尝试声明String const结果为预期的类型，找到“ my string”。 [英] Trying to declare a String const results in expected type, found "my string"

问题描述

我正在尝试在Rust中声明一个 String 常量，但出现编译器错误，我什至无法理解

I'm trying to declare a String constant in Rust, but I get a compiler error I just can't make sense of

const DATABASE : String::from("/var/lib/tracker/tracker.json");

这是我尝试编译时得到的内容：

and here's what I get when I try to compile it:

error: expected type, found `"/var/lib/tracker/tracker.json"`
  --> src/main.rs:19:31
   |
19 | const DATABASE : String::from("/var/lib/tracker/tracker.json");
   |                               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

error: expected one of `!`, `+`, `->`, `::`, or `=`, found `)`
  --> src/main.rs:19:64
   |
19 | const DATABASE : String::from("/var/lib/tracker/tracker.json");
   |                                                                ^ expected one of `!`, `+`, `->`, `::`, or `=` here

推荐答案

您应该阅读 Rust编程语言，第二版，特别是讨论常量的章节。声明 const 的正确语法为：

You should read The Rust Programming Language, second edition, specifically the chapter that discusses constants. The proper syntax for declaring a const is:

const NAME: Type = value;

在这种情况下：

const DATABASE: String = String::from("/var/lib/tracker/tracker.json");

但是，这是行不通的，因为分配字符串是不是在编译时就可以计算的东西。这就是 const 的意思。您可能要使用字符串slice ，特别是具有静态生存期的切片，它隐含在 const s和 static s中：

However, this won't work because allocating a string is not something that can be computed at compile time. That's what const means. You may want to use a string slice, specifically one with a static lifetime, which is implicit in consts and statics:

const DATABASE: &str = "/var/lib/tracker/tracker.json";

仅需要读取字符串的函数应接受& str ，因此这不太可能引起任何问题。

Functions that just need to read a string should accept a &str, so this is unlikely to cause any issues. It also has the nice benefit of requiring no allocation whatsoever, so it's pretty efficient.

如果您需要一个 String，它也具有不需要分配的好处。 ，您可能需要对其进行突变。在这种情况下，将其设置为全局会导致线程问题。相反，您应该只在需要时使用 String :: from（DATABASE）进行分配，并传入 String 。

If you need a String, it's likely that you will need to mutate it. In that case, making it global would lead to threading issues. Instead, you should just allocate when you need it with String::from(DATABASE) and pass in the String.

另请参见：

• 如何在编译时创建静态字符串

• How to create a static string at compile time

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