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[英]How can std::chrono::duration::duration() be constexpr?
[英]How to generalize std::chrono::duration(s)?
我为我的大学 class 编写了三个版本的算法。
一种是蛮力,另一种是贪婪,最后一种是启发式。
我希望能够测量每个算法完成所需的时间。
我正在使用<chrono>
库来实现这一点
现在我的代码如下所示:
#include <iostream>
#include <chrono>
#include <sstream>
using namespace std;
string getTimeElapsed(long time1, const string &unit1, long time2 = 0, const string &unit2 = "") {
stringstream s;
s << time1 << " [" << unit1 << "]";
if (time2) s << " " << time2 << " [" << unit2 << "]";
return s.str();
}
int main() {
auto begin = chrono::system_clock::now();
// algorithm goes here
auto solution = /* can be anything */
auto end = chrono::system_clock::now();
auto diff = end - begin;
string timeElapsed;
auto hours = chrono::duration_cast<chrono::hours>(diff).count();
auto minutes = chrono::duration_cast<chrono::minutes>(diff).count();
if (hours) {
minutes %= 60;
timeElapsed = getTimeElapsed(hours, "h", minutes, "min");
} else {
auto seconds = chrono::duration_cast<chrono::seconds>(diff).count();
if (minutes) {
seconds %= 60;
timeElapsed = getTimeElapsed(minutes, "min", seconds, "s");
} else {
auto milliseconds = chrono::duration_cast<chrono::milliseconds>(diff).count();
if (seconds) {
milliseconds %= 1000;
timeElapsed = getTimeElapsed(seconds, "s", milliseconds, "ms");
} else {
auto microseconds = chrono::duration_cast<chrono::microseconds>(diff).count();
if (milliseconds) {
microseconds %= 1000;
timeElapsed = getTimeElapsed(milliseconds, "ms", microseconds, "μs");
} else {
auto nanoseconds = chrono::duration_cast<chrono::nanoseconds>(diff).count();
if (microseconds) {
nanoseconds %= 1000;
timeElapsed = timeElapsed = getTimeElapsed(microseconds, "μs", nanoseconds, "ns");
} else timeElapsed = getTimeElapsed(nanoseconds, "ns");
}
}
}
}
cout << "Solution [" << solution << "] found in " << timeElapsed << endl;
return 0;
}
如您所见,堆叠的if-else
子句看起来非常难看,您可以在此处看到一个模式:
if (timeUnit) {
timeElapsed = /* process current time units */
} else {
/* step down a level and do the same for smaller time units */
}
我想让该过程成为递归 function。
但是,我不知道这种 function 的参数应该是什么,因为 chrono chrono::duration
是一个模板结构(?)
这个 function 看起来有点像这样:
string prettyTimeElapsed(diff, timeUnit) {
// recursion bound condition
if (timeUnit is chrono::nanoseconds) return getTimeElapsed(timeUnit, "ns");
auto smallerTimeUnit = /* calculate smaller unit using current unit */
if (timeUnit) return getTimeElapsed(timeUnit, ???, smallerTimeUnit, ???);
else return prettyTimeElapsed(diff, smallerTimeUnit);
}
我正在考虑这样做:
auto timeUnits = {chrono::hours(), chrono::minutes(), ..., chrono::nanoseconds()};
然后我可以将指针(甚至是索引)指向时间单位并将其传递给 function。
问题是我不知道如何概括这些结构。
CLion 高亮显示一个错误Deduced conflicting types (duration<[...], ratio<3600, [...]>> vs duration<[...], ratio<60, [...]>>) for initializer list element type
使用chrono
时最好的一般建议是仅在绝对必须时转义类型系统(使用.count()
)。 这可能与 C 或一些不了解计时的 C++ 库接口。 在 C++ 20 之前,这也意味着输出到 stream。
如果我们把自己留在类型系统中,我们可以得到很多很好的转换,而且总是正确的。
让我们更正问题中的代码以反映这一点:
#include <iostream>
#include <chrono>
#include <sstream>
std::string getTimeElapsed(long time1, const std::string &unit1, long time2 = 0, const std::string &unit2 = "") {
std::stringstream s;
s << time1 << " [" << unit1 << "]";
if (time2) s << " " << time2 << " [" << unit2 << "]";
