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< h1 id = "使用-use-及受限可见性" > < a class = "header" href = "#使用-use-及受限可见性" > 使用 use 及受限可见性< / a > < / h1 >
< p > 如果代码中，通篇都是 < code > crate::front_of_house::hosting::add_to_waitlist< / code > 这样的函数调用形式，我不知道有谁会喜欢，也许靠代码行数赚工资的人会很喜欢，但是强迫症肯定受不了，悲伤的是程序员大多都有强迫症。。。< / p >
< p > 因此我们需要一个办法来简化这种使用方式，在 Rust 中，可以使用 < code > use< / code > 关键字把路径提前引入到当前作用域中，随后的调用就可以省略该路径，极大地简化了代码。< / p >
< h2 id = "基本引入方式" > < a class = "header" href = "#基本引入方式" > 基本引入方式< / a > < / h2 >
< p > 在 Rust 中，引入模块中的项有两种方式：< a href = "https://course.rs/basic/crate-module/module.html#用路径引用模块" > 绝对路径和相对路径< / a > ，这两者在前面章节都有讲过，就不再赘述，先来看看使用绝对路径的引入方式。< / p >
< h4 id = "绝对路径引入模块" > < a class = "header" href = "#绝对路径引入模块" > 绝对路径引入模块< / a > < / h4 >
< pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > #![allow(unused)]
< / span > < span class = "boring" > fn main() {
< / span > mod front_of_house {
pub mod hosting {
pub fn add_to_waitlist() {}
}
}
use crate::front_of_house::hosting;
pub fn eat_at_restaurant() {
hosting::add_to_waitlist();
hosting::add_to_waitlist();
hosting::add_to_waitlist();
}
< span class = "boring" > }< / span > < / code > < / pre >
< p > 这里，我们使用 < code > use< / code > 和绝对路径的方式，将 < code > hosting< / code > 模块引入到当前作用域中，然后只需通过 < code > hosting::add_to_waitlist< / code > 的方式，即可调用目标模块中的函数，相比 < code > crate::front_of_house::hosting::add_to_waitlist()< / code > 的方式要简单的多，那么还能更简单吗？< / p >
< h4 id = "相对路径引入模块中的函数" > < a class = "header" href = "#相对路径引入模块中的函数" > 相对路径引入模块中的函数< / a > < / h4 >
< p > 在下面代码中，我们不仅要使用相对路径进行引入，而且与上面引入 < code > hosting< / code > 模块不同，直接引入该模块中的 < code > add_to_waitlist< / code > 函数：< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > #![allow(unused)]
< / span > < span class = "boring" > fn main() {
< / span > mod front_of_house {
pub mod hosting {
pub fn add_to_waitlist() {}
}
}
use front_of_house::hosting::add_to_waitlist;
pub fn eat_at_restaurant() {
add_to_waitlist();
add_to_waitlist();
add_to_waitlist();
}
< span class = "boring" > }< / span > < / code > < / pre >
< p > 很明显，三兄弟又变得更短了，不过，怎么觉得这句话怪怪的。。< / p >
< h4 id = "引入模块还是函数" > < a class = "header" href = "#引入模块还是函数" > 引入模块还是函数< / a > < / h4 >
< p > 从使用简洁性来说，引入函数自然是更甚一筹，但是在某些时候，引入模块会更好：< / p >
< ul >
< li > 需要引入同一个模块的多个函数< / li >
< li > 作用域中存在同名函数< / li >
< / ul >
< p > 在以上两种情况中，使用 < code > use front_of_house::hosting;< / code > 引入模块要比 < code > use front_of_house::hosting::add_to_waitlist;< / code > 引入函数更好。< / p >
< p > 例如，如果想使用 < code > HashMap< / code > ，那么直接引入该结构体是比引入模块更好的选择，因为在 < code > collections< / code > 模块中，我们只需要使用一个 < code > HashMap< / code > 结构体：< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > use std::collections::HashMap;
fn main() {
let mut map = HashMap::new();
map.insert(1, 2);
}< / code > < / pre >
< p > 其实严格来说，对于引用方式并没有需要遵守的惯例，主要还是取决于你的喜好，不过我们建议：< strong > 优先使用最细粒度（引入函数、结构体等）的引用方式，如果引起了某种麻烦（例如前面两种情况），再使用引入模块的方式< / strong > 。< / p >
< h2 id = "避免同名引用" > < a class = "header" href = "#避免同名引用" > 避免同名引用< / a > < / h2 >
< p > 根据上一章节的内容，我们只要保证同一个模块中不存在同名项就行，模块之间、包之间的同名，谁管得着谁啊，话虽如此，一起看看，如果遇到同名的情况该如何处理。< / p >
< h4 id = "模块函数" > < a class = "header" href = "#模块函数" > 模块::函数< / a > < / h4 >
< pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > #![allow(unused)]
< / span > < span class = "boring" > fn main() {
