比率是常数，但如果我想接受不同的比率作为参数怎么办? [英] Ratio<,> is constant, but what if I want to accept different ratios as an argument?

问题描述

也许是一个矛盾的问题:ratio<,> 根据定义, 编译时常量.

Perhaps an oxymoronic question: ratio<,> is, by definition, a compile-time constant.

但是，我想构造具有不同比率的持续时间，这些比率可由我的方法的调用者指定.我猜我应该使用比率和/或持续时间以外的其他东西，但是什么?

However, I would like to construct durations with different ratios that can be specified by the caller of my method. I'm guessing I should be using something other than ratio and/or duration, then, but what?

我想要一个可以在运行时设置的类成员，我希望它的类型为 ratio<,>.在代码中的某个点，该成员被设置/分配，它将被分配一个恒定的比率，但在我的班级中，我不想指定该比率应该是多少.

I want to have, say, a class member that can be set at runtime, and I would like it to be of type ratio<,>. At some point in the code, where this member gets set/assigned, it would be assigned a constant ratio, but in my class, I don't want to specify what that ratio should be.

推荐答案

好的，您需要一个 std::ratio，可以由调用者指定".共有三种不同的选项:

Ok, you need a std::ratio which "can be specified by the caller". There are three different options:

• 调用者想要在编译时间期间设置std::ratio模板参数.这很简单，只需设置参数，例如使用四分之一 = std::ratio<1, 4>.
• 调用者想要在运行时期间设置std::ratio模板参数.这根据定义是不可能的，std::ratio 是一个编译时常量.您必须实现自己的类来在运行时计算比率或使用某种库(我不知道有任何库，欢迎提出建议！).
• 调用者只想缩放 std::duration(这似乎是您使用 std::ratio 的目的).然后只需乘以一个数字.

• The caller wants to set std::ratio template parameters during compile time. This is trivial, simply set the parameters, e.g. using quarter = std::ratio<1, 4>.
• The caller wants to set std::ratio template parameters during runtime. This is not possible by definition, std::ratio is a compile-time constant. You will have to implement your own class for computing ratios during runtime or use some kind of library (I don't know of any, suggestions welcome!).
• The caller merely wants to scale a std::duration (which is what you seem to use your std::ratio for). Then just multiply it with a number.

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