return s.str();
}
int main() {
auto begin = std::chrono::system_clock::now();
// algorithm goes here
auto solution = "solution"; /* can be anything */
auto end = std::chrono::system_clock::now();
auto diff = end - begin;
std::string timeElapsed{""};
// Let's make the typing and reading easier for us but requires C++14
using namespace std::chrono_literals;
auto hours = std::chrono::duration_cast<std::chrono::hours>(diff);
auto minutes = std::chrono::duration_cast<std::chrono::minutes>(diff % 1h);
if (hours != 0h) {
// We need to escape the type system to call getTimeElapsed
timeElapsed = getTimeElapsed(hours.count(), "h", minutes.count(), "min");
} else {
auto seconds = std::chrono::duration_cast<std::chrono::seconds>(diff % 1min);
if (minutes != 0min) {
timeElapsed = getTimeElapsed(minutes.count(), "min", seconds.count(), "s");
} else {
auto milliseconds = std::chrono::duration_cast<std::chrono::milliseconds>(diff % 1s);
if (seconds != 0s) {
timeElapsed = getTimeElapsed(seconds.count(), "s", milliseconds.count(), "ms");
} else {
auto microseconds = std::chrono::duration_cast<std::chrono::microseconds>(diff % 1ms);
if (milliseconds != 0ms) {
timeElapsed = getTimeElapsed(milliseconds.count(), "ms", microseconds.count(), "μs");
} else {
auto nanoseconds = std::chrono::duration_cast<std::chrono::nanoseconds>(diff % 1us);
if (microseconds != 0us) {
timeElapsed = timeElapsed = getTimeElapsed(microseconds.count(), "μs", nanoseconds.count(), "ns");
} else timeElapsed = getTimeElapsed(nanoseconds.count(), "ns");
}
}
}
}
std::cout << "Solution [" << solution << "] found in " << timeElapsed << std::endl;
return 0;
}
现在,我们尽可能长时间地坚持使用chrono
。 调用getTimeElapsed
尚不兼容chrono
。
我并不完全满意,所以让我们在getTimeElapsed
中也支持duration
:
template <typename Duration1, typename Duration2>
std::string getTimeElapsed(Duration1 time1, const std::string &unit1, Duration2 time2, const std::string &unit2) {
std::stringstream s;
s << time1.count() << " [" << unit1 << "]";
if (time2 != Duration2::zero()) s << " " << time2.count() << " [" << unit2 << "]";
return s.str();
}
template <typename Duration1>
std::string getTimeElapsed(Duration1 time1, const std::string &unit1) {
std::stringstream s;
s << time1.count() << " [" << unit1 << "]";
return s.str();
}
我们需要两个版本的getTimeElapsed
,因为在最后一个else
中,我们只使用一个时间和单位参数对,这意味着我们不能满足两种Duration
类型的template
参数要求。 现在代码看起来好多了(只保留相关的更改):
...
if (hours != 0h) {
timeElapsed = getTimeElapsed(hours, "h", minutes, "min");
} else {
auto seconds = std::chrono::duration_cast<std::chrono::seconds>(diff % 1min);
if (minutes != 0min) {
timeElapsed = getTimeElapsed(minutes, "min", seconds, "s");
} else {
auto milliseconds = std::chrono::duration_cast<std::chrono::milliseconds>(diff % 1s);
if (seconds != 0s) {
timeElapsed = getTimeElapsed(seconds, "s", milliseconds, "ms");
} else {
auto microseconds = std::chrono::duration_cast<std::chrono::microseconds>(diff % 1ms);
if (milliseconds != 0ms) {
timeElapsed = getTimeElapsed(milliseconds, "ms", microseconds, "μs");
} else {
auto nanoseconds = std::chrono::duration_cast<std::chrono::nanoseconds>(diff % 1us);
if (microseconds != 0us) {
timeElapsed = timeElapsed = getTimeElapsed(microseconds, "μs", nanoseconds, "ns");
} else timeElapsed = getTimeElapsed(nanoseconds, "ns");
}
}
}
}
...
