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环境说明
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前言
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chrono
chrono 简介：
来源：http://www.cplusplus.com/reference/chrono/?kw=chrono
The elements in this header deal with time. This is done mainly by means of three concepts:
Durations
They measure time spans, like: one minute, two hours, or ten milliseconds.
In this library, they are represented with objects of the duration class template, that couples a count representation and a period precision (e.g., ten milliseconds has ten as count representation and milliseconds as period precision).
Time points
A reference to a specific point in time, like one's birthday, today's dawn, or when the next train passes.
In this library, objects of the time_point class template express this by using a duration relative to an epoch (which is a fixed point in time common to all time_point objects using the same clock).
Clocks
A framework that relates a time point to real physical time.
The library provides at least three clocks that provide means to express the current time as a time_point: system_clock, steady_clock and high_resolution_clock.
对我来看：
3种计时方式以及一个时间转换方式就ok，分别对应：
计时方式：
std::chrono::high_resolution_clock //high_resolution_clock is the clock with the shortest tick period. It may be a synonym for system_clock or steady_clock.
std::chrono::system_clock //Specifically, system_clock is a system-wide realtime clock.
std::chrono::steady_clock //steady_clock is specifically designed to calculate time intervals.
时间转换方式：
std::chrono::duration_cast
/*
Converts the value of dtn into some other duration type, taking into account differences in their periods.
The function does not use implicit conversions. Instead, all count values are internally converted into the widest representation (the common_type for the internal count types) and then casted to the destination type, all conversions being done explicitly with static_cast.
*/
chrono 大法中的坑（不同的时钟计时存在ms及us及ns级别的差别）
//windows下，std::chrono::system_clock 可能计时不准确，注意这里是“”“”可能“”“”
//windows下，high_resolution_clock 时间计时最精确，如果需要最高精确度，请使用此时钟
//总结：
//1 如果是秒级时间评估，std::chrono::system_clock 无所谓，如果是ms、us、ns级的时间评估(这里我做算法评估)，那么std::chrono::system_clock可能会出现较大的问题，这时候可用high_resolution_clock来解决此问题。
//2 如果计时应用在了多个线程，你需要把所有线程的计时加起来，才是你的真正耗时
//同时我猜测，出现此问题的原因就是不同时钟类型用的时间片(就是对应不同类型的一个时钟周期)是不一样的。当然，有一些cpu相关知识的朋友应该知道，最准确的计时是：时间片=cpu时钟周期（这是不合理的，理解到一部分就行了）
chrono 计时方法
//代码点一
std::chrono::time_point<std::chrono::high_resolution_clock> p0 = std::chrono::high_resolution_clock::now();
// ....... 若干代码
//代码点二
std::chrono::time_point<std::chrono::high_resolution_clock> p1 = std::chrono::high_resolution_clock::now();
//计算及打印耗时，用法不太标准,文中的1000 是换算到毫秒的意思
cout << "stitch high_resolution_clock time:" << (float)std::chrono::duration_cast<std::chrono::microseconds>(p1 - p0).count() / 1000 << "ms" << endl;
后记
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参考文献
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