Fix #146.

Author: ColinH

include/tao/json/binding/internal/object.hpp

@@ -136,14 +136,20 @@ namespace tao::json::binding::internal
          }
       }
 
+      template< typename A, template< typename... > class Traits, typename C >
+      [[nodiscard]] static bool require_encode( const C& x )
+      {
+         return ( N == for_nothing_value::encode ) || ( A::kind == member_kind::required ) || ( !A::template is_nothing< Traits >( x ) );
+      }
+
 #if defined( _MSC_VER )
 #pragma warning( push )
 #pragma warning( disable : 4127 )
 #endif
       template< typename A, template< typename... > class Traits, typename C >
       static void assign_member( basic_value< Traits >& v, const C& x )
       {
-         if( ( N == for_nothing_value::encode ) || ( !A::template is_nothing< Traits >( x ) ) ) {
+         if( require_encode< A, Traits >( x ) ) {
             v.try_emplace( A::template key< Traits >(), A::read( x ) );
          }
       }
@@ -199,10 +205,7 @@ namespace tao::json::binding::internal
       template< template< typename... > class Traits, typename C >
       [[nodiscard]] static std::size_t produce_size( const C& x )
       {
-         if constexpr( N == for_nothing_value::encode ) {
-            return sizeof...( As );
-         }
-         return ( std::size_t( !As::template is_nothing< Traits >( x ) ) + ... );
+         return ( std::size_t( require_encode< As, Traits >( x ) ) + ... );
       }
 
 #if defined( _MSC_VER )
@@ -213,7 +216,7 @@ namespace tao::json::binding::internal
       template< typename A, template< typename... > class Traits, typename Consumer, typename C >
       static void produce_member( Consumer& consumer, const C& x )
       {
-         if( ( N == for_nothing_value::encode ) || ( !A::template is_nothing< Traits >( x ) ) ) {
+         if( require_encode< A, Traits >( x ) ) {
             A::template produce_key< Traits >( consumer );
             A::template produce< Traits >( consumer, x );
             consumer.member();

src/test/json/binding_object.cpp

@@ -4,6 +4,7 @@
 #include "test.hpp"
 
 #include <tao/json.hpp>
+#include <tao/json/contrib/traits.hpp>
 
 namespace tao::json
 {
@@ -212,7 +213,26 @@ namespace tao::json
       TEST_ASSERT( s.i == 42 );
    }
 
-   // TODO: Test with different for_nothing_value (incl. consistency of size to consumer).
+   struct type_7
+   {
+      std::vector< int > a;
+      std::vector< int > b;
+   };
+
+   template<>
+   struct traits< type_7 >
+      : binding::basic_object< binding::for_unknown_key::fail,
+                               binding::for_nothing_value::suppress,
+                               TAO_JSON_BIND_REQUIRED( "a", &type_7::a ),
+                               TAO_JSON_BIND_OPTIONAL( "b", &type_7::b ) >
+   {};
+
+   void unit_test_7()
+   {
+      const type_7 t;
+      const auto s = produce::to_string( t );
+      TEST_ASSERT( s == "{\"a\":[]}" );
+   }
 
    void unit_test()
    {
@@ -222,6 +242,7 @@ namespace tao::json
       unit_test_4();
       unit_test_5();
       unit_test_6();
+      unit_test_7();
    }
 
 }  // namespace tao::json