const变量和静态变量之间有什么区别，我应该选择哪个？ [英] What is the difference between a const variable and a static variable and which should I choose?

问题描述

我从 RFC 246知道这一点：

• 常数声明恒定值。这些代表值，而不是存储器地址。这是最常见的事情，并且在今天几乎所有情况下都可以取代 static 。


• statics 声明全局变量。这些代表一个内存地址。它们将很少使用：主要用例是全局锁，全局原子计数器以及与旧版C库的接口。

• constants declare constant values. These represent a value, not a memory address. This is the most common thing one would reach for and would replace static as we know it today in almost all cases.
• statics declare global variables. These represent a memory address. They would be rarely used: the primary use cases are global locks, global atomic counters, and interfacing with legacy C libraries.

当我尝试维护表格时，我不知道两者之间实际上有什么区别。

I don't know what is actually different between the two when I try to maintain a table.

我应该选择哪个？

推荐答案

可变性

A const 变量是不可变的。您既不能重新分配也不能修改它：

Mutability

A const variable in Rust is immutable. You neither can reassign nor modify it:

struct Foo(u32);

const FOO: Foo = Foo(5);

fn main() {
    FOO = Foo(1); //illegal
    FOO.0 = 2; //illegal
}

A 静态变量可以可变，因此可以修改或重新分配。请注意，编写/修改全局 static 变量是不安全的，因此需要 unsafe 块：

A static variable can be mutable and therefore can either be modified or reassigned. Note that writing/modifying a global static variable is unsafe and therefore needs an unsafe block:

struct Foo(u32);
static FOO: Foo = Foo(5);
static mut FOO_MUT: Foo = Foo(3);

fn main() {
    unsafe {
        FOO = Foo(1); //illegal
        FOO.0 = 2; //illegal

        FOO_MUT = Foo(1);
        FOO_MUT.0 = 2;
    }
}

发生次数

编译二进制文件时，所有 const 个出现次数（在您的二进制文件中使用该 const 源代码）将直接替换为该值。

Occurrences

When you compile a binary, all const "occurrences" (where you use that const in your source code) will be replaced by that value directly.

静态 s将在二进制文件中有一个专用部分（ BSS部分，请参见在C和C ++？了解更多信息）。

statics will have a dedicated section in your binary where they will be placed (the BSS section, see Where are static variables stored in C and C++? for further information).

总而言之，坚持使用 const 。在不可能的情况下，因为稍后需要在程序中使用非 const 方法初始化变量，请使用 lazy_static！ 。

All in all, stick to a const whenever possible. When not possible, because you need to initialize a variable later in the program of with non-const methods, use lazy_static!.

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