const vs constexpr 变量 [英] const vs constexpr on variables

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，C++11 中首选哪种风格?

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，这意味着你不能修改它们.但是 only 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，only 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

至于你应该使用哪个?使用满足您需求的任何一个.你想确保你有一个编译时间常数，可以在需要编译时间常数的上下文中使用吗?您希望能够通过在运行时完成的计算来初始化它吗?等等.

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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