* Improved string conversion (write to stream if cppcore supports the type)

* Added tests to group 'cppcore'

include/cppcore/conversion/string.h

@@ -210,22 +210,13 @@ namespace std
 {
 
     /**
-     * @brief Operator overload to write value to stream that supports the to_string method, with stream parameter.
+     * @brief Operator overload to write value to stream.
      */
     template<typename T_char, typename T_traits, typename X>
     inline auto operator<<(basic_ostream<T_char, T_traits>& os, X&& x)
-        -> decltype(
-            std::forward<X>(x).to_string(std::declval<basic_ostream<T_char, T_traits>&>()),
-            std::declval<basic_ostream<T_char, T_traits>&>());
-
-    /**
-     * @brief Operator overload to write value to stream that supports the to_string method, with stream parameter.
-     */
-    template<typename T_char, typename T_traits, typename X>
-    inline auto operator<<(basic_ostream<T_char, T_traits>& os, X&& x)
-        -> decltype(
-            std::forward<X>(x).to_string(),
-            std::declval<basic_ostream<T_char, T_traits>&>());
+        -> std::enable_if_t<
+            cppcore::__impl::op_to_stream<std::decay_t<X>>::enable_streaming,
+            basic_ostream<T_char, T_traits>&>;
 
 }
 

include/cppcore/conversion/string.inl

@@ -224,6 +224,8 @@ namespace cppcore
         template<typename T>
         struct op_to_stream<T, std::enable_if_t<std::is_enum_v<T>>>
         {
+            static constexpr decltype(auto) enable_streaming = true;
+
             inline void operator()(std::ostream& os, const T& v) const
                 { os << enum_conversion<T>::to_string(v); }
         };
@@ -231,6 +233,8 @@ namespace cppcore
         template<typename T>
         struct op_to_stream<T, decltype(std::declval<T>().to_string(std::declval<std::ostream&>()), void())>
         {
+            static constexpr decltype(auto) enable_streaming = true;
+
             inline void operator()(std::ostream& os, const T& v) const
                 { v.to_string(os); }
         };
@@ -238,6 +242,8 @@ namespace cppcore
         template<typename T>
         struct op_to_stream<T, decltype(std::declval<T>().to_string(), void())>
         {
+            static constexpr decltype(auto) enable_streaming = true;
+
             inline void operator()(std::ostream& os, const T& v) const
                 { os << v.to_string(); }
         };
@@ -246,6 +252,8 @@ namespace cppcore
         template<typename T>
         struct op_to_stream<std::vector<T>, void>
         {
+            static constexpr decltype(auto) enable_streaming = true;
+
             inline void operator()(std::ostream& os, const std::vector<T>& v) const
             {
                 bool first = true;
@@ -263,6 +271,8 @@ namespace cppcore
         template<typename T>
         struct op_to_stream<std::list<T>, void>
         {
+            static constexpr decltype(auto) enable_streaming = true;
+
             inline void operator()(std::ostream& os, const std::list<T>& v) const
             {
                 bool first = true;
@@ -420,21 +430,12 @@ namespace std
 
     template<typename T_char, typename T_traits, typename X>
     inline auto operator<<(basic_ostream<T_char, T_traits>& os, X&& x)
-        -> decltype(
-            std::forward<X>(x).to_string(std::declval<basic_ostream<T_char, T_traits>&>()),
-            std::declval<basic_ostream<T_char, T_traits>&>())
-    {
-        std::forward<X>(x).to_string(os);
-        return os;
-    }
-
-    template<typename T_char, typename T_traits, typename X>
-    inline auto operator<<(basic_ostream<T_char, T_traits>& os, X&& x)
-        -> decltype(
-            std::forward<X>(x).to_string(),
-            std::declval<basic_ostream<T_char, T_traits>&>())
+        -> std::enable_if_t<
+            cppcore::__impl::op_to_stream<std::decay_t<X>>::enable_streaming,
+            basic_ostream<T_char, T_traits>&>
     {
-        os << std::forward<X>(x).to_string();
+        using op_type = cppcore::__impl::op_to_stream<std::decay_t<X>>;
+        op_type()(os, std::forward<X>(x));
         return os;
     }
 

test/CMakeLists.txt

@@ -45,7 +45,7 @@ ForEach                     ( FILE IN LISTS CPPCORE_TEST_SOURCE_FILES )
 
     # test
     If                          ( HAS_CMAKE_TESTS )
-        Add_CMake_Test              ( NAME ${TEST_NAME} TARGET ${TEST_NAME} )
+        Add_CMake_Test              ( NAME ${TEST_NAME} TARGET ${TEST_NAME} GROUP cppcore )
     Else                        ( )
         Add_Test                    ( NAME ${TEST_NAME} COMMAND ${TEST_NAME} WORKING_DIRECTORY ${CMAKE_CURRENT_SOURCE_DIR} )
     EndIf                       ( )

test/cppcore/conversion/string_tests.cpp

@@ -73,6 +73,13 @@ TEST(string_tests, vector_to_string)
         ::cppcore::to_string(std::vector<int> { 1, 2, 3, 4, 5 }));
 }
 
+TEST(string_tests, vector_to_stream)
+{
+    std::ostringstream ss;
+    ss << std::vector<int> { 1, 2, 3, 4, 5 };
+    EXPECT_EQ("1,2,3,4,5", ss.str());
+}
+
 TEST(string_tests, list_to_string)
 {
     EXPECT_EQ(
@@ -80,6 +87,13 @@ TEST(string_tests, list_to_string)
         ::cppcore::to_string(std::list<int> { 1, 2, 3, 4, 5 }));
 }
 
+TEST(string_tests, list_to_stream)
+{
+    std::ostringstream ss;
+    ss << std::list<int> { 1, 2, 3, 4, 5 };
+    EXPECT_EQ("1,2,3,4,5", ss.str());
+}
+
 TEST(string_tests, string_to_vector)
 {
     EXPECT_EQ(