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建造者模式

问题​

Rust 中建造者模式如何实现？

答案​

建造者模式在 Rust 中非常常见，因为 Rust 没有函数重载和默认参数。

基础实现​

struct Server {
    host: String,
    port: u16,
    max_connections: usize,
    timeout: std::time::Duration,
}

struct ServerBuilder {
    host: String,
    port: u16,
    max_connections: usize,
    timeout: std::time::Duration,
}

impl ServerBuilder {
    fn new(host: impl Into<String>, port: u16) -> Self {
        Self {
            host: host.into(),
            port,
            max_connections: 100,   // 默认值
            timeout: std::time::Duration::from_secs(30),
        }
    }

    fn max_connections(mut self, n: usize) -> Self {
        self.max_connections = n;
        self
    }

    fn timeout(mut self, d: std::time::Duration) -> Self {
        self.timeout = d;
        self
    }

    fn build(self) -> Server {
        Server {
            host: self.host,
            port: self.port,
            max_connections: self.max_connections,
            timeout: self.timeout,
        }
    }
}

// 使用
let server = ServerBuilder::new("localhost", 8080)
    .max_connections(200)
    .timeout(std::time::Duration::from_secs(60))
    .build();

类型状态 Builder（编译时安全）​

确保必填字段不被遗漏：

use std::marker::PhantomData;

// 状态标记
struct NoHost;
struct HasHost;
struct NoPort;
struct HasPort;

struct RequestBuilder<H, P> {
    host: Option<String>,
    port: Option<u16>,
    path: String,
    _h: PhantomData<H>,
    _p: PhantomData<P>,
}

impl RequestBuilder<NoHost, NoPort> {
    fn new() -> Self {
        Self {
            host: None, port: None,
            path: "/".into(),
            _h: PhantomData, _p: PhantomData,
        }
    }
}

impl<P> RequestBuilder<NoHost, P> {
    fn host(self, host: impl Into<String>) -> RequestBuilder<HasHost, P> {
        RequestBuilder {
            host: Some(host.into()), port: self.port,
            path: self.path,
            _h: PhantomData, _p: PhantomData,
        }
    }
}

impl<H> RequestBuilder<H, NoPort> {
    fn port(self, port: u16) -> RequestBuilder<H, HasPort> {
        RequestBuilder {
            host: self.host, port: Some(port),
            path: self.path,
            _h: PhantomData, _p: PhantomData,
        }
    }
}

// 只有设置了 host 和 port 才能 build
impl RequestBuilder<HasHost, HasPort> {
    fn build(self) -> String {
        format!("{}:{}{}", self.host.unwrap(), self.port.unwrap(), self.path)
    }
}

// 使用
let url = RequestBuilder::new()
    .host("example.com")
    .port(443)
    .build(); // ✅

// RequestBuilder::new().build(); // ❌ 编译错误！

常见面试问题​

Q1: Builder 模式和 Functional Options 模式如何选择？​

答案：

特性BuilderFunctional Options
Rust 中实现结构体 + 方法链闭包参数
类型安全可用 Typestate 保证较弱
可读性✅✅
推荐度⭐⭐⭐⭐⭐⭐⭐⭐（Go 中更常见）

Rust 社区首选 Builder 模式。

相关链接​