value_from supports conversion through helper types

include/boost/json/conversion.hpp

@@ -234,7 +234,7 @@ struct is_sequence_like;
 
     @see @ref value_from, @ref value_to
 */
-template<class T>
+template<class T, class Context>
 struct is_map_like;
 
 /** Determine if `T` can be treated like a tuple during conversions.
@@ -437,6 +437,12 @@ struct is_variant_like;
 template<class T>
 struct is_optional_like;
 
+template< class T, class Context = void >
+struct represent_as;
+
+template< class T, class Context = void >
+using represent_as_t = typename represent_as<T, Context>::type;
+
 } // namespace json
 } // namespace boost
 

include/boost/json/detail/parse_into.hpp

@@ -79,7 +79,8 @@ class converting_handler;
 
 // get_handler
 template< class V, class P >
-using get_handler = converting_handler< generic_conversion_category<V>, V, P >;
+using get_handler = converting_handler<
+    generic_conversion_category<V, no_context>, V, P>;
 
 template<error E> class handler_error_base
 {

include/boost/json/detail/value_from.hpp

@@ -22,9 +22,12 @@
 
 namespace boost {
 namespace json {
-
 namespace detail {
 
+template< class Ctx, class T >
+using value_from_attrs = conversion_attrs<
+    Ctx, remove_cvref<T>, value_from_conversion>;
+
 template< class Ctx, class T >
 struct append_tuple_element {
     array& arr;
@@ -41,6 +44,21 @@ struct append_tuple_element {
     }
 };
 
+template< class T, class Ctx >
+using to_representation_result = mp11::mp_if<
+    std::is_same<
+        remove_cvref<T>, typename value_from_attrs<Ctx, T>::representation >,
+    T&&,
+    typename value_from_attrs<Ctx, T>::representation>;
+
+template< class Ctx, class T >
+to_representation_result< T, Ctx >
+to_representation( T&& t )
+{
+    using R = to_representation_result< T, Ctx >;
+    return static_cast<R>( static_cast<T&&>(t) );
+}
+
 //----------------------------------------------------------
 // User-provided conversion
 
@@ -107,9 +125,11 @@ value_from_impl( map_like_conversion_tag, value& jv, T&& from, Ctx const& ctx )
     object& obj = jv.emplace_object();
     obj.reserve(detail::try_size(from, size_implementation<T>()));
     for (auto&& elem : from)
+    {
         obj.emplace(
-            get<0>(elem),
+            to_representation<Ctx>( get<0>(elem) ),
             value_from( get<1>(elem), ctx, obj.storage() ));
+    }
 }
 
 // ranges
@@ -257,13 +277,6 @@ value_from_impl( path_conversion_tag, value& jv, T&& from, Ctx const& )
     jv.emplace_string().assign(sv);
 }
 
-//----------------------------------------------------------
-// Contextual conversions
-
-template< class Ctx, class T >
-using value_from_category = conversion_category<
-    Ctx, T, value_from_conversion >;
-
 } // detail
 
 #ifndef BOOST_NO_CXX17_HDR_OPTIONAL

include/boost/json/detail/value_to.hpp

@@ -818,8 +818,8 @@ value_to_impl( Impl impl, value_to_tag<T>, value const& jv, Ctx const& ctx )
 }
 
 template< class Ctx, class T >
-using value_to_category = conversion_category<
-    Ctx, T, value_to_conversion >;
+using value_to_attrs = conversion_attrs<
+    Ctx, remove_cvref<T>, value_to_conversion>;
 
 } // detail
 

include/boost/json/fwd.hpp

@@ -45,7 +45,7 @@ struct is_string_like;
 template<class T>
 struct is_sequence_like;
 
-template<class T>
+template<class T, class Context = void>
 struct is_map_like;
 
 template<class T>

include/boost/json/impl/conversion.hpp

@@ -32,6 +32,9 @@ namespace boost {
 namespace json {
 namespace detail {
 
+struct no_context
+{};
+
 #ifdef __cpp_lib_nonmember_container_access
 using std::size;
 #endif
@@ -317,14 +320,14 @@ using native_conversion_category = mp11::mp_cond<
     std::is_same<T, string>, string_conversion_tag>;
 
