这是一段用于计时的C++代码(C++11)

struct Stopwatch{
	Stopwatch(chrono::nanoseconds& _res)
	:res{_res},
	 start{chrono::high_resolution_clock::now()} {}
	~Stopwatch()
	{
		res=chrono::high_resolution_clock::now()-start;
	}
	private:
		chrono::nanoseconds& res;
		chrono::time_point<chrono::high_resolution_clock> start;
};

报错了：
line 4: [Error] invalid initialization of non-const reference of type 'std::chrono::nanoseconds& {aka std::chrono::duration<long long int, std::ratio<1ll, 1000000000ll> >&}' from an rvalue of type '<brace-enclosed initializer list>'
所以怎么办