Made the 'data' member optional in node_ref - it's only created on-demand, so we don't waste extra memory every time we do Value tmp = v;

src/value/detail/node_data.cpp

@@ -8,6 +8,8 @@ namespace YAML
 {
 	namespace detail
 	{
+		std::string node_data::empty_scalar;
+
 		node_data::node_data(): m_isDefined(false), m_type(ValueType::Null)
 		{
 		}