< / span > use std::fmt;
use std::io;
fn function1() -> fmt::Result {
// --snip--
}
fn function2() -> io::Result< ()> {
// --snip--
}
< span class = "boring" > }< / span > < / code > < / pre >
< p > 上面的例子给出了很好的解决方案，使用模块引入的方式，具体的 < code > Result< / code > 通过 < code > 模块::Result< / code > 的方式进行调用。< / p >
< p > 可以看出，避免同名冲突的关键，就是使用< strong > 父模块的方式来调用< / strong > ，除此之外，还可以给予引入的项起一个别名。< / p >
< h4 id = "as-别名引用" > < a class = "header" href = "#as-别名引用" > < code > as< / code > 别名引用< / a > < / h4 >
< p > 对于同名冲突问题，还可以使用 < code > as< / code > 关键字来解决，它可以赋予引入项一个全新的名称：< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > #![allow(unused)]
< / span > < span class = "boring" > fn main() {
< / span > use std::fmt::Result;
use std::io::Result as IoResult;
fn function1() -> Result {
// --snip--
}
fn function2() -> IoResult< ()> {
// --snip--
}
< span class = "boring" > }< / span > < / code > < / pre >
< p > 如上所示，首先通过 < code > use std::io::Result< / code > 将 < code > Result< / code > 引入到作用域，然后使用 < code > as< / code > 给予它一个全新的名称 < code > IoResult< / code > ，这样就不会再产生冲突：< / p >
< ul >
< li > < code > Result< / code > 代表 < code > std::fmt::Result< / code > < / li >
< li > < code > IoResult< / code > 代表 < code > std:io::Result< / code > < / li >
< / ul >
< h2 id = "引入项再导出" > < a class = "header" href = "#引入项再导出" > 引入项再导出< / a > < / h2 >
< p > 当外部的模块项 < code > A< / code > 被引入到当前模块中时，它的可见性自动被设置为私有的，如果你希望允许其它外部代码引用我们的模块项 < code > A< / code > ，那么可以对它进行再导出：< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > #![allow(unused)]
< / span > < span class = "boring" > fn main() {
< / span > mod front_of_house {
pub mod hosting {
pub fn add_to_waitlist() {}
}
}
pub use crate::front_of_house::hosting;
pub fn eat_at_restaurant() {
hosting::add_to_waitlist();
hosting::add_to_waitlist();
hosting::add_to_waitlist();
}
< span class = "boring" > }< / span > < / code > < / pre >
< p > 如上，使用 < code > pub use< / code > 即可实现。这里 < code > use< / code > 代表引入 < code > hosting< / code > 模块到当前作用域，< code > pub< / code > 表示将该引入的内容再度设置为可见。< / p >
< p > 当你希望将内部的实现细节隐藏起来或者按照某个目的组织代码时，可以使用 < code > pub use< / code > 再导出，例如统一使用一个模块来提供对外的 API， ， ， ， < / p >
< h2 id = "使用第三方包" > < a class = "header" href = "#使用第三方包" > 使用第三方包< / a > < / h2 >
< p > 之前我们一直在引入标准库模块或者自定义模块，现在来引入下第三方包中的模块，关于如何引入外部依赖，我们在 < a href = "https://course.rs/first-try/cargo.html#package-配置段落" > Cargo 入门< / a > 中就有讲，这里直接给出操作步骤：< / p >
< ol >
< li > 修改 < code > Cargo.toml< / code > 文件，在 < code > [dependencies]< / code > 区域添加一行：< code > rand = "0.8.3"< / code > < / li >
< li > 此时，如果你用的是 < code > VSCode< / code > 和 < code > rust-analyzer< / code > 插件， ， ， （ < / li >
< / ol >
< p > 好了，此时，< code > rand< / code > 包已经被我们添加到依赖中，下一步就是在代码中使用：< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > use rand::Rng;
fn main() {
let secret_number = rand::thread_rng().gen_range(1..101);
}< / code > < / pre >
< p > 这里使用 < code > use< / code > 引入了第三方包 < code > rand< / code > 中的 < code > Rng< / code > 特征，因为我们需要调用的 < code > gen_range< / code > 方法定义在该特征中。< / p >
< h4 id = "cratesiolibrs" > < a class = "header" href = "#cratesiolibrs" > crates.io， < / a > < / h4 >
< p > Rust 社区已经为我们贡献了大量高质量的第三方包，你可以在 < code > crates.io< / code > 或者 < code > lib.rs< / code > 中检索和使用，从目前来说查找包更推荐 < code > lib.rs< / code > ，搜索功能更强大，内容展示也更加合理，但是下载依赖包还是得用< code > crates.io< / code > 。< / p >
