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

问题描述

以下定义之间有区别吗？

Is there a difference between the following definitions?

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

如果不是，C ++ 11中首选哪种样式？

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

推荐答案

我相信有区别。让我们重命名它们，以便我们可以更容易地谈论它们：

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 是常量，这意味着您无法修改它们。但仅 PI2 是编译时常量。 应在编译时初始化。 PI1 可能在编译时或运行时初始化。此外，在需要编译时常量的上下文中可以使用仅 c $ c $> PI2 。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 compiled 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

您应该使用哪个？使用满足您需要的。你想确保你有一个编译时常数，可以在需要编译时常数的上下文中使用？你想要能够在运行时完成一个计算来初始化它吗？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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