很好,但是,我们仍然邀请用户发送他们想要的任何内容getTimeElapsed
,除非他们碰巧有一个.count()
成员,否则会导致编译器错误。 让我们稍微限制一下我们的template
:
template <typename Rep1, typename Ratio1, typename Rep2, typename Ratio2>
std::string getTimeElapsed(std::chrono::duration<Rep1, Ratio1> time1, const std::string &unit1, std::chrono::duration<Rep2, Ratio2> time2, const std::string &unit2) {
std::stringstream s;
s << time1.count() << " [" << unit1 << "]";
if (time2 != time2.zero()) s << " " << time2.count() << " [" << unit2 << "]";
return s.str();
}
template <typename Rep, typename Ratio>
std::string getTimeElapsed(std::chrono::duration<Rep, Ratio> time1, const std::string &unit1) {
std::stringstream s;
s << time1.count() << " [" << unit1 << "]";
return s.str();
}
我们不需要为此更改调用代码。 我相信这足以帮助您了解如何在更通用的上下文中使用std::chrono::duration
,这是您遇到的一个子问题。
现在我们可以开始解决您的问题,我认为(通过阅读评论)实际上是“我怎样才能整理嵌套的if
语句并只打印前两个非零单元。”
这并不像最初出现的那么简单。 在我看来,递归很少是答案。 将其视为单元类型的循环也对其进行了过度设计,您需要编写一些代码来从元组中获取当前类型的索引,将其加一,然后使用它来索引相同的元组以获得更高分辨率的下一个单元。 然后,当所有这些都说完了,你仍然需要知道要打印什么单位来为值提供它的上下文。 我宁愿看到getTimeElapsed
写成如下:
std::string getTimeElapsed(std::chrono::nanoseconds elapsed, size_t maxUnits = 2)
{
using namespace std::chrono_literals;
std::ostringstream formatted("");
int usedUnits{};
auto hours = std::chrono::duration_cast<std::chrono::hours>(elapsed);
if (hours != 0h)
{
formatted << hours.count() << " [h] ";
++usedUnits;
}
auto minutes = std::chrono::duration_cast<std::chrono::minutes>(elapsed % 1h);
if (minutes != 0min)
{
formatted << minutes.count() << " [min] ";
++usedUnits;
}
auto seconds = std::chrono::duration_cast<std::chrono::seconds>(elapsed % 1min);
if (seconds != 0min && usedUnits != maxUnits)
{
formatted << seconds.count() << " [s] ";
++usedUnits;
}
auto milliseconds = std::chrono::duration_cast<std::chrono::milliseconds>(elapsed % 1s);
if (milliseconds != 0ms && usedUnits != maxUnits)
{
formatted << milliseconds.count() << " [ms] ";
++usedUnits;
}
auto microseconds = std::chrono::duration_cast<std::chrono::microseconds>(elapsed % 1ms);
if (microseconds != 0us && usedUnits != maxUnits)
{
formatted << microseconds.count() << " [us] ";
++usedUnits;
}
auto nanoseconds = std::chrono::duration_cast<std::chrono::nanoseconds>(elapsed % 1us);
if (nanoseconds != 0us && usedUnits != maxUnits)
{
formatted << nanoseconds.count() << " [us] ";
++usedUnits;
}
return formatted.str();
}
将总经过时间作为std::chrono::nanoseconds
nanoseconds (从end - begin
)并将其传递给getTimeElapsed
。 我们现在进行与以前相同的计算以获取组件单位,但还要跟踪我们计算了多少单位。 如果elapsed
为1'000'000'000ns,则结果为“1 [s]”; 如果elapsed
为 1'234'568ns,则结果为“1 [ms] 234 [us]”。 有尾随空格,但我会把它留给你来解决。
这也意味着我们不再需要我们之前重构的template
,但我添加它们是为了展示我在整个重构过程中的思考过程。 最终程序如下所示:
#include <chrono>
#include <iostream>
#include <sstream>
std::string getTimeElapsed(std::chrono::nanoseconds elapsed, size_t maxUnits = 2)
{
using namespace std::chrono_literals;
std::ostringstream formatted("");
int usedUnits{};
auto hours = std::chrono::duration_cast<std::chrono::hours>(elapsed);
