UDT serialization of described enums

include/boost/json/impl/serializer.hpp

@@ -10,6 +10,7 @@
 #ifndef BOOST_JSON_IMPL_SERIALIZER_HPP
 #define BOOST_JSON_IMPL_SERIALIZER_HPP
 
+#include <boost/describe/enum_to_string.hpp>
 #include <boost/json/conversion.hpp>
 #include <cstddef>
 
@@ -585,6 +586,58 @@ do_obj6:
     return w.suspend(writer::state::obj6, cur, pt);
 }
 
+template<class T, bool StackEmpty>
+BOOST_FORCEINLINE
+bool
+write_impl(described_enum_conversion_tag, writer& w, stream& ss)
+{
+#ifdef BOOST_DESCRIBE_CXX14
+    using Integer = typename std::underlying_type<T>::type;
+
+#if defined(_MSC_VER)
+# pragma warning( push )
+# pragma warning( disable : 4127 )
+#endif
+    if(StackEmpty || w.st_.empty())
+#if defined(_MSC_VER)
+# pragma warning( pop )
+#endif
+    {
+        BOOST_ASSERT( w.p_ );
+        T const* pt = reinterpret_cast<T const*>(w.p_);
+        char const* const name = describe::enum_to_string(*pt, nullptr);
+        if( name )
+        {
+            string_view const sv = name;
+            w.cs0_ = { sv.data(), sv.size() };
+            return write_string(w, ss);
+        }
+        else
+        {
+            Integer n = static_cast<Integer>(*pt);
+            w.p_ = &n;
+            return write_impl<Integer, true>(w, ss);
+        }
+    }
+    else
+    {
+        writer::state st;
+        w.st_.peek(st);
+        if( st == writer::state::lit )
+            return write_impl<Integer, false>(w, ss);
+        else
+            return resume_string(w, ss);
+    }
+#else // BOOST_DESCRIBE_CXX14
+    (void)w;
+    (void)ss;
+    static_assert(
+        !std::is_same<T, T>::value,
+        "described enums require C++14 support");
+    return false;
+#endif // BOOST_DESCRIBE_CXX14
+}
+
 template<class T, bool StackEmpty>
 bool
 write_impl(writer& w, stream& ss)

test/serializer.cpp

@@ -11,6 +11,7 @@
 #include <boost/json/serializer.hpp>
 
 #include <boost/describe/class.hpp>
+#include <boost/describe/enum.hpp>
 #include <boost/json/detail/stack.hpp>
 #include <boost/json/serialize.hpp>
 #include <boost/json/null_resource.hpp>
@@ -36,6 +37,15 @@ struct my_struct
 };
 BOOST_DESCRIBE_STRUCT(my_struct, (), (s, n, d));
 
+enum class my_enum
+{
+    option_one,
+    option_two,
+    option_three,
+    option_four,
+};
+BOOST_DESCRIBE_ENUM(my_enum, option_one, option_two, option_three);
+
 } // namespace serializer_test_ns
 
 BOOST_STATIC_ASSERT( std::is_nothrow_destructible<serializer>::value );
@@ -821,12 +831,18 @@ public:
                     "a string", {12, true}),
                 R"(["a string",[12,true]])");
         }
-        {
 #ifdef BOOST_DESCRIBE_CXX14
+        {
             serializer_test_ns::my_struct s{"some string", 1424, 12.4};
             check_udt(s, R"({"s":"some string","n":1424,"d":1.24E1})");
-#endif // BOOST_DESCRIBE_CXX14
         }
+        {
+            check_udt(
+                serializer_test_ns::my_enum::option_three,
+                R"("option_three")");
+            check_udt( serializer_test_ns::my_enum(100), "100" );
+        }
+#endif // BOOST_DESCRIBE_CXX14
     }
 
     void