< p > 你可以在网站上搜索 < code > rand< / code > 包，看看它的文档使用方式是否和我们之前引入方式相一致：在网上找到想要的包，然后将你想要的包和版本信息写入到 < code > Cargo.toml< / code > 中。< / p >
< h2 id = "使用--简化引入方式" > < a class = "header" href = "#使用--简化引入方式" > 使用 < code > {}< / code > 简化引入方式< / a > < / h2 >
< p > 对于以下一行一行的引入方式：< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > #![allow(unused)]
< / span > < span class = "boring" > fn main() {
< / span > use std::collections::HashMap;
use std::collections::BTreeMap;
use std::collections::HashSet;
use std::cmp::Ordering;
use std::io;
< span class = "boring" > }< / span > < / code > < / pre >
< p > 可以使用 < code > {}< / code > 来一起引入进来，在大型项目中，使用这种方式来引入，可以减少大量 < code > use< / code > 的使用：< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > #![allow(unused)]
< / span > < span class = "boring" > fn main() {
< / span > use std::collections::{HashMap,BTreeMap,HashSet};
use std::{cmp::Ordering, io};
< span class = "boring" > }< / span > < / code > < / pre >
< p > 对于下面的同时引入模块和模块中的项：< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > #![allow(unused)]
< / span > < span class = "boring" > fn main() {
< / span > use std::io;
use std::io::Write;
< span class = "boring" > }< / span > < / code > < / pre >
< p > 可以使用 < code > {}< / code > 的方式进行简化:< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > #![allow(unused)]
< / span > < span class = "boring" > fn main() {
< / span > use std::io::{self, Write};
< span class = "boring" > }< / span > < / code > < / pre >
< h4 id = "self" > < a class = "header" href = "#self" > self< / a > < / h4 >
< p > 上面使用到了模块章节提到的 < code > self< / code > 关键字，用来替代模块自身，结合上一节中的 < code > self< / code > ，可以得出它在模块中的两个用途：< / p >
< ul >
< li > < code > use self::xxx< / code > ，表示加载当前模块中的 < code > xxx< / code > 。此时 < code > self< / code > 可省略< / li >
< li > < code > use xxx::{self, yyy}< / code > ，表示，加载当前路径下模块 < code > xxx< / code > 本身，以及模块 < code > xxx< / code > 下的 < code > yyy< / code > < / li >
< / ul >
< h2 id = "使用--引入模块下的所有项" > < a class = "header" href = "#使用--引入模块下的所有项" > 使用 < code > *< / code > 引入模块下的所有项< / a > < / h2 >
< p > 对于之前一行一行引入 < code > std::collections< / code > 的方式，我们还可以使用< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > #![allow(unused)]
< / span > < span class = "boring" > fn main() {
< / span > use std::collections::*;
< span class = "boring" > }< / span > < / code > < / pre >
< p > 以上这种方式来引入 < code > std::collections< / code > 模块下的所有公共项，这些公共项自然包含了 < code > HashMap< / code > ， < code > HashSet< / code > 等想手动引入的集合类型。< / p >
< p > 当使用 < code > *< / code > 来引入的时候要格外小心，因为你很难知道到底哪些被引入到了当前作用域中，有哪些会和你自己程序中的名称相冲突：< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > use std::collections::*;
struct HashMap;
fn main() {
let mut v = HashMap::new();
v.insert("a", 1);
}< / code > < / pre >
< p > 以上代码中，< code > std::collections::HashMap< / code > 被 < code > *< / code > 引入到当前作用域，但是由于存在另一个同名的结构体，因此 < code > HashMap::new< / code > 根本不存在，因为对于编译器来说，本地同名类型的优先级更高。< / p >
< p > 在实际项目中，这种引用方式往往用于快速写测试代码，它可以把所有东西一次性引入到 < code > tests< / code > 模块中。< / p >
< h2 id = "受限的可见性" > < a class = "header" href = "#受限的可见性" > 受限的可见性< / a > < / h2 >
< p > 在上一节中，我们学习了< a href = "https://course.rs/basic/crate-module/module.html#代码可见性" > 可见性< / a > 这个概念，这也是模块体系中最为核心的概念，控制了模块中哪些内容可以被外部看见，但是在实际使用时，光被外面看到还不行，我们还想控制哪些人能看，这就是 Rust 提供的受限可见性。< / p >
