operator==

template<class Fty>
    bool operator==(const function<Fty>& f, null_ptr_type npc);
template<class Fty>
    bool operator==(null_ptr_type npc, const function<Fty>& f);

[Added with TR1]

The operators both take an argument that is a reference to a function object and an argument that is a null pointer. Both return true only if the function object is empty.