[boost] [ptr_container] associative container insert takes reference instead of const reference as key ?

Hello , I've found that ptr_unordered_map.insert has following signature (from code and latest doc) std::pair<iterator,bool> insert( key_type& k, T* x ); that forbids calls like ptr_unordered_map.insert( myobj->id(), myobj) ,I wonder if this is by design , or some other reason ? why other overload takes const key_type& k as key type? and insert always call insert_impl which have const reference as key type ? any help would be appreciated.