closures - 如何在 Rust 的结构中存储闭包？

Question

在 Rust 1.0 之前，我可以使用这种过时的闭包语法编写一个结构:

struct Foo {
    pub foo: |usize| -> usize,
}

现在我可以做类似的事情:

struct Foo<F: FnMut(usize) -> usize> {
    pub foo: F,
}

但是我创建的 Foo 对象的类型是什么？

let foo: Foo<???> = Foo { foo: |x| x + 1 };

我也可以使用引用:

struct Foo<'a> {
    pub foo: &'a mut FnMut(usize) -> usize,
}

我认为这比较慢，因为

1. 指针解引用
2. 对于实际最终使用的 FnMut 类型没有专门化

Answer 1

补充 the existing answer还有一些用于演示目的的代码:

未装箱闭包

使用通用类型:

struct Foo<F>
where
    F: Fn(usize) -> usize,
{
    pub foo: F,
}

impl<F> Foo<F>
where
    F: Fn(usize) -> usize,
{
    fn new(foo: F) -> Self {
        Self { foo }
    }
}

fn main() {
    let foo = Foo { foo: |a| a + 1 };
    (foo.foo)(42);
    
    (Foo::new(|a| a + 1).foo)(42);
}

盒装特征对象

struct Foo {
    pub foo: Box<dyn Fn(usize) -> usize>,
}

impl Foo {
    fn new(foo: impl Fn(usize) -> usize + 'static) -> Self {
        Self { foo: Box::new(foo) }
    }
}

fn main() {
    let foo = Foo {
        foo: Box::new(|a| a + 1),
    };
    (foo.foo)(42);
    
    (Foo::new(|a| a + 1).foo)(42);
}

性状对象引用

struct Foo<'a> {
    pub foo: &'a dyn Fn(usize) -> usize,
}

impl<'a> Foo<'a> {
    fn new(foo: &'a dyn Fn(usize) -> usize) -> Self {
        Self { foo }
    }
}

fn main() {
    let foo = Foo { foo: &|a| a + 1 };
    (foo.foo)(42);
    
    (Foo::new(&|a| a + 1).foo)(42);
}

函数指针

struct Foo {
    pub foo: fn(usize) -> usize,
}

impl Foo {
    fn new(foo: fn(usize) -> usize) -> Self {
        Self { foo }
    }
}

fn main() {
    let foo = Foo { foo: |a| a + 1 };
    (foo.foo)(42);
    
    (Foo::new(|a| a + 1).foo)(42);
}

---

what's the type of a Foo object I create?

这是一个无法命名的自动生成的类型。

I could also use a reference [...] slower because [...] the pointer deref [...] no specialization

也许吧，但这对调用者来说会容易得多。

另见:

• How do I call a function through a member variable?
• Returning a closure from a function
• How to return an anonymous type from a trait method without using Box?
• Closures as a type in a Rust struct
• Types of unboxed closures being unique to each
• Why does passing a closure to function which accepts a function pointer not work?
• What does "dyn" mean in a type?