< p > 例如，在 Rust 中，包是一个模块树，我们可以通过 < code > pub(crate) item;< / code > 这种方式来实现：< code > item< / code > 虽然是对外可见的，但是只在当前包内可见，外部包无法引用到该 < code > item< / code > 。< / p >
< p > 所以，如果我们想要让某一项可以在整个包中都可以被使用，那么有两种办法：< / p >
< ul >
< li > 在包根中定义一个非 < code > pub< / code > 类型的 < code > X< / code > (父模块的项对子模块都是可见的，因此包根中的项对模块树上的所有模块都可见)< / li >
< li > 在子模块中定义一个 < code > pub< / code > 类型的 < code > Y< / code > ，同时通过 < code > use< / code > 将其引入到包根< / li >
< / ul >
< pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > #![allow(unused)]
< / span > < span class = "boring" > fn main() {
< / span > mod a {
pub mod b {
pub fn c() {
println!("{:?}",crate::X);
}
#[derive(Debug)]
pub struct Y;
}
}
#[derive(Debug)]
struct X;
use a::b::Y;
fn d() {
println!("{:?}",Y);
}
< span class = "boring" > }< / span > < / code > < / pre >
< p > 以上代码充分说明了之前两种办法的使用方式，但是有时我们会遇到这两种方法都不太好用的时候。例如希望对于某些特定的模块可见，但是对于其他模块又不可见：< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > #![allow(unused)]
< / span > < span class = "boring" > fn main() {
< / span > // 目标：`a` 导出 `I`、`bar` and `foo`，其他的不导出
pub mod a {
pub const I: i32 = 3;
fn semisecret(x: i32) -> i32 {
use self::b::c::J;
x + J
}
pub fn bar(z: i32) -> i32 {
semisecret(I) * z
}
pub fn foo(y: i32) -> i32 {
semisecret(I) + y
}
mod b {
mod c {
const J: i32 = 4;
}
}
}
< span class = "boring" > }< / span > < / code > < / pre >
< p > 这段代码会报错，因为与父模块中的项对子模块可见相反，子模块中的项对父模块是不可见的。这里 < code > semisecret< / code > 方法中，< code > a< / code > -> < code > b< / code > -> < code > c< / code > 形成了父子模块链，那 < code > c< / code > 中的 < code > J< / code > 自然对 < code > a< / code > 模块不可见。< / p >
< p > 如果使用之前的可见性方式，那么想保持 < code > J< / code > 私有，同时让 < code > a< / code > 继续使用 < code > semisecret< / code > 函数的办法是将该函数移动到 < code > c< / code > 模块中，然后用 < code > pub use< / code > 将 < code > semisecret< / code > 函数进行再导出：< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > #![allow(unused)]
< / span > < span class = "boring" > fn main() {
< / span > pub mod a {
pub const I: i32 = 3;
use self::b::semisecret;
pub fn bar(z: i32) -> i32 {
semisecret(I) * z
}
pub fn foo(y: i32) -> i32 {
semisecret(I) + y
}
mod b {
pub use self::c::semisecret;
mod c {
const J: i32 = 4;
pub fn semisecret(x: i32) -> i32 {
x + J
}
}
}
}
< span class = "boring" > }< / span > < / code > < / pre >
< p > 这段代码说实话问题不大，但是有些破坏了我们之前的逻辑，如果想保持代码逻辑，同时又只让 < code > J< / code > 在 < code > a< / code > 内可见该怎么办？< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > #![allow(unused)]
< / span > < span class = "boring" > fn main() {
< / span > pub mod a {
pub const I: i32 = 3;
fn semisecret(x: i32) -> i32 {
use self::b::c::J;
x + J
}
pub fn bar(z: i32) -> i32 {
semisecret(I) * z
}
pub fn foo(y: i32) -> i32 {
semisecret(I) + y
}
mod b {
pub(in crate::a) mod c {
pub(in crate::a) const J: i32 = 4;
}
}
}
< span class = "boring" > }< / span > < / code > < / pre >
< p > 通过 < code > pub(in crate::a)< / code > 的方式，我们指定了模块 < code > c< / code > 和常量 < code > J< / code > 的可见范围都只是 < code > a< / code > 模块中，< code > a< / code > 之外的模块是完全访问不到它们的。< / p >
< h4 id = "限制可见性语法" > < a class = "header" href = "#限制可见性语法" > 限制可见性语法< / a > < / h4 >
< p > < code > pub(crate)< / code > 或 < code > pub(in crate::a)< / code > 就是限制可见性语法，前者是限制在整个包内可见，后者是通过绝对路径，限制在包内的某个模块内可见，总结一下：< / p >
