变量上的 const 与 constexpr

问题描述

以下定义有区别吗?

const     double PI = 3.141592653589793;
constexpr double PI = 3.141592653589793;

如果不是，那么在 C++11 中更喜欢哪种风格?

If not, which style is preferred in C++11?

推荐答案

我相信是有区别的.让我们重命名它们，以便我们可以更轻松地讨论它们:

I believe there is a difference. Let's rename them so that we can talk about them more easily:

const     double PI1 = 3.141592653589793;
constexpr double PI2 = 3.141592653589793;

PI1 和 PI2 都是常量，意味着你不能修改它们.但是仅 PI2 是编译时常量.它应在编译时初始化.PI1 可以在编译时或运行时初始化.此外，仅 PI2 可以在需要编译时常量的上下文中使用.例如:

Both PI1 and PI2 are constant, meaning you can not modify them. However only PI2 is a compile-time constant. It shall be initialized at compile time. PI1 may be initialized at compile time or run time. Furthermore, only PI2 can be used in a context that requires a compile-time constant. For example:

constexpr double PI3 = PI1;  // error

但是:

constexpr double PI3 = PI2;  // ok

和:

static_assert(PI1 == 3.141592653589793, "");  // error

但是:

static_assert(PI2 == 3.141592653589793, "");  // ok

至于您应该使用哪个?使用满足您需求的任何一种.你想确保你有一个编译时常量可以在需要编译时常量的上下文中使用吗?您是否希望能够使用在运行时完成的计算来初始化它?等

As to which you should use? Use whichever meets your needs. Do you want to ensure that you have a compile time constant that can be used in contexts where a compile-time constant is required? Do you want to be able to initialize it with a computation done at run time? Etc.

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