静态constexpr变量与函数 [英] static constexpr variable vs function

问题描述

将浮点常量声明为 static constexpr 变量和一个函数之间有区别，如下例所示，还是只是一个风格问题？

Is there a difference between declaring floating point constant as a static constexpr variable and a function as in example below, or is it just a matter of style?

class MY_PI
{
public:
    static constexpr float MY_PI_VAR = 3.14f;
    static constexpr float MY_PI_FUN() { return 3.14f; }
}

推荐答案

constexpr 函数

函数有一个优点，自由变量没有（直到C ++ 14）：它们可以很容易模板没有一些类样板。这意味着您可以根据模板参数精确地获得 pi ：

template<typename T>
constexpr T pi();

template<>
constexpr float pi() { return 3.14f; }

template<>
constexpr double pi() { return 3.1415; }

int main()
{
    constexpr float a = pi<float>();
    constexpr double b = pi<double>();
}

但是，如果你决定使用 static 成员函数而不是自由函数，它不会比 static 成员变量更短或更容易写。

However, if you decide to use a static member function instead of a free function, it won't be shorter nor easir to write than a static member variable.

使用变量的主要优点是...好。你想要一个常数，对吧？

The main advantage of using a variable is that... well. You want a constant, right? It clarifies the intent and that may be one of the most important points here.

constexpr float a = constants<float>::pi;

或者这样，如果你的类只是代表 pi ：

Or like this if your class is only meant to represent pi:

constexpr double = pi<double>::value;

在第一种情况下，您可能更喜欢使用变量，因为它会更短，真的表明你正在使用一个常数，而不是试图计算的东西。如果你只有一个类代表pi，你可以使用一个自由的 constexpr 函数，而不是一个整体类。这将使IMHO更简单。

In the first case, you may prefer to use variables since it will be shorter to write and that will really show that you are using a constant and not trying to compute something. If you just have a class representing pi, you could however go with a free constexpr function instead of a whole class. It would IMHO be simpler.

但是，请注意，如果选择使用C ++ 14而不是C + +11，您将能够编写以下类型的 constexpr 变量模板：

However, note that if you choose to use C++14 instead of C++11, you will be able to write the following kind of constexpr variable templates:

template<typename T>
constexpr T pi = T(3.1415);

这样就可以这样编写代码：

That will allow you to write your code like this:

constexpr float a = pi<float>;

在C ++ 14中，这可能是首选的做事方式。如果您使用的是旧版本的标准，则前两个段落仍然有效。

In C++14, this may be the preferred way to do things. If you are using an older version of the standard, the first two paragraphs still hold.

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