< ul >
< li > < code > pub< / code > 意味着可见性无任何限制< / li >
< li > < code > pub(crate)< / code > 表示在当前包可见< / li >
< li > < code > pub(self)< / code > 在当前模块可见< / li >
< li > < code > pub(super)< / code > 在父模块可见< / li >
< li > < code > pub(in < path> )< / code > 表示在某个路径代表的模块中可见，其中 < code > path< / code > 必须是父模块或者祖先模块< / li >
< / ul >
< h4 id = "一个综合例子" > < a class = "header" href = "#一个综合例子" > 一个综合例子< / a > < / h4 >
< pre class = "playground" > < code class = "language-rust edition2021" > // 一个名为 `my_mod` 的模块
mod my_mod {
// 模块中的项默认具有私有的可见性
fn private_function() {
println!("called `my_mod::private_function()`");
}
// 使用 `pub` 修饰语来改变默认可见性。
pub fn function() {
println!("called `my_mod::function()`");
}
// 在同一模块中，项可以访问其它项，即使它是私有的。
pub fn indirect_access() {
print!("called `my_mod::indirect_access()`, that\n> ");
private_function();
}
// 模块也可以嵌套
pub mod nested {
pub fn function() {
println!("called `my_mod::nested::function()`");
}
#[allow(dead_code)]
fn private_function() {
println!("called `my_mod::nested::private_function()`");
}
// 使用 `pub(in path)` 语法定义的函数只在给定的路径中可见。
// `path` 必须是父模块（ ） （ ）
pub(in crate::my_mod) fn public_function_in_my_mod() {
print!("called `my_mod::nested::public_function_in_my_mod()`, that\n > ");
public_function_in_nested()
}
// 使用 `pub(self)` 语法定义的函数则只在当前模块中可见。
pub(self) fn public_function_in_nested() {
println!("called `my_mod::nested::public_function_in_nested");
}
// 使用 `pub(super)` 语法定义的函数只在父模块中可见。
pub(super) fn public_function_in_super_mod() {
println!("called my_mod::nested::public_function_in_super_mod");
}
}
pub fn call_public_function_in_my_mod() {
print!("called `my_mod::call_public_funcion_in_my_mod()`, that\n> ");
nested::public_function_in_my_mod();
print!("> ");
nested::public_function_in_super_mod();
}
// `pub(crate)` 使得函数只在当前包中可见
pub(crate) fn public_function_in_crate() {
println!("called `my_mod::public_function_in_crate()");
}
// 嵌套模块的可见性遵循相同的规则
mod private_nested {
#[allow(dead_code)]
pub fn function() {
println!("called `my_mod::private_nested::function()`");
}
}
}
fn function() {
println!("called `function()`");
}
fn main() {
// 模块机制消除了相同名字的项之间的歧义。
function();
my_mod::function();
// 公有项，包括嵌套模块内的，都可以在父模块外部访问。
my_mod::indirect_access();
my_mod::nested::function();
my_mod::call_public_function_in_my_mod();
// pub(crate) 项可以在同一个 crate 中的任何地方访问
my_mod::public_function_in_crate();
// pub(in path) 项只能在指定的模块中访问
// 报错！函数 `public_function_in_my_mod` 是私有的
//my_mod::nested::public_function_in_my_mod();
// 试一试 ^ 取消该行的注释
// 模块的私有项不能直接访问，即便它是嵌套在公有模块内部的
// 报错！`private_function` 是私有的
//my_mod::private_function();
// 试一试 ^ 取消此行注释
// 报错！`private_function` 是私有的
//my_mod::nested::private_function();
// 试一试 ^ 取消此行的注释
// 报错！ `private_nested` 是私有的
//my_mod::private_nested::function();
// 试一试 ^ 取消此行的注释
}< / code > < / pre >
< h2 id = "课后练习" > < a class = "header" href = "#课后练习" > 课后练习< / a > < / h2 >
< blockquote >
< p > < a href = "https://practice-zh.course.rs/crate-module/use-pub.html" > Rust By Practice< / a > ，支持代码在线编辑和运行，并提供详细的< a href = "https://github.com/sunface/rust-by-practice/blob/master/solutions/crate-module/use-pub.md" > 习题解答< / a > 。< / p >
< / blockquote >
< / main >
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