Many types can be deduced and not explicitly mapped to a JSON type(string, number, bool, object, array) This allows one to use them directly in places that don't require a member name(e.g. json_array's element type) or with the json_link<Name, Type> mapping type. The order of deduction is as follows

types with existing json_data_contract's specialized for them
Well known types

Type	Mapped To	Notes
std:string_view	json_string_raw
std::string	json_string
bool	json_bool
Integer	json_number	Uses std::numeric_limits. json_number optimized for Signed/Unsigned integers
Enum	json_number	Used std::is_enum and std::underlying_type
Floating point	json_number	Uses std::numeric_limits
Associative Container	json_key_value_map	Has begin()/end()/key_type/mapped_type and constructable with two iterators
readable values		The value_type in the readable mapping of T with a json_null wrapped around T's deduced mapping. See the readable value cookbook item
Containers	json_array	Excluding associative containers. Uses value_type as the type for each element

Containers - map to json_array with the element type as the detected type of the value_type. Must have the methods begin(), end(), type alias value_type, and can be constructed with two iterators. Same Iterator requirements as std:: vector's two iterator constructor.