template< class R, class... ArgTypes >
bool operator==( const std::function<R(ArgTypes...)>& f, std::nullptr_t ); (1) (since C++11) template< class R, class... ArgTypes >
bool operator==( std::nullptr_t, const std::function<R(ArgTypes...)>& f ); (2) (since C++11) template< class R, class... ArgTypes >
bool operator!=( const std::function<R(ArgTypes...)>& f, std::nullptr_t ); (3) (since C++11) template< class R, class... ArgTy