简短的转换，从秒转换为std :: chrono :: steady_clock :: duration? [英] Terse conversion from double seconds to std::chrono::steady_clock::duration?

问题描述

我可以这样做:

double period_in_seconds = 3.4;

auto as_duration = std::chrono::duration_cast<std::chrono::steady_clock::duration>(std::chrono::duration<double>(period_in_seconds));

但是我相信您会同意这是非常荒谬的.有麻烦的方法吗?

But I'm sure you'll agree that is quite ridiculous. Is there a terser way?

在明显的使用命名空间std :: chrono 旁边.

推荐答案

有充分的理由狂热地劝阻在文件范围(尤其是在标头中)不使用 using namespace s.但是，在功能范围内，特别是对于< chrono> lib，使用名称空间std :: chrono 的是您的朋友.

There is good reason to be fanatic about discouraging use of using namespaces at file scope, especially in headers. However at function scope, especially with the <chrono> lib, using namespace std::chrono is your friend.

这一步可以帮助您:

double period_in_seconds = 3.4;
auto as_duration = std::chrono::duration_cast<std::chrono::steady_clock::duration>(std::chrono::duration<double>(period_in_seconds));

收件人:

double period_in_seconds = 3.4;
using namespace std::chrono;
auto as_duration = duration_cast<steady_clock::duration>(duration<double>(period_in_seconds));

当从浮点持续时间转换为整数持续时间时，我建议使用 round .这不仅会给预期的"答案更多的频率，而且比 duration_cast 的冗长程度略低. std :: chrono :: round 是C ++ 17的新功能.但是您可以从此处或此处并立即开始使用.另外，如果将 {} 用于构造函数，则有助于将对象构造与函数调用区分开，从而使代码更易于阅读:

I recommend the use of round when casting from a floating point duration to an integral duration. Not only will this give the "expected" answer more often, but it is slightly less verbose than duration_cast. std::chrono::round is new with C++17. But you can grab it from various places like here or here and start using it today. Also if you use {} for constructors it helps distinguish object construction from function calls, making the code a little easier to read:

double period_in_seconds = 3.4;
using namespace std::chrono;
auto as_duration = round<steady_clock::duration>(duration<double>{period_in_seconds});

最后，即使在本地范围内，也可以毫不犹豫地创建自定义的持续时间，以提高可读性:

Finally, don't hesitate to create a custom duration, even if in a local scope to improve readability:

double period_in_seconds = 3.4;
using namespace std::chrono;
using dsec = duration<double>;
auto as_duration = round<steady_clock::duration>(dsec{period_in_seconds});

并且如果由于某种原因(例如样式指南放错了位置)而不能使用命名空间std :: chrono 使用本地函数，那么您可以用更具体的类型别名并使用声明来弥补这一点:

And if for some reason (say a misplaced style guide) you can't using a function-local using namespace std::chrono, then you can make up for that with more specific type aliases and using declarations:

double period_in_seconds = 3.4;
using dsec = std::chrono::duration<double>;
using duration = std::chrono::steady_clock::duration;
using std::chrono::round;
auto as_duration = round<duration>(dsec{period_in_seconds});

如果此代码旨在用于文件作用域，并且您不希望这些用法在文件作用域(这是完全可以理解的)，则只需创建一个函数来封装它们:

If this code is intended to be at file scope and you don't want these usings at file scope (which is perfectly understandable), just create a function to encapsulate them:

constexpr
std::chrono::steady_clock::duration
get_period(double period_in_seconds)
{
    using dsec = std::chrono::duration<double>;
    using duration = std::chrono::steady_clock::duration;
    using std::chrono::round;
    return round<duration>(dsec{period_in_seconds});
}

auto constexpr as_duration = get_period(3.4);

上面，我什至演示了这个特定的示例都可以在编译时完成. clang -O3 -S 将上述内容简化为以下程序集:

Above I've even demonstrated that this particular example can all be done at compile time. clang -O3 -S reduces the above down to the following assembly:

    .globl  _as_duration            ## @as_duration
    .p2align    3
_as_duration:
    .quad   3400000000              ## 0xcaa7e200

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