闭包什么时候实现 Fn、FnMut 和 FnOnce? [英] When does a closure implement Fn, FnMut and FnOnce?

问题描述

闭包实现Fn、FnMut和FnOnce特征的具体条件是什么?

What are the specific conditions for a closure to implement the Fn, FnMut and FnOnce traits?

即:

• 闭包什么时候不实现 FnOnce 特性?
• 闭包什么时候不实现 FnMut trait?
• 闭包什么时候不实现 Fn trait?

• When does a closure not implement the FnOnce trait?
• When does a closure not implement the FnMut trait?
• When does a closure not implement the Fn trait?

例如，改变其主体上的闭包状态会使编译器无法在其上实现 Fn.

For instance, mutating the state of the closure on it's body makes the compiler not implement Fn on it.

推荐答案

每个 trait 代表越来越多的关于闭包/函数的限制性属性，由它们的 call_... 方法的签名表示，尤其是 self 的类型:

The traits each represent more and more restrictive properties about closures/functions, indicated by the signatures of their call_... method, and particularly the type of self:

• FnOnce (self) 是可以调用一次的函数
• FnMut (&mut self) 是可以调用的函数，前提是它们可以&mut 访问其环境
• Fn (&self) 是可以调用的函数，前提是它们只有 & 访问其环境

• FnOnce (self) are functions that can be called once
• FnMut (&mut self) are functions that can be called if they have &mut access to their environment
• Fn (&self) are functions that can be called if they only have & access to their environment

一个闭包<代码>|...|... 将自动实现尽可能多的那些.

A closure |...| ... will automatically implement as many of those as it can.

• 所有闭包都实现了 FnOnce:一个不能被调用一次的闭包不配得这个名字.请注意，如果一个闭包只实现了 FnOnce，那么它只能被调用一次.
• 不移出捕获的闭包实现了 FnMut，允许它们被多次调用(如果对函数对象进行无别名访问).
• 不需要对其捕获进行唯一/可变访问的闭包实现了 Fn，使它们基本上可以在任何地方被调用.

• All closures implement FnOnce: a closure that can't be called once doesn't deserve the name. Note that if a closure only implements FnOnce, it can be called only once.
• Closures that don't move out of their captures implement FnMut, allowing them to be called more than once (if there is unaliased access to the function object).
• Closures that don't need unique/mutable access to their captures implement Fn, allowing them to be called essentially everywhere.

这些限制直接来自于 self 的类型和将闭包脱糖"为结构体；在我的博客文章中描述了在 Rust 中寻找闭包.

These restrictions follow directly from the type of self and the "desugaring" of closures into structs; described in my blog post Finding Closure in Rust.

有关闭包的信息，请参阅闭包:可以捕获其特征的匿名函数Rust 编程语言中的环境.

For information on closures, see Closures: Anonymous Functions that Can Capture Their Environment in The Rust Programming Language.

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