if (hours != 0h)
{
formatted << hours.count() << " [h] ";
++usedUnits;
}
auto minutes = std::chrono::duration_cast<std::chrono::minutes>(elapsed % 1h);
if (minutes != 0min)
{
formatted << minutes.count() << " [min] ";
++usedUnits;
}
auto seconds = std::chrono::duration_cast<std::chrono::seconds>(elapsed % 1min);
if (seconds != 0min && usedUnits != maxUnits)
{
formatted << seconds.count() << " [s] ";
++usedUnits;
}
auto milliseconds = std::chrono::duration_cast<std::chrono::milliseconds>(elapsed % 1s);
if (milliseconds != 0ms && usedUnits != maxUnits)
{
formatted << milliseconds.count() << " [ms] ";
++usedUnits;
}
auto microseconds = std::chrono::duration_cast<std::chrono::microseconds>(elapsed % 1ms);
if (microseconds != 0us && usedUnits != maxUnits)
{
formatted << microseconds.count() << " [us] ";
++usedUnits;
}
auto nanoseconds = std::chrono::duration_cast<std::chrono::nanoseconds>(elapsed % 1us);
if (nanoseconds != 0us && usedUnits != maxUnits)
{
formatted << nanoseconds.count() << " [us] ";
++usedUnits;
}
return formatted.str();
}
int main() {
auto begin = std::chrono::system_clock::now();
// algorithm goes here
auto solution = "solution"; /* can be anything */
auto end = std::chrono::system_clock::now();
auto diff = end - begin;
using namespace std::chrono_literals;
std::cout << "Solution [" << solution << "] found in " << getTimeElapsed(1'234'567ns) << std::endl;
return 0;
}
如果您想更进一步并且永远不需要逃避类型系统,那么我建议您查看Howard Hinnant 的date
库。 该库是 C++20 中新的chrono
功能的基础,并将字符串格式化引入表中。 只需以适合您的任何方式从库中包含date.h
并修改getTimeElapsed
如下:
std::string getTimeElapsed(std::chrono::nanoseconds elapsed, size_t maxUnits = 2)
{
using namespace std::chrono_literals;
std::ostringstream formatted("");
int usedUnits{};
auto hours = std::chrono::duration_cast<std::chrono::hours>(elapsed);
if (hours != 0h)
{
formatted << hours << " ";
++usedUnits;
}
auto minutes = std::chrono::duration_cast<std::chrono::minutes>(elapsed % 1h);
if (minutes != 0min)
{
formatted << minutes << " ";
++usedUnits;
}
auto seconds = std::chrono::duration_cast<std::chrono::seconds>(elapsed % 1min);
if (seconds != 0min && usedUnits != maxUnits)
{
formatted << seconds << " ";
++usedUnits;
}
auto milliseconds = std::chrono::duration_cast<std::chrono::milliseconds>(elapsed % 1s);
if (milliseconds != 0ms && usedUnits != maxUnits)
{
formatted << milliseconds << " ";
++usedUnits;
}
auto microseconds = std::chrono::duration_cast<std::chrono::microseconds>(elapsed % 1ms);
if (microseconds != 0us && usedUnits != maxUnits)
{
formatted << microseconds << " ";
++usedUnits;
}
auto nanoseconds = std::chrono::duration_cast<std::chrono::nanoseconds>(elapsed % 1us);
if (nanoseconds != 0us && usedUnits != maxUnits)
{
formatted << nanoseconds << " ";
++usedUnits;
}
return formatted.str();
}
使用与以前相同的值,结果现在将是:“1ms 234us”代表 1'234'567ns。
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