 // generic conversions
-template< class T >
+template< class T, class Ctx >
 using generic_conversion_category = mp11::mp_cond<
     std::is_same<T, bool>,     bool_conversion_tag,
     std::is_integral<T>,       integral_conversion_tag,
     std::is_floating_point<T>, floating_point_conversion_tag,
     is_null_like<T>,           null_like_conversion_tag,
     is_string_like<T>,         string_like_conversion_tag,
-    is_map_like<T>,            map_like_conversion_tag,
+    is_map_like<T, Ctx>,       map_like_conversion_tag,
     is_sequence_like<T>,       sequence_conversion_tag,
     is_tuple_like<T>,          tuple_conversion_tag,
     is_described_class<T>,     described_class_conversion_tag,
@@ -338,38 +341,92 @@ using generic_conversion_category = mp11::mp_cond<
 template< class T >
 using nested_type = typename T::type;
 template< class T1, class T2 >
-using conversion_category_impl_helper = mp11::mp_eval_if_not<
+using conversion_category_helper = mp11::mp_eval_if_not<
     std::is_same<detail::no_conversion_tag, T1>,
     T1,
     mp11::mp_eval_or_q, T1, mp11::mp_quote<nested_type>, T2>;
+
+template< class Ctx >
+using fix_context = mp11::mp_if< std::is_same<Ctx, no_context>, void, Ctx>;
+
+template<class T, class Ctx>
+using representation_or_void = mp11::mp_eval_or<void, represent_as_t, T, Ctx>;
+
+template< class U >
+using is_not_void = mp11::mp_bool< !std::is_same<void, U>::value >;
+
+template< class T, class Ctxs >
+struct representation_helper
+{
+    using size = mp11::mp_size<Ctxs>;
+
+    template< class I >
+    using exists = mp11::mp_less<I, size>;
+
+    template< class Ctx >
+    using step = representation_or_void<T, Ctx>;
+    using reps = mp11::mp_transform<step, Ctxs>;
+    using r_index = mp11::mp_find_if< reps, is_not_void >;
+
+    using type = mp11::mp_eval_if<
+        mp11::mp_not< exists<r_index> >,
+        T,
+        mp11::mp_at, reps, r_index>;
+};
+
+template< class T, class Ctx >
+struct conversion_representation_impl
+    : representation_helper< T, mp11::mp_list<Ctx, void> >
+{};
+
+template< class T >
+struct conversion_representation_impl<T, no_context>
+    : representation_helper< T, mp11::mp_list<void> >
+{};
+
+template< class T, class... Ctxs >
+struct conversion_representation_impl< T, std::tuple<Ctxs...> >
+    : representation_helper< T, mp11::mp_list<remove_cvref<Ctxs>..., void> >
+{};
+
+template< class T, class Ctx >
+using conversion_representation
+    = typename conversion_representation_impl<T, Ctx>::type;
+
 template< class Ctx, class T, class Dir >
-struct conversion_category_impl
+struct conversion_attrs
 {
-    using type = mp11::mp_fold<
+    using representation = conversion_representation<T, Ctx>;
+
+    using category = mp11::mp_fold<
         mp11::mp_list<
-            mp11::mp_defer<user_conversion_category, Ctx, T, Dir>,
-            mp11::mp_defer<native_conversion_category, T>,
-            mp11::mp_defer<generic_conversion_category, T>>,
+            mp11::mp_defer<user_conversion_category, Ctx, representation, Dir>,
+            mp11::mp_defer<native_conversion_category, representation>,
+            mp11::mp_defer<generic_conversion_category, representation, Ctx>>,
         no_conversion_tag,
-        conversion_category_impl_helper>;
+        conversion_category_helper>;
 };
-template< class Ctx, class T, class Dir >
-using conversion_category =
-    typename conversion_category_impl< Ctx, T, Dir >::type;
 
 template< class T >
 using any_conversion_tag = mp11::mp_not<
     std::is_same< T, no_conversion_tag > >;
 
+template< class Ctx, class T, class Dir >
+using conversion_category = typename conversion_attrs<Ctx, T, Dir>::category;
+
 template< class T, class Dir, class... Ctxs >
-struct conversion_category_impl< std::tuple<Ctxs...>, T, Dir >
+struct conversion_attrs< std::tuple<Ctxs...>, T, Dir >
 {
+    using size = mp11::mp_size_t< sizeof...(Ctxs) >;
     using ctxs = mp11::mp_list< remove_cvref<Ctxs>... >;
-    using cats = mp11::mp_list<
-        conversion_category<remove_cvref<Ctxs>, T, Dir>... >;
 
     template< class I >
-    using exists = mp11::mp_less< I, mp11::mp_size<cats> >;
+    using exists = mp11::mp_less<I, size>;
+
+    using representation = conversion_representation< T, std::tuple<Ctxs...> >;
+
+    using cats = mp11::mp_list<
+        conversion_category<remove_cvref<Ctxs>, representation, Dir>... >;
 
     using context2 = mp11::mp_find< cats, full_context_conversion_tag >;
     using context1 = mp11::mp_find< cats, context_conversion_tag >;
@@ -379,14 +436,11 @@ struct conversion_category_impl< std::tuple<Ctxs...>, T, Dir >
         exists<context1>, context1,
         exists<context0>, context0,
         mp11::mp_true, mp11::mp_find_if< cats, any_conversion_tag > >;
-    using type = mp11::mp_eval_or<
+    using category = mp11::mp_eval_or<
         no_conversion_tag,
         mp11::mp_at, cats, index >;
 };
 
