BSON: allow and discard values and object entries of type `value_t::discarded`

include/nlohmann/detail/output/binary_writer.hpp

@@ -9,7 +9,6 @@
 #include <nlohmann/detail/input/binary_reader.hpp>
 #include <nlohmann/detail/output/output_adapters.hpp>
 
-
 namespace nlohmann
 {
 namespace detail
@@ -864,6 +863,8 @@ class binary_writer
                 assert(false);
                 return 0ul;
             // LCOV_EXCL_STOP
+            case value_t::discarded:
+                return 0ul;
             case value_t::object:
                 return header_size + calc_bson_object_size(*j.m_value.object);
             case value_t::array:
@@ -898,6 +899,8 @@ class binary_writer
                 assert(false);
                 return;
             // LCOV_EXCL_STOP
+            case value_t::discarded:
+                return;
             case value_t::object:
                 return write_bson_object_entry(name, *j.m_value.object);
             case value_t::array:

single_include/nlohmann/json.hpp

@@ -7853,7 +7853,6 @@ class binary_reader
 // #include <nlohmann/detail/output/output_adapters.hpp>
 
 
-
 namespace nlohmann
 {
 namespace detail
@@ -8708,6 +8707,8 @@ class binary_writer
                 assert(false);
                 return 0ul;
             // LCOV_EXCL_STOP
+            case value_t::discarded:
+                return 0ul;
             case value_t::object:
                 return header_size + calc_bson_object_size(*j.m_value.object);
             case value_t::array:
@@ -8742,6 +8743,8 @@ class binary_writer
                 assert(false);
                 return;
             // LCOV_EXCL_STOP
+            case value_t::discarded:
+                return;
             case value_t::object:
                 return write_bson_object_entry(name, *j.m_value.object);
             case value_t::array:

test/src/unit-bson.cpp

@@ -376,6 +376,31 @@ TEST_CASE("BSON")
             CHECK(json::from_bson(result, true, false) == j);
         }
 
+        SECTION("discarded values are not serialized")
+        {
+            json j = json::value_t::discarded;
+            const auto result = json::to_bson(j);
+            CHECK(result.empty());
+        }
+
+        SECTION("discarded members are not serialized")
+        {
+            json j =
+            {
+                { "entry", json::value_t::discarded }
+            };
+
+            std::vector<uint8_t> expected =
+            {
+                0x05, 0x00, 0x00, 0x00, // size (little endian)
+                // no entries
+                0x00 // end marker
+            };
+
+            const auto result = json::to_bson(j);
+            CHECK(result == expected);
+        }
+
 
         SECTION("non-empty object with object member")
         {