in include/spotify/json/codec/object.hpp [250:262]
void add_field(
const std::string &name,
bool required,
value_type object_type::*member,
codec_type &&codec) {
using member_ptr = value_type (object_type::*);
using field_type = member_var_field<member_ptr, typename std::decay<codec_type>::type>;
_fields.save(name, required, std::shared_ptr<detail::field>(new field_type(
required,
_fields.num_required_fields(),
std::forward<codec_type>(codec),
member)));
}