-struct no_context
-{};
-
 template <class T, class Dir>
 using can_convert = mp11::mp_not<
     std::is_same<
@@ -456,10 +510,10 @@ template< class T, class Dir, class... Ctxs >
 struct supported_context< std::tuple<Ctxs...>, T, Dir >
 {
     using Ctx = std::tuple<Ctxs...>;
-    using impl = conversion_category_impl<Ctx, T, Dir>;
-    using index = typename impl::index;
+    using Attrs = conversion_attrs<Ctx, T, Dir>;
+    using index = typename Attrs::index;
     using next_supported = supported_context<
-        mp11::mp_at< typename impl::ctxs, index >, T, Dir >;
+        mp11::mp_at< typename Attrs::ctxs, index >, T, Dir >;
     using type = typename next_supported::type;
 
     static
@@ -508,12 +562,14 @@ struct is_sequence_like
         mp11::mp_valid<detail::begin_iterator_category, T>>
 { };
 
-template<class T>
+template<class T, class Ctx>
 struct is_map_like
     : mp11::mp_all<
         is_sequence_like<T>,
         mp11::mp_valid_and_true<detail::is_value_type_pair, T>,
-        is_string_like<detail::key_type<T>>,
+        is_string_like<
+            detail::conversion_representation<
+                detail::remove_cvref< detail::key_type<T> >, Ctx>>,
         mp11::mp_valid_and_true<detail::has_unique_keys, T>>
 { };
 

include/boost/json/impl/serializer.hpp

@@ -818,7 +818,7 @@ template<class T, bool StackEmpty>
 bool
 write_impl(writer& w, stream& ss)
 {
-    using cat = detail::generic_conversion_category<T>;
+    using cat = detail::generic_conversion_category<T, detail::no_context>;
     return write_impl<T, StackEmpty>( cat(), w, ss );
 }
 

include/boost/json/value_from.hpp

@@ -83,11 +83,17 @@ value_from(
     Context const& ctx,
     value& jv)
 {
-    using bare_T = detail::remove_cvref<T>;
+    using Attrs = detail::value_from_attrs<Context, T>;
     BOOST_STATIC_ASSERT(detail::conversion_round_trips<
-        Context, bare_T, detail::value_from_conversion>::value);
-    using cat = detail::value_from_category<Context, bare_T>;
-    detail::value_from_impl( cat(), jv, std::forward<T>(t), ctx );
+        Context,
+        typename Attrs::representation,
+        detail::value_from_conversion>::value);
+
+    detail::value_from_impl(
+        typename Attrs::category(),
+        jv,
+        detail::to_representation<Context>( static_cast<T&&>(t) ),
+        ctx );
 }
 
 /** Convert an object of type `T` to @ref value.

include/boost/json/value_to.hpp

@@ -89,11 +89,16 @@ T
 value_to( value const& jv, Context const& ctx )
 {
     BOOST_STATIC_ASSERT(! std::is_reference<T>::value);
-    using bare_T = detail::remove_cvref<T>;
+
+    using Attrs = detail::value_to_attrs<Context, T>;
     BOOST_STATIC_ASSERT(detail::conversion_round_trips<
-        Context, bare_T, detail::value_to_conversion>::value);
-    using cat = detail::value_to_category<Context, bare_T>;
-    return detail::value_to_impl( cat(), value_to_tag<bare_T>(), jv, ctx );
+        Context,
+        typename Attrs::representation,
+        detail::value_to_conversion>::value);
+
+    using bare_T = detail::remove_cvref<T>;
+    return detail::value_to_impl(
+        typename Attrs::category(), value_to_tag<bare_T>(), jv, ctx);
 }
 
 /** Convert a @ref value to an object of type `T`.
@@ -223,12 +228,16 @@ typename result_for<T, value>::type
 try_value_to( value const& jv, Context const& ctx )
 {
     BOOST_STATIC_ASSERT(! std::is_reference<T>::value);
-    using bare_T = detail::remove_cvref<T>;
+
+    using Attrs = detail::value_to_attrs<Context, T>;
     BOOST_STATIC_ASSERT(detail::conversion_round_trips<
-        Context, bare_T, detail::value_to_conversion>::value);
-    using cat = detail::value_to_category<Context, bare_T>;
+        Context,
+        typename Attrs::representation,
+        detail::value_to_conversion>::value);
+
+    using bare_T = detail::remove_cvref<T>;
     return detail::value_to_impl(
-        cat(), try_value_to_tag<bare_T>(), jv, ctx );
+        typename Attrs::category(), try_value_to_tag<bare_T>(), jv, ctx );
 }
 
 /** Convert a @ref value to a `boost::system::result<T>`.