variables - 常量和静态变量有什么区别，我应该选择哪个？

Question

我从 RFC 246 知道这个:

• 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.

当我尝试维护一个表时，我不知道两者之间到底有什么不同。

我应该选择哪个？

Answer 1

可变性

Rust 中的 constant 是不可变的。您既不能重新分配也不能修改它:

struct Foo(u32);

const FOO: Foo = Foo(5);
const mut FOO: Foo = Foo(5); // illegal

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

static 变量可以是可变的，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，因此需要一个 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;
    }
}

出现次数

当您编译二进制文件时，所有 const“出现”(您在源代码中使用该 const 的地方)都将直接替换为该值。

static 将在您的二进制文件中有一个专门的部分用于放置它们(BSS 部分，请参阅 Where are static variables stored in C and C++? 了解更多信息)。

---

总而言之，尽可能坚持const。如果不可能，因为您需要稍后在程序中使用非 const 方法初始化变量，请使用 lazy_static! .

内部可变性

虽然 const 和 static 都可以使用内部可变性，但您永远不应该使用 const 来实现。这是一个例子

use std::sync::atomic::{AtomicU32, Ordering};

static STATIC: AtomicU32 = AtomicU32::new(0);
const CONST: AtomicU32 = AtomicU32::new(0);

fn print() {
    println!("static: {}", STATIC.load(Ordering::Relaxed));
    println!("const:  {}", CONST.load(Ordering::Relaxed));
}

fn main() {
    STATIC.store(3, Ordering::Relaxed);
    CONST.store(3, Ordering::Relaxed);

    print();
}

这可以在没有任何警告的情况下正常编译，但会导致不良行为。输出:

static: 3
const:  0

使用clippy时，会出现如下两个警告:

warning: a `const` item should never be interior mutable
 --> src/main.rs:4:1
  |
4 | const CONST: AtomicU32 = AtomicU32::new(0);
  | -----^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  | |
  | make this a static item (maybe with lazy_static)
  |
  = note: `#[warn(clippy::declare_interior_mutable_const)]` on by default
  = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#declare_interior_mutable_const

warning: a `const` item with interior mutability should not be borrowed
 --> src/main.rs:8:27
  |
8 |     println!("const: {}", CONST.load(Ordering::Relaxed));
  |                           ^^^^^
  |
  = note: `#[warn(clippy::borrow_interior_mutable_const)]` on by default
  = help: assign this const to a local or static variable, and use the variable here
  = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#borrow_interior_mutable_const

warning: a `const` item with interior mutability should not be borrowed
  --> src/main.rs:13:5
   |
13 |     CONST.store(3, Ordering::Relaxed);
   |     ^^^^^
   |
   = help: assign this const to a local or static variable, and use the variable here
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#borrow_interior_mutable_const