A nullable value is also called and option type or readable type. This includes std::optional, std::unique_ptr, std::shared_ptr, and pointers. In order to use them without a custom Constructor type for each mapping, or in deduced situations (json_link<...>) they must be mapped via the daw::nullable_value_traits type mapping.

A specialization must have the following, assuming T is the specialized type(e.g std::optional<int>):

• value_type type alias to the inner type. e.g ( std::optional<U> -> U )
• is_nullable A variable that must be static constexpr bool is_nullable = true for nullable types. If false it will mark the type T as not nullable
• static value_type const & read( T const & ) A function that reads the value from the nullable type and returns a const ref to it. A value_type return will work too
• T operator( )( daw::construct_nullable_with_empty_t ) const A member function that takes no args and returns an empty value.
• T operator( )( daw::construct_nullable_with_value_t, Args... ) const A member function that when passed appropriate args constructs a value. It should be SFINAE'able and constrain on valid arguments.
• static bool has_value( T const & ) returns if the value is present or not.