哪位高手能看懂typedef ReturnT (ConsumerT:PtrMember)()

谁能看懂typedef ReturnT (ConsumerT::*PtrMember)();

    template <typename ConsumerT, typename ReturnT>
    class Callback: public CallbackBase<ReturnT>
    {
        private:
        typedef ReturnT (ConsumerT::*PtrMember)();

        public:
        Callback(ConsumerT* const object, PtrMember member) : object(object), member(member) {}

        Callback(const Callback<ConsumerT,ReturnT> &e) : object(e.object), member(e.member) {}

        ReturnT operator()()
        {
            return (const_cast<ConsumerT*>(object)->*member)();
        }

        private:
        ConsumerT* const object;
        const PtrMember  member;
    };