如何概括标准::时间::d?

使用chrono时最好的一般建议是只在绝对必要时转义类型系统(使用.count()(。这可能是与 C 或一些不理解 chrono 的C++库接口。在 C++ 20 之前，这也意味着输出到流。

如果我们把自己保持在类型系统中，我们可以得到很多总是正确的很好的转换。

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chrono。调用getTimeElapsed还chrono兼容。

我不完全满意，所以让我们也支持durationgetTimeElapsed：

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，因为在上一个else中，我们只使用一个时间和单位参数对，这意味着我们无法满足两种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");
}
}
}
}
...

伟大！但是，我们仍然邀请用户发送他们想要getTimeElapsed的任何内容，除非他们碰巧有.count()成员，否则将导致编译器错误。 让我们稍微约束一下我们的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();
}

我们不需要更改此调用代码。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，这其中包含您有一个子问题。

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"我如何整理嵌套的if语句并仅打印出前两个非零单元。

这并不像最初看起来那么简单。在我看来，递归很少是答案。将其视为单元类型的循环也是过度工程化的，您需要编写一些代码来从元组获取当前类型的索引，将其增加 1，然后使用它来索引相同的元组以获得下一个具有更高分辨率的单元。然后，当所有这些说完并完成时，您仍然需要知道要打印哪个单位才能为值提供上下文。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();
}

将总运行时间作为std::chrono::nanoseconds(您已经从end - begin中获取(并将其传递给getTimeElapsed。我们现在执行与以前相同的计算来获取组件单位，但也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是 1'234'568ns，则结果是"1 [ms] 234 [us]"。有尾随空间，但我会留给你修复。

这也意味着我们不再需要之前重构过的template，但我添加了它们来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;
}

如果你想更进一步，永远不需要逃避类型系统，那么我建议看看Howard Hinnant的date库。此库是 C++20 中新的chrono功能的基础，并为表带来了字符串格式。只需以适合您的任何方式包含库中的date.h，并按如下方式修改getTimeElapsed：

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，现在的结果将是："1ms 234us"，表示1'234'567ns。