为什么我可以在引用上调用 File.take() ? [英] Why can I call File.take() on a reference?

问题描述

当我查看 File的文档，我看到 take 方法采用 self，而不是 &self.但是我仍然可以在借用的引用上调用该方法:

fn foo(file: &File) {让 _ = file.take(1);//为什么这样做?println!("仍然可以使用文件:{:?}", file);}

我认为 self 会传递所有权，但我什至可以在调用 take 后使用 file，因此所有权显然保留在 中foo.

如果我自己使用方法在自定义结构上执行此操作，则不起作用:

struct Foo;impl Foo {fn foo(self: foo) { }}fn 主(){让 foo = &Foo;foo.foo();//错误:无法移出借用的内容}

完全符合预期.

同样，根据文档，File 据我所知没有实现任何特殊特征.引起我注意的是 Read 有一个 by_ref() 方法，但我没有调用它但一切仍然有效.

这是怎么回事?(使用 rustc 1.3.0-dev)

解决方案

take 方法来自 Read 特性.该特性在 File 上实现，因此有一个方法 File::take(self, u64) ->以<Self>为例，但该特征也在 &File 上实现(该实现甚至列在您链接到的页面上).对于那个实现，Self 类型是 &File，所以 its take 方法需要一个引用.>

When I look at File's docs, I see that the take method takes a self, not a &self. But I'm still able to call the method on a borrowed reference:

fn foo(file: &File) {
    let _ = file.take(1); // why does this work?
    println!("can still use the file: {:?}", file);
}

I thought a self passes ownership but I'm even able to use file after calling take so ownership clearly stays inside foo.

If I do it myself on a custom struct with a method, it doesn't work:

struct Foo;

impl Foo {
    fn foo(self: Foo) { }
}

fn main() {
    let foo = &Foo;
    foo.foo(); // error: cannot move out of borrowed content
}

which is fully expected.

Likewise according to the docs, File does not implement any special traits as far as I can tell. What did get my attention is that Read has a by_ref() method but I don't call it yet everything still works.

What's going on here? (using rustc 1.3.0-dev)

解决方案

The take method comes from the Read trait. That trait is implemented on File, so there is a method File::take(self, u64) -> Take<Self>, but the trait is also implemented on &File (the impl is even listed on the very page you link to). For that impl, the Self type is &File, so its take method takes a reference.

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