fix references used in visit

1. Fix references in return type
2. Pass nullptr to the visitor with the same value category as the argument.
    This is done for uniformity.

Author: grisumbras

include/boost/json/impl/visit.hpp

@@ -10,49 +10,60 @@
 #ifndef BOOST_JSON_IMPL_VISIT_HPP
 #define BOOST_JSON_IMPL_VISIT_HPP
 
+#include <boost/type_traits/copy_cv_ref.hpp>
+
 namespace boost {
 namespace json {
+namespace detail {
 
-template<class Visitor>
+template<class Visitor, class Value>
 auto
 visit(
     Visitor&& v,
-    value& jv) -> decltype(
-        std::declval<Visitor>()(nullptr))
+    Value&& jv) -> decltype(
+        static_cast<Visitor&&>(v)(
+            std::declval< copy_cv_ref_t<std::nullptr_t, Value&&> >() ) )
 {
+    auto&& vis_ref = static_cast<Visitor&&>(v);
+    auto&& arg_ref = static_cast<Value&&>(jv);
     switch(jv.kind())
     {
     default: // unreachable()?
-    case kind::null:    return std::forward<Visitor>(v)(nullptr);
-    case kind::bool_:   return std::forward<Visitor>(v)(jv.get_bool());
-    case kind::int64:   return std::forward<Visitor>(v)(jv.get_int64());
-    case kind::uint64:  return std::forward<Visitor>(v)(jv.get_uint64());
-    case kind::double_: return std::forward<Visitor>(v)(jv.get_double());
-    case kind::string:  return std::forward<Visitor>(v)(jv.get_string());
-    case kind::array:   return std::forward<Visitor>(v)(jv.get_array());
-    case kind::object:  return std::forward<Visitor>(v)(jv.get_object());
+    case kind::bool_:   return vis_ref( arg_ref.get_bool() );
+    case kind::int64:   return vis_ref( arg_ref.get_int64() );
+    case kind::uint64:  return vis_ref( arg_ref.get_uint64() );
+    case kind::double_: return vis_ref( arg_ref.get_double() );
+    case kind::string:  return vis_ref( arg_ref.get_string() );
+    case kind::array:   return vis_ref( arg_ref.get_array() );
+    case kind::object:  return vis_ref( arg_ref.get_object() );
+    case kind::null: {
+        std::nullptr_t np;
+        using Ref = copy_cv_ref_t<std::nullptr_t, Value&&>;
+        return vis_ref( static_cast<Ref>(np) );
+    }
     }
 }
 
+} // namespace detail
+
+template<class Visitor>
+auto
+visit(
+    Visitor&& v,
+    value& jv) -> decltype(
+        static_cast<Visitor&&>(v)( std::declval<std::nullptr_t&>() ) )
+{
+    return detail::visit( static_cast<Visitor&&>(v), jv );
+}
+
 template<class Visitor>
 auto
 visit(
     Visitor&& v,
     value const& jv) -> decltype(
-        std::declval<Visitor>()(nullptr))
+        static_cast<Visitor&&>(v)( std::declval<std::nullptr_t const&>() ) )
 {
-    switch (jv.kind())
-    {
-    default: // unreachable()?
-    case kind::null:    return std::forward<Visitor>(v)(nullptr);
-    case kind::bool_:   return std::forward<Visitor>(v)(jv.get_bool());
-    case kind::int64:   return std::forward<Visitor>(v)(jv.get_int64());
-    case kind::uint64:  return std::forward<Visitor>(v)(jv.get_uint64());
-    case kind::double_: return std::forward<Visitor>(v)(jv.get_double());
-    case kind::string:  return std::forward<Visitor>(v)(jv.get_string());
-    case kind::array:   return std::forward<Visitor>(v)(jv.get_array());
-    case kind::object:  return std::forward<Visitor>(v)(jv.get_object());
-    }
+    return detail::visit( static_cast<Visitor&&>(v), jv );
 }
 
 } // namespace json

include/boost/json/visit.hpp

@@ -32,14 +32,14 @@ auto
 visit(
     Visitor&& v,
     value& jv) -> decltype(
-        std::declval<Visitor>()(nullptr));
+        static_cast<Visitor&&>(v)( std::declval<std::nullptr_t&>() ) );
 
 template<class Visitor>
 auto
 visit(
     Visitor &&v,
     value const &jv) -> decltype(
-        std::declval<Visitor>()(nullptr));
+        static_cast<Visitor&&>(v)( std::declval<std::nullptr_t const&>() ) );
 /** @} */
 
 } // namespace json

test/visit.cpp

@@ -25,15 +25,15 @@ class visit_test
         struct f
         {
             kind k;
-            bool operator()(std::nullptr_t) { return k == kind::null; }
-            bool operator()(bool) { return k == kind::bool_; }
-            bool operator()(std::int64_t) { return k == kind::int64; }
-            bool operator()(std::uint64_t) { return k == kind::uint64; }
-            bool operator()(double) { return k == kind::double_; }
-            bool operator()(string const&) { return k == kind::string; }
-            bool operator()(array const&) { return k == kind::array; }
-            bool operator()(object const&) { return k == kind::object; }
-            bool operator()(...) { return false; }
+            bool operator()(std::nullptr_t const&) const { return k == kind::null; }
+            bool operator()(bool const&) const { return k == kind::bool_; }
+            bool operator()(std::int64_t const&) const { return k == kind::int64; }
+            bool operator()(std::uint64_t const&) const { return k == kind::uint64; }
+            bool operator()(double const&) const { return k == kind::double_; }
+            bool operator()(string const&) const { return k == kind::string; }
+            bool operator()(array const&) const { return k == kind::array; }
+            bool operator()(object const&) const { return k == kind::object; }
+            bool operator()(...) const { return false; }
         };
         value const v(t);
         BOOST_TEST(visit(f{k}, v));
@@ -46,7 +46,7 @@ class visit_test
         struct f
         {
             kind k;
-            bool operator()(std::nullptr_t) { return k == kind::null; }
+            bool operator()(std::nullptr_t&) { return k == kind::null; }
             bool operator()(bool&) { return k == kind::bool_; }
             bool operator()(std::int64_t&) { return k == kind::int64; }
             bool operator()(std::uint64_t&) { return k == kind::uint64; }