1962,7 @@ TEST_CASE("all CBOR first bytes", "[!throws]")
 
         try
         {
-            json::from_cbor(std::vector<uint8_t>(1, byte));
+            auto res = json::from_cbor(std::vector<uint8_t>(1, byte));
         }
         catch (const json::parse_error& e)
         {

test/src/unit-readme.cpp

@@ -305,7 +305,7 @@ TEST_CASE("README", "[hide]")
             // }
 
             // calculate a JSON patch from two JSON values
-            json::diff(j_result, j_original);
+            auto res = json::diff(j_result, j_original);
             // [
             //   { "op":" replace", "path": "/baz", "value": ["one", "two", "three"] },
             //   { "op":"remove","path":"/hello" },

test/src/unit-ubjson.cpp

@@ -2128,7 +2128,7 @@ TEST_CASE("all UBJSON first bytes", "[!throws]")
 
         try
         {
-            json::from_ubjson(std::vector<uint8_t>(1, byte));
+            auto res = json::from_ubjson(std::vector<uint8_t>(1, byte));
         }
         catch (const json::parse_error& e)
         {