[boost] [property map] Document Error?

Hello, I'm reading the doc of Boost Property Map, in the part for identity_property_map, it is said, in the 1st table, that the type "boost::property_traits<identity_property_map>::value_type" is void, and that of key_type is also void. But in the file property_map.hpp, I found, struct identity_property_map // ... { typedef std::size_t key_type; typedef std::size_t value_type; // ... } It seems that the key_type and value_type should both be std::size_t. Am I misunderstanding or is it a document error? Thanks. Max