C++ Chrono - 以毫秒为单位的两个时间点之间的差异？
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Chrono - The difference between two points in time in milliseconds?
提问by waas1919
How can I get (using the std::chrono library) the difference between two points in time in milliseconds?
我怎样才能（使用 std::chrono 库）以毫秒为单位获得两个时间点之间的差异？
I could do that using this:
我可以用这个来做到这一点：
std::chrono::time_point<std::chrono::system_clock> now = std::chrono::system_clock::now();
std::chrono::time_point<std::chrono::system_clock> foo = now + std::chrono::milliseconds(100);
std::chrono::duration<float> difference = foo - now;
const int milliseconds = difference.count() * 1000;
How can I get this time in milliseconds, so I can use the duration as a unsigned int, and not a float and then multiply by 1000?
我怎样才能以毫秒为单位获得这个时间，所以我可以将持续时间用作无符号整数，而不是浮点数，然后乘以 1000？
回答by lisyarus
std::chrono::durationhas two template parameters, the second being exactly the unit of measure. You can invoke std::chrono::duration_castto cast from one duration type to another. Also, there is a predefined duration type for milliseconds: std::chrono::milliseconds. Composing this together:
std::chrono::duration有两个模板参数，第二个正是度量单位。您可以调用std::chrono::duration_cast从一种持续时间类型转换为另一种持续时间类型。此外，还有一个预定义的毫秒持续时间类型：std::chrono::milliseconds. 一起组成这个：
auto milliseconds = std::chrono::duration_cast<std::chrono::milliseconds>(foo - now);
To get the actual number of milliseconds, use duration::count:
要获得实际的毫秒数，请使用duration::count：
auto ms = milliseconds.count();
Its return type is duration::rep, which for standard duration types like std::chrono::millisecondsis a signed integer of unspecified size.
它的返回类型是duration::rep，对于标准持续时间类型，它std::chrono::milliseconds是一个未指定大小的有符号整数。
回答by zoska
chrono::duration_cast<chrono::milliseconds>(end_time - start_time).count()
回答by johnjohnlys
http://www.cplusplus.com/reference/chrono/duration_cast/
http://www.cplusplus.com/reference/chrono/duration_cast/
std::chrono::duration_cast<std::chrono::milliseconds>();
回答by AceFunk
I had issues with the duration printing out with letters like e-09. Here's how I fixed it:
我在打印 e-09 之类的字母时遇到了持续时间问题。这是我修复它的方法：
auto start = std::chrono::high_resolution_clock::now();
< run your function or code here >
auto finish = std::chrono::high_resolution_clock::now();
std::chrono::duration<double, std::milli> elapsed = finish - start;
std::cout << "Elapsed Time: " << elapsed.count() << " seconds" << std::endl;
And now I get desired results:
现在我得到了想要的结果：
Elapsed Time: 34.406 seconds
