json_nullable types

nullable types are supported by using the json_nullable mapping type or appending the type name with _null to use the convenience wrapper. The requirement of the underlying type is that it will be default constructable for null values or support the Nullable Concept.

Named _null mappings accept a JsonNullable template argument that controls how an empty value is serialized when it is a class member:

• JsonNullable::Nullable (the default) permits the entire member to be omitted.
• JsonNullable::NullVisible always emits the member, using null for an empty value.

Take the following JSON

[
  {
    "member0": 5,
    "member1": "hello"
  },
  {
    "member1": "world",
    "member2": true
  }
]

The JSON document is an array of an object with two members, an optional int, a string, and an optional bool.

To see a working example using this code, refer to cookbook_optional_values1_test.cpp.

 ++
struct MyOptionalStuff1 {
  std::optional<int> member0;
  std::string member1;
  std::unique_ptr<bool> member2;
};
 
namespace daw::json {
  template<>
  struct json_data_contract<MyOptionalStuff1> {
    using type = json_member_list<
      json_number_null<"member0", std::optional<int>>, 
      json_string<"member1">,
      json_bool_null<
        "member2", 
        std::unique_ptr<bool>, 
        options::bool_opt( options::LiteralAsStringOpt::Never ),
        JsonNullable::Nullable,
        UniquePtrConstructor<bool>
      >
    >;
 
    static inline auto to_json_data( MyOptionalStuff1 const &value ) {
      return std::forward_as_tuple( value.member0, value.member1, value.member2 );
    }
  };
}