为什么固定大小的数组可以放在堆栈上，而str却不能呢? [英] Why can fixed-size arrays be on the stack, but str cannot?

问题描述

回答 Rust的`String`有什么区别?和`str`?描述&str和String如何相互关联.

Answers to What are the differences between Rust's `String` and `str`? describe how &str and String relate to each other.

令人惊讶的是，与固定大小的数组相比，str的局限性更大，因为它不能声明为局部变量.编译

What is surprising is that a str is more limited than a fixed-sized array, because it cannot be declared as a local variable. Compiling

let arr_owned = [0u8; 32];
let arr_slice = &arr_owned;

let str_slice = "apple";
let str_owned = *str_slice;

在Rust 1.32.0中，我得到了

in Rust 1.32.0, I get

error[E0277]: the size for values of type `str` cannot be known at compilation time
 --> src/lib.rs:6:9

令人困惑，因为编译器可以知道"apple"的大小，因此它不是str类型的一部分.

which is confusing, because the size of "apple" can be known by the compiler, it is just not part of the str type.

在Vec<T><-> [T; N]和String<-> str所拥有的类型之间存在不对称性的语言原因吗? str[N]类型(仅包含经证明有效的UTF-8编码字符串的[u8; N]类型的缩写)可以代替str而不会破坏大量现有代码吗?

Is there a linguistic reason for the asymmetry between Vec<T> <-> [T; N] and String <-> str owned types? Could an str[N] type, which would be a shortand to a [u8; N] that only contains provably valid UTF-8 encoded strings, replace str without breaking lots of existing code?

推荐答案

Vec<T><-> [T; N]和String<-str

asymmetry between Vec<T> <-> [T; N] and String <-> str

那是因为您在这里有些困惑.这种关系是这样的:

That's because you confused something here. The relationships are rather like this:

• Vec<T>⇔[T]
• String⇔str

• Vec<T> ⇔ [T]
• String ⇔ str

在所有这四种类型中，长度信息是在运行时而不是编译时存储的.固定大小的数组([T; N])在这方面有所不同:它们在编译时存储长度，而不在运行时存储长度！

In all those four types, the length information is stored at runtime, not compile time. Fixed size arrays ([T; N]) are different in that regard: they store the length at compile time, but not runtime!

实际上，[T]和str都不能存储在堆栈中，因为它们都没有大小.

And indeed, both [T] and str can't be stored on the stack, because they are both unsized.

str[N]类型(仅包含经证明有效的UTF-8编码字符串的[u8; N]的简写形式)是否可以替换str而又不破坏大量现有代码?

Could an str[N] type, which would be a shorthand to a [u8; N] that only contains provably valid UTF-8 encoded strings, replace str without breaking lots of existing code?

它不会替代str，但确实是一个有趣的补充！但是，可能有一些原因导致它尚不存在，例如因为Unicode的长度字符串通常并不真正相关.特别是，采用具有三个字节的Unicode字符串"通常没有意义.

It wouldn't replace str, but it could be an interesting addition indeed! But there are probably reasons why it doesn't exist yet, e.g. because the length of a Unicode string is usually not really relevant. In particular, it usually doesn't make sense to "take a Unicode string with exactly three bytes".

这篇关于为什么固定大小的数组可以放在堆栈上，而str却不能呢?的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！