初学rust,闭包引用外部变量
Tag rust, 闭包, on by view 234

今天又遇到一个新的问题,那就是在闭包函数中,无法引用闭包函数之外的变量。

fn main() {
    let cfg = config::cfg::get_config();
    let filename = cfg.las.as_ref().unwrap().access_log.as_ref().unwrap();
    // println!("{}",filename);
    // let mut p = poly::Poly::new();
    let mut s = String::from("_");
    let mut log_watcher = LogWatcher::register(filename.to_string()).unwrap();

    log_watcher.watch(|line: String| {
        let (_, token) = parse::lex::parser(&line);
        // p.push(LineStat {});
        print!("{}", s);
        process::report::send(token);
    });
}

上面代码中,watch传入参数为一个闭包函数,其中需要引用main函数中的局部变量s,但是会报错

error[E0308]: mismatched types
  --> src/main.rs:22:23
   |
22 |       log_watcher.watch(|line: String| {
   |  _______________________^
23 | |         let (_, token) = parse::lex::parser(&line);
24 | |         // p.push(LineStat {});
25 | |         print!("{}", s);
26 | |         process::report::send(token);
27 | |     });
   | |_____^ expected fn pointer, found closure
   |
   = note: expected fn pointer `fn(std::string::String)`
                 found closure `[closure@src/main.rs:22:23: 27:6]`
note: closures can only be coerced to `fn` types if they do not capture any variables

查看logwatcher源码,watch该版本(0.1.0)方法定义如下

pub fn watch(&mut self, callback: fn (line: String)) {
    ....
}

不过我后来发现logwatcher版本升级到(0.1.1)之后,该方法变为

pub fn watch<F: ?Sized>(&mut self, callback: &mut F)
where
    F: FnMut(String) -> LogWatcherAction,
{
    ....
}

如此可以正常调用,代码如下

fn main() {
    let cfg = config::cfg::get_config();
    let filename = cfg.las.as_ref().unwrap().access_log.as_ref().unwrap();
    let mut watcher = LogWatcher::register(filename.to_string()).unwrap();
    let mut poly: Poly = Poly::new();

    watcher.watch(&mut |line: String| {
        poly.clone().push(line);
        LogWatcherAction::None
    })
}

由此可见,闭包与函数还是有区别的。但是回调函数定义为Fn/FnMut类型就可以同时接受闭包和函数类型的回调参数。举例如下

let a = String::from("abc");

let mut x = || println!("x {}", a);

fn y() {
    println!("y")
}

fn wrap(c: &mut Fn()) {
    c()
}

wrap(&mut x); // pass a closure
wrap(&mut y); // pass a function

同样可以参考 stackoverflow 的这个提问