value_from supports conversion through helper types

doc/Jamfile

@@ -14,8 +14,8 @@ import boostbook ;
 import ../../../tools/docca/docca.jam ;
 import path ;
 
-local include-prefix = [ path.root $(__file__:D) [ path.pwd ] ] ;
-include-prefix = [ path.native $(include-prefix:D)/include ] ;
+local include-prefix = [ path.join $(__file__:D) .. ]  ;
+include-prefix = [ path.native $(include-prefix)/include ] ;
 docca.pyreference reference.qbk
     : [ glob-tree-ex ../include/boost/json : *.hpp *.ipp : detail impl ]
       externals.hpp

include/boost/json/conversion.hpp

@@ -437,6 +437,15 @@ struct is_variant_like;
 template<class T>
 struct is_optional_like;
 
+template< class T, class Context = void, class = void >
+struct represent_as
+{
+    using type = T;
+};
+
+template< class T, class Context = void >
+using represent_as_t = typename represent_as<T>::type;
+
 } // namespace json
 } // namespace boost
 

include/boost/json/detail/value_from.hpp

@@ -41,6 +41,45 @@ struct append_tuple_element {
     }
 };
 
+template< class T, class Rep, class = void >
+struct representation_helper
+{
+    using result_to = Rep;
+
+    static
+    result_to
+    to( T&& t )
+    {
+        return static_cast<result_to>( static_cast<T&&>(t) );
+    }
+};
+
+template< class T, class Rep >
+struct representation_helper<
+    T,
+    Rep,
+    typename std::enable_if<
+        std::is_same<remove_cvref<T>, Rep>::value>::type >
+{
+    using result_to = T&&;
+
+    static
+    result_to
+    to( T&& t )
+    {
+        return static_cast<T&&>(t);
+    }
+};
+
+template< class Ctx, class T >
+typename representation_helper<
+    T, represent_as_t<remove_cvref<T>, Ctx> >::result_to
+to_representation( T&& t )
+{
+    using Rep = represent_as_t<remove_cvref<T>, Ctx>;
+    return representation_helper<T, Rep>::to( static_cast<T&&>(t) );
+}
+
 //----------------------------------------------------------
 // User-provided conversion
 
@@ -107,9 +146,11 @@ 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

include/boost/json/impl/conversion.hpp

@@ -516,7 +516,8 @@ 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<
+            represent_as_t<detail::remove_cvref< detail::key_type<T> >>>,
         mp11::mp_valid_and_true<detail::has_unique_keys, T>>
 { };
 

include/boost/json/value_from.hpp

@@ -84,10 +84,15 @@ value_from(
     value& jv)
 {
     using bare_T = detail::remove_cvref<T>;
+    using Rep = typename represent_as<bare_T>::type;
     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, Rep, detail::value_from_conversion>::value);
+    using cat = detail::value_from_category<Context, Rep>;
+    detail::value_from_impl(
+        cat(),
+        jv,
+        detail::representation_helper<T, Rep>::to( static_cast<T&&>(t) ),
+        ctx );
 }
 
 /** Convert an object of type `T` to @ref value.

test/value_from.cpp

@@ -258,6 +258,60 @@ tag_invoke(
 }
 
 
+struct id
+{
+    static constexpr auto& id1 = "Id#1";
+    static constexpr auto& id2 = "Id#2";
+
+    std::size_t n;
+};
+
+bool
+operator<(id l, id r) noexcept
+{
+    return l.n < r.n;
+}
+
+struct id_string_repr
+{
+    std::size_t n;
+
+    id_string_repr(id x) noexcept
+        : n(x.n)
+    {}
+
+    id_string_repr(boost::json::string_view sv)
+    {
+        if( sv.data() == id::id1 )
+            n = 1;
+        else if( sv.data() == id::id2 )
+            n = 2;
+        else
+            n = std::size_t(-1);
+    }
+
+    operator id() const noexcept
+    {
+        return {n};
+    }
+
+    operator boost::json::string_view() const noexcept
+    {
+        switch(n)
+        {
+        case 1: return boost::json::string_view(id::id1);
+        case 2: return boost::json::string_view(id::id2);
+        default: return boost::json::string_view("unknown");
+        }
+    }
+};
+
+struct T14
+{
+    id i;
+};
+BOOST_DESCRIBE_STRUCT(T14, (), (i))
+
 } // namespace value_from_test_ns
 
 template<class T>
@@ -296,6 +350,12 @@ struct is_described_class<::value_from_test_ns::T11>
     : std::true_type
 { };
 
+template<>
+struct represent_as<::value_from_test_ns::id>
+{
+    using type = ::value_from_test_ns::id_string_repr;
+};
+
 namespace {
 
 template< class T, class... Context >
@@ -404,6 +464,23 @@ class value_from_test
             value b = value_from( a, ctx... );
             BOOST_TEST(b.is_null());
         }
+        {
+            value jv = value_from( value_from_test_ns::id{1}, ctx... );
+            BOOST_TEST( jv == value("Id#1") );
+
+            jv = value_from( value_from_test_ns::id{2}, ctx... );
+            BOOST_TEST( jv == value("Id#2") );
+
+            jv = value_from(
+                std::vector<value_from_test_ns::id>{ {1}, {2}, {2}, {1} },
+                ctx... );
+            BOOST_TEST(( jv == value{"Id#1", "Id#2", "Id#2", "Id#1"} ));
+
+            jv = value_from(
+                std::tuple<value_from_test_ns::id, int>{ {1}, 12 },
+                ctx... );
+            BOOST_TEST(( jv == value{"Id#1", 12} ));
+        }
     }
 
     template< class... Context >
@@ -447,6 +524,12 @@ class value_from_test
             BOOST_TEST(a.size() == c.as_array().size());
             BOOST_TEST(b.as_array().size() == c.as_array().size());
         }
+        {
+            value jv = value_from(
+                std::map<value_from_test_ns::id, int>{ {{1}, 42}, {{2}, 43} },
+                ctx... );
+            BOOST_TEST(( jv == object{ {"Id#1", 42}, {"Id#2", 43} } ));
+        }
     }
 
     template< class... Context >
@@ -505,6 +588,9 @@ class value_from_test
         ::value_from_test_ns::E1 e1 = ::value_from_test_ns::E1::a;
         BOOST_TEST( value_from( e1, ctx... ) == "a" );
 
+        jv = value_from( value_from_test_ns::T14{ {1} }, ctx... );
+        BOOST_TEST(( jv == object{ {"i", "Id#1"} } ));
+
         e1 = ::value_from_test_ns::E1::b;
         BOOST_TEST( value_from( e1, ctx... ) == "b" );
 
@@ -524,6 +610,10 @@ class value_from_test
         BOOST_TEST( jv == (value{1, 2, 3, nullptr, 5}) );
 
         BOOST_TEST( value_from( std::nullopt, ctx... ).is_null() );
+
+        jv = value_from(
+            std::optional<value_from_test_ns::id>( {1} ), ctx... );
+        BOOST_TEST( jv == value("Id#1") );
 #endif
     }
 
@@ -546,6 +636,12 @@ class value_from_test
         jv = value_from( v, ctx... );
         BOOST_TEST(jv == "T5");
 
+        jv = value_from(
+            std::variant<int, value_from_test_ns::id>(
+                value_from_test_ns::id{2} ),
+            ctx... );
+        BOOST_TEST( jv == value("Id#2") );
+
         BOOST_TEST( value() == value_from( std::monostate(), ctx... ) );
 #endif // BOOST_NO_CXX17_HDR_VARIANT
     }